Last week (Thursday 03/14/2008) to be precise, I received my Antares refractor telescope which I had purchased online through the Agena Astro-products web-site. The scope was delivered by UPS in a box that was just under 7' long. Even the UPS driver was wondering and said he rarely saw boxes like these. The scope was well packed. The optical tube was in a plastic bag and semi-circular foam cutouts were used to hold the tube in position. The scope is indeed very long and I am glad that I went for the 1300mm focal length and not for the 1500mm version. The optical tube came with the the dew shield, the finder base, the rings and the dovetail plate all assembled together. It meant that the OTA could be taken out of the box and put right on a mount. But there was another box about a foot long in the main box. This one contained the finder and the finder rings with a quick release bracket. All components --- the OTA, trhe finder had all the caps, etc.
I am not very happy with the cap that fits the focuser. First of all, the focuser is a 2" Crayford style and has an adapter for 1.25" eyepieces. I would have preferred that Antares spend a little more and gave a 2" as well as 1.25" cap. Most amateurs that I know have both eyepieces. Another cap that annoyed me was the cap that fit the dew shield i.e. lens side of the OTA. This is a press-fit style of cap and when I tried removing the cap, the rubber gasket at the end of the dew shield came off. This is another cap that Antares needs to improve. The cap here has to be one that fits from outside of the tube and not from the inside.
Next, I removed the finder, attached the quick release bracket and mounted the finder onto the OTA. So the tube assembly was complete. It took me less than 15mins from the time I opened the box to getting everything assembled. I already had my Orion Sky-view-Pro mount ready and attached the dovetail plate of the OTA to the mount. Everything fit well and there were no surprises there. Even though the tube is so long, it is not very heavy. As a result, I had to use just one counter-weight as opposed to two that are supplied with this mount. Then I attached the diagonal which I had and finally an eyepiece. A few more minutes spent on balancing the scope and I was ready to go. I pointed the scope to a distant tower. The tower was visible clearly and looked amazing. The image was crisp, clear and there was no sign of violet fringing. Next, I bumped up the magnification by switching from a 40mm to 20mm eyepiece. This ones a wide-field having 65degrees AFOV. With 65X magnification, I could easily make out details on that tower. An amateur astronomer is however not satisfied with pointing the scope to a distant tower. After all, the reason I bought the scope was to point it to astronomical objects and not to use it as a spotting scope.
So I waited for night fall and around 11:00pm that night, I was able to see the moon through one of my windows. Finally I had something of significance to look at. I pointed the scope to the moon and there it was, a magnificent work of asteroids and meteorites over millions and billions of years --- the caters, the hills, the valleys, the shadows. And it looked so wonderful. Even when pointing the scope through about 3/4" glass of my window, I was able to see things clearly. This was good news. It meant that the optics were good, really good. I was impatient for the next day (Friday) when we are going to have a star-party at the Onan observatory. Next morning, I got up, went to work and came back home running (or rather driving fast) for the star party. Coming home, I saw that the star party was canceled. What ???? Star party canceled !!! I couldn't believe it. Yes, it was canceled due to the forecasted cloudy weather. Things are usually always this way --- whenever someone buys a new piece of equipment, it guarantees that the skies will be cloudy. This is nothing bu Murphy's law at play here. Feeling sad and cursing the skies, I went out for a drive. As I went away from the city, the skies became clear. Now this was a classic example of Murphy's law at it's worst. First the party was canceled due to cloudy skies and after that the skies cleared up. Man, I was really pissed off. As the night wore on, the skies were crystal clear.
Next day (Saturday) I waited for the alternate star party (if Friday is cloudy, there is an alternate party on Saturdays). I was reading the club-forums, watching the weather, almost all day. Just couldn't wait to get out to the party and enjoy the new scope. On the forums, there was a small discussion about how yesterday's night was clear and that the star party should have been ON but all that was in the past. The weather forecast on TV, the clear-sky clock and the various websites were showing about 20-50% cloudy and I was really thinking that I should have got out yesterday night. As expected, later that day even the Saturday night's star party was canceled and I had all my hopes dashed. I was looking out of the window and the skies were looking less than 20% cloudy. This seemed a case just like the night before when the skies eventually turned out to be clear and the star-party was canceled. Dejected, I watched the idiot box (TV) for sometime while continuing to browse the Internet. Then the club web-site announced that the star-party was ON. I was completely elated and started packing. Made a quick list of things that I needed to carry, dismantled the scope and the mount and loaded up the car. I also took a star-chart and was on my way to Onan. I reached Onan around 08:00pm or so and quickly unloaded my car. Removed the mount and quickly set it up. Attached the counter-weight and then the scope. Since I was not going to do astro-photography, I did just a rough alignment of the polar (RA) axis. The axis angle was already setup to 45degrees so all I had to do was to to point it to the pole star.
After completing the setup, it was time for the "first light" and it had to be the moon. Trying to point the scope at the moon (which was almost overhead), it quickly became apparent that the eyepiece end of the tube was going very low. In fact the eyepiece was less than 2' from the ground. This was a pain. I had completely forgotten that the mount had extensible legs so out came the legs and the eyepiece was now more than 3' from the ground. This was much better. The moon now looked wonderful, amazing --- whatever adjective you want to apply to the earth's only natural satellite. There was absolutely no trace of the violet fringing around the moon's limb. I bumped up the magnification to 65X (20mm eyepiece) and thing were still better. A couple of people from the club also saw through the telescope and were surprised at the lack of color. A few even asked me if it was a semi-apochromatic and I had to tell them that the scope was a simple crown/flint cemented-doublet. Increasing the magnification to 130X (10mm Plossl) made it even better. Even though the 10mm eyepiece is not very good, the moon appeared flawless. close examination of the limb did reveal some minor violet fringing which was to be expected. I wish i had an eyepiece with a focal length in the range of 6-8mm so I could have done more testing at higher magnifications.
Next, it was time for the warlord planet Mars. Mars appeared like a small reddish disc and at low magnification (32.5X), I could barely make out it's disc. At 65X, the disc was clearly visible and so was a small whitish area which I think was the polar cap. At 130X, the polar cap was distinctly visible. However, I must admit that I was somewhat disappointed with Mars as I had expected more. Later on, I turned the scope to the Lord of the rings --- Saturn. Saturn is amazing to look even from a small telescope. With such a long focus instrument, Saturn was a beauty to watch. with the magnification at 130X, the planet was wonderful. It soon became apparent that I needed eyepieces that could deliver more power. The rings were clearly visible and so was the shadow of the rings on the planet. The Cassini division was somewhat visible and I strongly suspect that with more magnification it would be better visible.
Overall, my impressions of the instrument are really good. I left the day wishing that I had better eyepieces and a finder so that I could make more of this wonderful instrument. I am very satisfied with this scope.
Tuesday, March 25, 2008
My Accord crosses 200K miles
Today my 1993 Honda Accord crossed 200K miles. The current odometer reading stands at 200023miles (that's 321906kms for you metric folks). To give some perspective that is 0.837418 (roughly 5/6) times the average distance to the moon. The car would have to run another approximate 38834 miles (or 62649kms) for it to run the distance to the moon. Even though these numbers may seem impressive, this is still 0.21518% of the Earth-to-Sun distance which is 149597870 kms. If these things seem astronomical (well they are astronomical numbers), then we can come back to earth. The circumference of the earth at the equator is 40075.02kms so the car has traveled 8.03258 times around the earth. Now that feels good right ? It would have to travel a total of 360675.18kms to complete the ninth trip. That would mean another 38769.18kms (or 24090miles) of travel. Sticking with the United States, the distance from Los Angeles, CA to Manhattan, NY is 2778 miles (per Google maps). So the round trip distance would be 5556miles. Hence the car has made 36.00126 round trips between the two coasts. If one wanted to drive from Fairbanks, Alaska all the way to Miami, Florida, the car would have done 20.49 round trips i.e. 20 round and another one-way trip.
Let us look at things from other angles. The car gives me approx 29 - 30 miles per gallon in mixed city/highway driving. Assuming a rating of 30mpg, the car has consumed 6667.43 gallons (or 25238.9807 liters) of gasoline fuel to travel that distance. Maybe when the car was new and was driven more frequently on the highways, the mpg would be higher so this number might be a little bit exaggerated. even than, the car has consumed 6500 gallons of fuel. Just out of curiosity, if the car had a diesel engine, the car would have consumed 6667.43/1.3 = 5128.79 gallons of diesel fuel. This is because all things being equal, a diesel engine typically gets 30% more mpg.
More ways to look at the same thing --- the car has gone through at least 50 oil changes (assuming 4000 miles per oil change), 200 gallons of motor oil (the crankcase holds just under 4 gallons of oil) and 5 sets of tires (40000 miles per tyre set) --- thats 20 tires in all.
Now for some history lesson --- I bought this car almost a year back in the second week of April, when the car had 190993 miles on it. This means that I have driven this baby for a total of 9030 miles in about 11.5 months, which translates to approx 785 miles per month. Another way to look at this is that I have shaved (or should it be saved) 9030 miles off my other car --- 2004 Volkswagen Passat. That car has slightly more than 56000 miles so without the Accord and assuming that my driving would have been similar, the Passat would be over 65000 miles by now. The Accord car was bought from a student who owned it for several years.
Overall, I am very satisfied with this car. It is a fun to drive, stick shift (manual transmission) car with a peppy engine all the while giving a decent mileage. To top it off, the car is still gong strong at 15+ years and 200K miles.
A great many thanks to Honda for making a car worth driving !!!!
Let us look at things from other angles. The car gives me approx 29 - 30 miles per gallon in mixed city/highway driving. Assuming a rating of 30mpg, the car has consumed 6667.43 gallons (or 25238.9807 liters) of gasoline fuel to travel that distance. Maybe when the car was new and was driven more frequently on the highways, the mpg would be higher so this number might be a little bit exaggerated. even than, the car has consumed 6500 gallons of fuel. Just out of curiosity, if the car had a diesel engine, the car would have consumed 6667.43/1.3 = 5128.79 gallons of diesel fuel. This is because all things being equal, a diesel engine typically gets 30% more mpg.
More ways to look at the same thing --- the car has gone through at least 50 oil changes (assuming 4000 miles per oil change), 200 gallons of motor oil (the crankcase holds just under 4 gallons of oil) and 5 sets of tires (40000 miles per tyre set) --- thats 20 tires in all.
Now for some history lesson --- I bought this car almost a year back in the second week of April, when the car had 190993 miles on it. This means that I have driven this baby for a total of 9030 miles in about 11.5 months, which translates to approx 785 miles per month. Another way to look at this is that I have shaved (or should it be saved) 9030 miles off my other car --- 2004 Volkswagen Passat. That car has slightly more than 56000 miles so without the Accord and assuming that my driving would have been similar, the Passat would be over 65000 miles by now. The Accord car was bought from a student who owned it for several years.
Overall, I am very satisfied with this car. It is a fun to drive, stick shift (manual transmission) car with a peppy engine all the while giving a decent mileage. To top it off, the car is still gong strong at 15+ years and 200K miles.
A great many thanks to Honda for making a car worth driving !!!!
Saturday, March 15, 2008
Ideal eyepiece collection
Since I have decided to pursue my astronomy hobby again, one of the things that I need to do is to update my equipment arsenal. Having just bought a telescope (a superb 105mm f/12.4 refractor) and having completed a 6" f/5 mirror (and I have also started grinding a 12" f/5 mirror), I decided that now would be a good time to augment my eyepieces. While a telescope gets most attention in astronomy, an eyepiece is also a very important part. A bad eyepiece can ruin a good telescope. Upgrading the eyepiece to a better one may be the easiest (and probably the cheapest) way to enhance the "output" of a telescope. Thinking along these lines, I decided to upgrade my eyepieces.
Before moving further, the first step was to take stock of what I have. All in all, I have three eyepieces currently --- a 10mm Plossl, a 20mm wide field and a 40mm Plossl. All three have a 1.25" diameter barrel. The 10mm is a standard Plossl with approx 52degrees Apparent Field of View (AFOV for short), the 20mm eyepiece has 65degrees AFOV and the 40mm Plossl has a 45degrees AFOV. when used with my long focus refractor, these serve as a low power (32.5X, 1.6degrees TFOV), medium power (65X, 1.0degrees TFOV) and a high power (130X, 0.4degrees TFOV). Although having three powers is considered as ideal, the quality of the 10mm eyepiece leaves a lot to be desired. This eyepiece was received as a freebie along with some ATM components that I purchased a while back. I would certainly like to replace this eyepiece. And with the advent of some decent quality low-cost ultra-super-ultimate-enhanced wide-angle eyepieces (eyepieces probably use more adjectives to describe them than any other object in the Universe), I decided it was time to upgrade. So in this article I am setting out to describe what should be my ideal eyepiece collection. I will try to have a broad range of powers, wide and flat fields, and a suitability for my long focus refractor as well as for the short-focus scope that I am trying to build. The eyepieces must also serve any future scopes (most likely a grab-and-go scope and/or a large-aperture dobsonian) that I might buy.
In order to decide the extreme range of powers, I had to do some basic calculations of exit-pupil and maximum magnifications. Most of my observations are done from a partly dark suburban park like area so I doubt that my eye-pupil size exceeds 6mm. Further since I plan to complete my short focus 6" f/5 scope eventually, the eyepiece that gives lowest possible power and widest possible true fields with these scopes would be essential. For a scope having f/5 as focal ratio and 6mm exit pupil, the lowest power I can use is 25X. Accurate measurements indicate that the mirror has a focal length of about 775mm (30.5"). An eyepiece having 31mm focal-length would give the lowest possible useful magnification on this scope while allowing me to use a full 6mm eye-pupil. With the refractor, the minimum useful magnification is only 18X. Since the focal length of this scope is 1300mm, that translates to an eyepiece of approx 74mm. The thing to note here is that the long focus refractor is not designed to provide wide field views so I doubt I will ever buy an eyepiece of 74mm or even anything having more than 50mm focal length. So for the lowest possible magnifications, the shorty scope can use an eyepiece with not more than 31mm focal length while the refractor not more than 74mm.
On the other hand, for the highest possible magnifications, for the shorty, I would consider 30X/inch as the maximum useful (note the word "useful" here, not "theoretical") magnification since this scope is primarily to be used as wide field rather than a high powered scope. That converts to about 180X of useful magnification. With its' 775mm focal length, an eyepiece of 4.3mm would be required to do the trick. For the refractor,a magnification of 50X/inch is probably the upper limit. Considering 50x/inch of aperture as the upper limit, it would mean a maximum magnification of 205X (4.1" x 50 = 205X). With its 1300mm focal length, a 6.3mm eyepiece would give the required magnification.
Below is a table that lists the minimum, moderate and maximum magnifications for both the scopes. I am defining moderate magnifications as half the maximum magnifications (i.e. 25X/inch for the 'fractor and 15X/inch for the shorty).
----------------------------------------------------------------
Scope Minimum Moderate Maximum
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6" f/5 31X (31mm) 90X (8.6mm) 180X (4.3mm)
4" f/12.4 18X (74mm) 103X (13mm) 205X (6.3mm)
----------------------------------------------------------------
Now my goal is to find a range of eyepieces that allow me to use the maximum possible range of magnifications while providing with the widest possible fields and a flat-field. I will also try to lookup particular brand reviews on Cloudy-nights and other websites and try to narrow down my selection.
Starting on the highest end, having a 6-7mm eyepiece would be a good start. There are reasons I do not want to buy a 4.3mm eyepiece ---- Firstly, eyepieces having focal lengths less than 6mm usually have extremely short eye-reliefs. Secondly, it is very difficult to eliminate most aberrations from an eyepiece having such short focal lengths. Finally, considering that there are very few nights which offer the best "seeing", I suppose using a slightly lower powered eyepiece in conjunction with a 1.5X barlow lens may be more desirable. That would allow me to use somewhat less magnifications when the seeing is normal (i.e. not so good) while still allowing a high magnification for those "perfect" nights. Thus, rather than using an eyepiece specially for this highest possible magnification for the perfect night, I would prefer to use a more "general" eyepice along with a 1.5X - 2X barlow. For e.g. using 6.5mm eyepiece would give a magnification of 119X for the shorty scope and 200X for the refractor. Using this eyepeice in combination with a 1.5X barlow lens would give me 179X on the shorty. Another reason for the same is that a 4.3mm eyepiece would be almost useless with the refractor as it would mean a magnification of 302X (73X/inch of aperture) which would be beyond what that instrument can handle. Since this eyepiece is to be used for high powers, having very wide fields is not a condition. A wide field would be nice to have but it is not essential. Thus, buying an eyepiece of 6-7mm focal length with 60-80degrees AFOV and using it with a 1.5X barlow lens would be a good start on the high power end.
On the lower end, a 74mm eyepiece is out of question. Not only would an eyepiece with that kind of focal length be extremely rare, it is likely to be expensive also. Further, since the reason I purchased the refractor is not for wide-sky views but rather for planetary studies which require moderate to high powers. An eyepiece of around 40-45mm focal length would be ideal as it would give a low power on the refractor and while the exit-pupil on the shorty would exceed 6mm (8.8mm to 7.8mm), I could still use it when I am in a dark location. In addition using the 1.5X barlow would give me
For the intermediate powers, I am planning to get use eyepieces. I plan to keep the 20mm eyepiece that I have. It will give me 65X of the refractor and 39X on the shorty. With the 1.5X barlow, I would get 98X and 58X. In the table below, I am listing various commonly used eyepieces and the magnification/TFOV that I can get when used with the shorty and the 'fractor.
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Eyepiece 6" f/5 shorty 105mm f/12.4 refractor Remarks
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6.5mm 80deg AFOV 119X/0.67 179X/0.45 200X/0.40 -------
10mm 52deg AFOV 78X/0.67 116X/0.45 130X/0.40 195X/0.27
13mm 80deg AFOV 60X/1.34 90X/0.90 100X/0.80 150X/0.53
16mm 80deg AFOV 48X/1.65 73X/1.10 81X/0.99 122X/0.66
20mm 67deg AFOV 39X/1.73 58X/1.15 65X/1.03 98X/0.69
25mm 80deg AFOV 31X/2.58 47X/1.72 52X/1.54 78X/1.03
30mm 80deg AFOV 26X/3.10 39X/2.06 43X/1.85 65X/1.23
40mm 44deg AFOV -------- 29X/1.51 32X/1.35 49X/0.90
1.5X barlow lens
----------------------------------------------------------------------------------
This is the ideal list of eyepieces that I would like to have. Since I already own three eyepieces, now the task before me is to read through reviews and select eyepieces that match or closely match the above criteria. I realize that the tables above are not displaying properly. I will try to find out a way so that the tables look as they were intended to.
Before moving further, the first step was to take stock of what I have. All in all, I have three eyepieces currently --- a 10mm Plossl, a 20mm wide field and a 40mm Plossl. All three have a 1.25" diameter barrel. The 10mm is a standard Plossl with approx 52degrees Apparent Field of View (AFOV for short), the 20mm eyepiece has 65degrees AFOV and the 40mm Plossl has a 45degrees AFOV. when used with my long focus refractor, these serve as a low power (32.5X, 1.6degrees TFOV), medium power (65X, 1.0degrees TFOV) and a high power (130X, 0.4degrees TFOV). Although having three powers is considered as ideal, the quality of the 10mm eyepiece leaves a lot to be desired. This eyepiece was received as a freebie along with some ATM components that I purchased a while back. I would certainly like to replace this eyepiece. And with the advent of some decent quality low-cost ultra-super-ultimate-enhanced wide-angle eyepieces (eyepieces probably use more adjectives to describe them than any other object in the Universe), I decided it was time to upgrade. So in this article I am setting out to describe what should be my ideal eyepiece collection. I will try to have a broad range of powers, wide and flat fields, and a suitability for my long focus refractor as well as for the short-focus scope that I am trying to build. The eyepieces must also serve any future scopes (most likely a grab-and-go scope and/or a large-aperture dobsonian) that I might buy.
In order to decide the extreme range of powers, I had to do some basic calculations of exit-pupil and maximum magnifications. Most of my observations are done from a partly dark suburban park like area so I doubt that my eye-pupil size exceeds 6mm. Further since I plan to complete my short focus 6" f/5 scope eventually, the eyepiece that gives lowest possible power and widest possible true fields with these scopes would be essential. For a scope having f/5 as focal ratio and 6mm exit pupil, the lowest power I can use is 25X. Accurate measurements indicate that the mirror has a focal length of about 775mm (30.5"). An eyepiece having 31mm focal-length would give the lowest possible useful magnification on this scope while allowing me to use a full 6mm eye-pupil. With the refractor, the minimum useful magnification is only 18X. Since the focal length of this scope is 1300mm, that translates to an eyepiece of approx 74mm. The thing to note here is that the long focus refractor is not designed to provide wide field views so I doubt I will ever buy an eyepiece of 74mm or even anything having more than 50mm focal length. So for the lowest possible magnifications, the shorty scope can use an eyepiece with not more than 31mm focal length while the refractor not more than 74mm.
On the other hand, for the highest possible magnifications, for the shorty, I would consider 30X/inch as the maximum useful (note the word "useful" here, not "theoretical") magnification since this scope is primarily to be used as wide field rather than a high powered scope. That converts to about 180X of useful magnification. With its' 775mm focal length, an eyepiece of 4.3mm would be required to do the trick. For the refractor,a magnification of 50X/inch is probably the upper limit. Considering 50x/inch of aperture as the upper limit, it would mean a maximum magnification of 205X (4.1" x 50 = 205X). With its 1300mm focal length, a 6.3mm eyepiece would give the required magnification.
Below is a table that lists the minimum, moderate and maximum magnifications for both the scopes. I am defining moderate magnifications as half the maximum magnifications (i.e. 25X/inch for the 'fractor and 15X/inch for the shorty).
----------------------------------------------------------------
Scope Minimum Moderate Maximum
----------------------------------------------------------------
6" f/5 31X (31mm) 90X (8.6mm) 180X (4.3mm)
4" f/12.4 18X (74mm) 103X (13mm) 205X (6.3mm)
----------------------------------------------------------------
Now my goal is to find a range of eyepieces that allow me to use the maximum possible range of magnifications while providing with the widest possible fields and a flat-field. I will also try to lookup particular brand reviews on Cloudy-nights and other websites and try to narrow down my selection.
Starting on the highest end, having a 6-7mm eyepiece would be a good start. There are reasons I do not want to buy a 4.3mm eyepiece ---- Firstly, eyepieces having focal lengths less than 6mm usually have extremely short eye-reliefs. Secondly, it is very difficult to eliminate most aberrations from an eyepiece having such short focal lengths. Finally, considering that there are very few nights which offer the best "seeing", I suppose using a slightly lower powered eyepiece in conjunction with a 1.5X barlow lens may be more desirable. That would allow me to use somewhat less magnifications when the seeing is normal (i.e. not so good) while still allowing a high magnification for those "perfect" nights. Thus, rather than using an eyepiece specially for this highest possible magnification for the perfect night, I would prefer to use a more "general" eyepice along with a 1.5X - 2X barlow. For e.g. using 6.5mm eyepiece would give a magnification of 119X for the shorty scope and 200X for the refractor. Using this eyepeice in combination with a 1.5X barlow lens would give me 179X on the shorty. Another reason for the same is that a 4.3mm eyepiece would be almost useless with the refractor as it would mean a magnification of 302X (73X/inch of aperture) which would be beyond what that instrument can handle. Since this eyepiece is to be used for high powers, having very wide fields is not a condition. A wide field would be nice to have but it is not essential. Thus, buying an eyepiece of 6-7mm focal length with 60-80degrees AFOV and using it with a 1.5X barlow lens would be a good start on the high power end.
On the lower end, a 74mm eyepiece is out of question. Not only would an eyepiece with that kind of focal length be extremely rare, it is likely to be expensive also. Further, since the reason I purchased the refractor is not for wide-sky views but rather for planetary studies which require moderate to high powers. An eyepiece of around 40-45mm focal length would be ideal as it would give a low power on the refractor and while the exit-pupil on the shorty would exceed 6mm (8.8mm to 7.8mm), I could still use it when I am in a dark location. In addition using the 1.5X barlow would give me
For the intermediate powers, I am planning to get use eyepieces. I plan to keep the 20mm eyepiece that I have. It will give me 65X of the refractor and 39X on the shorty. With the 1.5X barlow, I would get 98X and 58X. In the table below, I am listing various commonly used eyepieces and the magnification/TFOV that I can get when used with the shorty and the 'fractor.
----------------------------------------------------------------------------------
Eyepiece 6" f/5 shorty 105mm f/12.4 refractor Remarks
----------------------------------------------------------------------------------
6.5mm 80deg AFOV 119X/0.67 179X/0.45 200X/0.40 -------
10mm 52deg AFOV 78X/0.67 116X/0.45 130X/0.40 195X/0.27
13mm 80deg AFOV 60X/1.34 90X/0.90 100X/0.80 150X/0.53
16mm 80deg AFOV 48X/1.65 73X/1.10 81X/0.99 122X/0.66
20mm 67deg AFOV 39X/1.73 58X/1.15 65X/1.03 98X/0.69
25mm 80deg AFOV 31X/2.58 47X/1.72 52X/1.54 78X/1.03
30mm 80deg AFOV 26X/3.10 39X/2.06 43X/1.85 65X/1.23
40mm 44deg AFOV -------- 29X/1.51 32X/1.35 49X/0.90
1.5X barlow lens
----------------------------------------------------------------------------------
This is the ideal list of eyepieces that I would like to have. Since I already own three eyepieces, now the task before me is to read through reviews and select eyepieces that match or closely match the above criteria. I realize that the tables above are not displaying properly. I will try to find out a way so that the tables look as they were intended to.
Friday, March 14, 2008
Recent telescope purchase
Recent telescope purchase
Recently, 03/02/2008 to be exact, I purchased an Antares 105mm f/12.4 (F=1300mm) achromatic refractor made by Antares Optical (Canada). I had been interested in this scope for quite sometime especially since the time I purchased a partially built 4" scope of similar dimensions. Since, my scope was out of use due to a broken focuser and since non-standard parts were used for making the scope (PVC tubes, etc.), it was very difficult to get the scope back in working condition. What got me more interested was that the scope I was looking at and the scope that I owned, both had Vixen spec objective lenses made by the same company. I have always been pleased with the performance of my telescope and so was looking for something along similar lines. A slightly bigger telescope (say a 5" scope) would have been better however, with the long focus scopes that I am interested in, the lengths quickly become too unwieldy. A 5" scope with a similar focal ratio would be about 62" long or about 12" longer than this scope. In addition bigger scopes need even longer focal ratios to keep down chromatic aberration so something that was at lest f/13 (read 65" long) would be needed. That would have proved to be too difficult for my mount which is a Orion SV-Pro mount.
Recent years have seen rebirth of the refractor, especially the short focus scopes with a fluorite doublet or a triplet lens i.e apochromatic or APO for short. Also new to the market are the short focus refractors but with a regular 2-element achromatic lens, such as the Orion 80/100/120mm f/5 scopes. These are different from the fluorite 2-element lenses in the sense that these use the standard crown/flint lens combinations and are usually air-spaced. The Fluorite doublets on the other hand use an extra low dispersion lens that reduces the chromatic aberration even further. The APO lenses are in a complete different class than these two as the three element design combined with the extra-low dispersion elements produce a virtually color-free image. The fluorite 2-element are however costly to produce than the regular 2-element lenses while the APO lenses are way too expensive. The best part about these Fluorite and APO lenses is that the chromatic aberration can be reduced significantly even with a short focal ratio (< f/7). This is important because a shorter tube means a simpler mount will do the trick. It also makes the scope portable enough to be carried in carry on luggage on an airliner.
While the short focus refractors have seen their advantages, I am not a vary big fan of them. Although the short tubes are portable, the long focus scopes have their advantages. First and foremost is that longer focal ratios mean not just reduced aberrations, not just chromatic but also spherical and other aberrations that affect a scope. It takes a lot of work to get a short focus scope to correct for all these aberrations. Many of the cheaper scopes are not corrected very well while on the higher end, the scopes that correct (or compensate) these aberrations get too expensive. A long focus refractor can correct for many of the aberrations with even a simple crown/flint doublet and still be priced reasonably. Another advantage of these is that a higher magnification can be achieved with eyepieces that do not have extremely short focal-lengths. A 100mm diameter f/12 refractor (with 1200mm focal length) would yield 150mm magnification with a 8mm eyepiece while another refractor with similar diameter but with a focal ratio of f/5 (500mm focal length) would need need a 3.3mm eyepiece to get the same magnification. A 3.3mm eyepiece is likely to have a very short eye-relief compared to the 8mm eyepiece. Let me point out one more thing here --- while magnification is not everything, a higher magnification is needed for certain astronomical objects, mainly planets.
When I looked at the Antares Optical web-site about sometime back, they had a new product --- a 105mm (4.1") refractor which they advertised as "in the spirit of Unitron". This was in reference to some marvelous long-focus refractors made by Unitron a couple of decades ago. Unitron does not make anything similar now. This was interesting as very few manufacturers make long focus scopes. The longest focus scope in 100-120mm range that I have found from a major manufacturer is the C4 f/9.8 (102mm diameter objective and 1000mm focal length). I have long been a fan of refractors and more so of those that have focal ratios greater than f/12 or so. Looking at Antares site, they were selling the 105mm objective with three focal ratios --- f/9.5 (1000mm), f/12.4 (1300mm) and f/14.3 (1500mm). I went looking for reviews on the 1300mm and 1500mm scopes and found very few. More reviews of the 1500mm scope were found and they all pointed to negligible chromatic aberration. I looked up the dealers on Antares web-site and called a couple of them. One of these dealers, Agena Astro-products located in southern California, seemed very helpful. The owner, Manish was kind enough to answer my questions, even on a weekend. I browsed their web-site while continuing to read reviews for the scopes. My talks with the owner were mainly concerning the trade-off between the longer tube of the f/14 scope (1500mm) vs chromatic aberration of the f/12.4 (1300mm) scope. Going through the reviews and based on my talks with various people, nest day I placed order with Agena for an Antares 105mm diameter f/12.4 (1300mm focal length) scope. I think this is a good balance in the sense that the longer focal ratio (compared with some other scopes) will ensure far less aberrations than shorties (f/5 - f/7) while ensuring that the tube won't be as long as the 1500mm focal length scopes.
Now begins that waiting game. Days and days of waiting for the scope to arrive. I will keep you all posted as and when the telescope arrives.
Recently, 03/02/2008 to be exact, I purchased an Antares 105mm f/12.4 (F=1300mm) achromatic refractor made by Antares Optical (Canada). I had been interested in this scope for quite sometime especially since the time I purchased a partially built 4" scope of similar dimensions. Since, my scope was out of use due to a broken focuser and since non-standard parts were used for making the scope (PVC tubes, etc.), it was very difficult to get the scope back in working condition. What got me more interested was that the scope I was looking at and the scope that I owned, both had Vixen spec objective lenses made by the same company. I have always been pleased with the performance of my telescope and so was looking for something along similar lines. A slightly bigger telescope (say a 5" scope) would have been better however, with the long focus scopes that I am interested in, the lengths quickly become too unwieldy. A 5" scope with a similar focal ratio would be about 62" long or about 12" longer than this scope. In addition bigger scopes need even longer focal ratios to keep down chromatic aberration so something that was at lest f/13 (read 65" long) would be needed. That would have proved to be too difficult for my mount which is a Orion SV-Pro mount.
Recent years have seen rebirth of the refractor, especially the short focus scopes with a fluorite doublet or a triplet lens i.e apochromatic or APO for short. Also new to the market are the short focus refractors but with a regular 2-element achromatic lens, such as the Orion 80/100/120mm f/5 scopes. These are different from the fluorite 2-element lenses in the sense that these use the standard crown/flint lens combinations and are usually air-spaced. The Fluorite doublets on the other hand use an extra low dispersion lens that reduces the chromatic aberration even further. The APO lenses are in a complete different class than these two as the three element design combined with the extra-low dispersion elements produce a virtually color-free image. The fluorite 2-element are however costly to produce than the regular 2-element lenses while the APO lenses are way too expensive. The best part about these Fluorite and APO lenses is that the chromatic aberration can be reduced significantly even with a short focal ratio (< f/7). This is important because a shorter tube means a simpler mount will do the trick. It also makes the scope portable enough to be carried in carry on luggage on an airliner.
While the short focus refractors have seen their advantages, I am not a vary big fan of them. Although the short tubes are portable, the long focus scopes have their advantages. First and foremost is that longer focal ratios mean not just reduced aberrations, not just chromatic but also spherical and other aberrations that affect a scope. It takes a lot of work to get a short focus scope to correct for all these aberrations. Many of the cheaper scopes are not corrected very well while on the higher end, the scopes that correct (or compensate) these aberrations get too expensive. A long focus refractor can correct for many of the aberrations with even a simple crown/flint doublet and still be priced reasonably. Another advantage of these is that a higher magnification can be achieved with eyepieces that do not have extremely short focal-lengths. A 100mm diameter f/12 refractor (with 1200mm focal length) would yield 150mm magnification with a 8mm eyepiece while another refractor with similar diameter but with a focal ratio of f/5 (500mm focal length) would need need a 3.3mm eyepiece to get the same magnification. A 3.3mm eyepiece is likely to have a very short eye-relief compared to the 8mm eyepiece. Let me point out one more thing here --- while magnification is not everything, a higher magnification is needed for certain astronomical objects, mainly planets.
When I looked at the Antares Optical web-site about sometime back, they had a new product --- a 105mm (4.1") refractor which they advertised as "in the spirit of Unitron". This was in reference to some marvelous long-focus refractors made by Unitron a couple of decades ago. Unitron does not make anything similar now. This was interesting as very few manufacturers make long focus scopes. The longest focus scope in 100-120mm range that I have found from a major manufacturer is the C4 f/9.8 (102mm diameter objective and 1000mm focal length). I have long been a fan of refractors and more so of those that have focal ratios greater than f/12 or so. Looking at Antares site, they were selling the 105mm objective with three focal ratios --- f/9.5 (1000mm), f/12.4 (1300mm) and f/14.3 (1500mm). I went looking for reviews on the 1300mm and 1500mm scopes and found very few. More reviews of the 1500mm scope were found and they all pointed to negligible chromatic aberration. I looked up the dealers on Antares web-site and called a couple of them. One of these dealers, Agena Astro-products located in southern California, seemed very helpful. The owner, Manish was kind enough to answer my questions, even on a weekend. I browsed their web-site while continuing to read reviews for the scopes. My talks with the owner were mainly concerning the trade-off between the longer tube of the f/14 scope (1500mm) vs chromatic aberration of the f/12.4 (1300mm) scope. Going through the reviews and based on my talks with various people, nest day I placed order with Agena for an Antares 105mm diameter f/12.4 (1300mm focal length) scope. I think this is a good balance in the sense that the longer focal ratio (compared with some other scopes) will ensure far less aberrations than shorties (f/5 - f/7) while ensuring that the tube won't be as long as the 1500mm focal length scopes.
Now begins that waiting game. Days and days of waiting for the scope to arrive. I will keep you all posted as and when the telescope arrives.
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