Small, cheap and fast and . . .?

Let me get to the bottom line - I was testing a Celestron 102mm F5 against an Orion ShortTube 80mm F5 for brightness and quality and while I would give the edge - in brightness - to the 102mm, the differenece was slight. I didn't see a difference in quality.

Ok . . . backing up . . .

I've used small, fast, cheap refractors for several years off and on and like them. Yes, these are just achromatics and yes, their F5 focal ratios mean they are not going to be all that sharp. I expect a purple fringe around the moon, planets and the brightest stars. That said, I have a nagging feeling we can get too demanding on our optics, not focusing enough on what they do reveal and honing in on their relatively minor faults.

I'm also beginning to wonder if we don't cut the size business far too fine.

Just beginning - I'm not at all sure on either of these points - well, more confident about the quality issue. My testing yesterday and this morning brought both issues into focus, but primarily the size one. I just received Friday - near the start of two days of sleet, rain, and snow - a used Celestron 102 F5 refractor - what they sell as a "wide view spotting scope." The price was right - $120 for the optical tube, mounting rings, 6X30 finder, 45-degree prism diagonal, and a 20 mm Plossl I think is quite nice. (You can get these for about $210 new, but without the rings and something less of a finder.)

With snow continuing to fall, I set up the 102 inside the next day, beside an Orion ShortTube 80 - roughly the equivalent in price and quality and also an F5 scope. (Orion has these on sale right now for $149.) Because of my planned use for the new scope - straight through viewing at low power in the revolving beach chair with p-mount (see this post and this one) - I was hoping to see a very noticeable difference in image brightness, even in daylight. I aimed it into the woods where it was plenty dull, so I could judge this.

Shock. The little 80mm ShortTube did better - I mean really, noticeably better. I was crestfallen. Did I get a lousy Celestron? The light gathering area of the 80mm lens is 5,024 square millimeters. The comparable figure for the 102 is 8,167. That's an increase of 3,143 sq. mm - or about about a 63 percent increase in light gathering surface which should count for something. Again, in theory it should mean I can see stars that are half a magnitude fainter. Since people regularly evaluate variable stars to one-tenth of a mgnitude, half a magnitude should be more than discernible to an experienced observer. I switched eyepieces. i tried to balance the power difference between the two with different eyepieces. The 80mm kept winning.

I set them aside, but when the sleet and rain stopped, I took them outside in daylight for a side-by-side test that didn't involve looking through windows. Same result! Ugh. So I switched diagonals. Ouch! That was it big time. The 45-degree correct image diagonal that came with the 102mm was really poor. (Or maybe's it's dirty - I haven't examined it closely yet.)

In any event, suddenly the 102 was outperforming the 80 - which now had the poor diagonal in it - and when I put good diagonals in both . . . well, it was too close to call.

My Clear Sky Clock said I should have a window of opportunity about 6 am this morning. I was up at 4:30 and it was already clear! Great. I tended to the puppies and got out in time for some legitmate, though hurried, checks. (I was racing the dawn -lovely, chill morning with about an inch of crisp, crunchy snow on the ground still.)

With good diagonals in both scopes and with eyepieces that roughly matched in power - 25.5X in the 102, and 26.6X in the 80 - well, that's what the math says. I never believe these numbers are that precise. But this is important because a difference in power makes a difference in field of view and in the dimmest objects you can see,

I tested on Albireo and the stars around it, on M57, the Ring Nebula, and on Mizar, that wonderful little field of stars in the handle of the Big Dipper. My results? An edge to the 102mm, but it's subjective. I could find no single star in the 102 that I could not also see in the 80mm. With the two double stars I examined, I looked for the faintest star in the field - the one that seemed to be on the edge of the scope's capability - and in each case I found the same faint stars with each scope. Now, I'm quite sure this really wasn't the faintest one the scope was capable of revealing - I believe it was a 10.1 magnitude star when I should be seeing another half magnitude, or so. Hard to say exactly because so muh depend son weather conditions and light pollution. When looking at M57 I tried to evaluate how easily I could see the hole in the middle. Not easily - but again, roughly the same results.

Bottom line - I remain deeply suspicious of the value of relatively small changes in objective diameter in telescopes. I believe they are there - I just believe that they only show up at the extreme and the way most people observe, I wonder if they would even have time to notice. Double the objective diameter and there's a real gain that anyone can see. But go, for example, from a six-inch to an eight-inch reflector and I'm just not that impressed. And I'm not that impressed at the difference between the 80mm and the 102mm - as long as they are both achromatics.

I don't consider this the last word on this scope, however. I still need to use it straight through in my observing chair and I need to do some more demanding tests, such as counting stars in an open cluster like M37. I would also like to see if there's a detectable difference in performance when the scopes are turned on a globular, such as M13.

Finally - and more importantly - I want to do a quality check. This scope is almost exactly the same size - same tube diameter andlength, that is - as the Orion 80ED. The difference is the 80ED is an apochromatic and operates at F7.5. Just operating at F7.5 could make a significant difference, but I know the quality is there in other ways. So which do I want to use in my chair? Well, the one that allow s me to see the faintest objects. So the big question is does a quality, longer focal length 80mm lens outperform a lower quality, shorter focal length 102mm lens when it comes to seeing dim objects? Oh - and I really need to try them both on extended dim objects - galaxies and nebula, such as M1. I suspect the higher quality 80mm will win out on faint stars simply because it will deliver a sharper image. But we'll see.

While I really want the answer to this - I'm also getting tired of testing. If these scopes are real close in performance, then I don't really care about results. I don't wnat to spend all my valuable observing time testing equipment - I want to observe! ;-)

Comments? Please send them to me and I'll post them here - send them to: gregstone@verizon.net

Hi Clay:

You're right - the gap between 80 and 102 is about 63% (in area) , the gap from 102 to 150 is about 116%, so i would expect it to be more noticeable. The gap from the 6-inch (150) to the 8-inch (200) is 78% - bigger, then, than the gap from 80 to 100, but not as big as the gap from 100 to 150. So yes, comparing the 102 with a 6-inch should show significant jump. Certainly you should have noticed more than I noticed with the 80-to-100 comparison.

I agree that "aperture rules." But I question just how much it does. I find your statement that you "definitely. . . enjoy the quality of the views thru my 80ED better than those thru the 102" interesting. That's the question I need to explore. I have no doubt doubt the 80ED delivers a sharper image. I also don't doubt that this can result in a sense that you would rather look through it than the 102. But I still want to put this to a more objective test - I want to see whether the better optics of the 80 ED result in more information reaching your brain - or whether the information is simply "better" in some other way? To me the real test will be on M13 and other globs where separating source of light that are close together is important.

In theory the 102 can separate two stars of equal magnitude roughly 2 seconds apart. The 80 mm should only be able to separate them if they are abut 2.6 seconds apart, But - the superior optics in the 80 ED may have it performing closer to its theoretical limits that the optics in the 102. That, I suspect will be the case, but I need to see. And then there the question of an extended source like a galaxy or nebula or comet. Here, I would expect the 102, with significantly more light gathering area, to do a little better. But I'll try to keep an open mind and open eyes and test my belief with what I actually see ;-)

Bottom line - I want to decide which scope to mount on the p-mount and use straight through with the chair. It will be the one with which I can see the most - period. For example, I would like to do a lot of galaxy hunting in Leo and Virgo this spring with it and that will be tough, I can see these galaxies better, of course, with the 15-inch Obsession and better still - another two magnitudes or more better - with the video camera attached to the 15. But I do have reasons why I want to be able to see them from the chair using smaller scopes, so . . . the forecast says it won't be clear tonight until after 10- hmmmm, and about then Leo and maybe Virgo will be high enough, so . . . stay tuned.

On Mar 18, 2007, at 10:59 AM, Clayton Cooper wrote:

Hi Greg,

Thanks for the link to your interesting write-up. Not too long ago I looked at the same objects thru my 102mm refractor, my 6" f/5 Dob and my 8" f/5 Dob. It was readily apparent to me in a very visible way that "aperture rules" when doing this comparison. With each increase in aperture size, fainter stars could be seen in the field and the views were brighter. I would probably draw the same conclusions that you did if comparing the 80mm to the 102, because here we're talking about a much smaller aperture difference here. I can definitely tell you that I enjoy the quality of the views thru my 80ED better than those thru the 102. Try comparing the 102 to a 6" short focus reflector if you have one - I think that you'll agree that the differences are definitely noticible in this case. I had my 6" f/5 Dob out last night and I really enjoyed it - this may be my favorite grab 'n go scope because it's relatively light, the Dob mounting is very stable, and it shows me a lot while I'm out there. I made my 6" f/5 Dob using a C6N OTA that I bought on AstroMart and it performs pretty well for a mass produced OTA.

Regards,
Clay

Hi Clay:

You're right - the gap between 80 and 102 is about 63% (in area) , the gap from 102 to 150 is about 116%, so i would expect it to be more noticeable. The gap from the 6-inch (150) to the 8-inch (200) is 78% - bigger, then, than the gap from 80 to 100, but not as big as the gap from 100 to 150. So yes, comparing the 102 with a 6-inch should show significant jump. Certainly you should have noticed more than I noticed with the 80-to-100 comparison.

I agree that "aperture rules." But I question just how much it does. I find your statement that you "definitely. . . enjoy the quality of the views thru my 80ED better than those thru the 102" interesting. That's the question I need to explore. I have no doubt doubt the 80ED delivers a sharper image. I also don't doubt that this can result in a sense that you would rather look through it than the 102. But I still want to put this to a more objective test - I want to see whether the better optics of the 80 ED result in more information reaching your brain - or whether the information is simply "better" in some other way? To me the real test will be on M13 and other globs where separating source of light that are close together is important.

In theory the 102 can separate two stars of equal magnitude roughly 2 seconds apart. The 80 mm should only be able to separate them if they are abut 2.6 seconds apart, But - the superior optics in the 80 ED may have it performing closer to its theoretical limits that the optics in the 102. That, I suspect will be the case, but I need to see. And then there the question of an extended source like a galaxy or nebula or comet. Here, I would expect the 102, with significantly more light gathering area, to do a little better. But I'll try to keep an open mind and open eyes and test my belief with what I actually see ;-)

Bottom line - I want to decide which scope to mount on the p-mount and use straight through with the chair. It will be the one with which I can see the most - period. For example, I would like to do a lot of galaxy hunting in Leo and Virgo this spring with it and that will be tough, I can see these galaxies better, of course, with the 15-inch Obsession and better still - another two magnitudes or more better - with the video camera attached to the 15. But I do have reasons why I want to be able to see them from the chair using smaller scopes, so . . . the forecast says it won't be clear tonight until after 10- hmmmm, and about then Leo and maybe Virgo will be high enough, so . . . stay tuned.

oops a second thought - well , mroe thoughts . . .

Hi Clay -

This is a second response - I should add that my experiments on this are all driven by the business of looking straight through and what the p-mount can handle. For example, if I thought it could handle the extra weight of a 120mm I would probably go for the one Orion sells - a 120mm F5. but it weighs 8.6 pounds. Adding the weight of a red dot finder and eyepiece and it's at the limit of what I can put on the p-mount - more likely exceeding the limit. There are issues here with this mounting of balance as well. But let's say I find the 102 exceeds the 80ED for my purposes - then I might become serious about a 120mm.

On the other hand, if the 80 ED does better than the 102, then a better choice for me may be the 100ED. At 7 pounds I'm pretty sure I can handle it. And complicating all this will be the performance of a zoom lens I expect to get by Tuesday, If that lives up to expectation, it will have a significant impact on the issue of total weight and changing eyepieces. but, of course, the zoom has it's drawbacks too.

And underneath all this I'm doing a slow burn getting upset with my own obsession on equipment because I'm the guy who wants to stop screwing with equipment and spend more time observing ;-)