Moon with astronomical telescope and 1dsm3

[PM01]

Just look at the link for the pics. :)

http://photography-on-the.net/forum/...534639&page=17 (http://photography-on-the.net/forum/showthread.php?t=534639&page=17)

[Bernoulli]

PM01 -

I really enjoyed reading the last several pages of that thread and seeing some of the beautiful images of the moon. Very sharp!

I'd appreciate any insights you could give of using long lenses vs. astronomical telescopes.

[PM01]

They each have their place. If you need AF, IS and aperture control, then go with the Canon lenses.

But as for optical quality, the high end telescopes are in a different upper realm altogether. They're not "rushed" on a production line to meet numbers and are meant to be used at extremely high magnifications - so the lens must be made to a higher critical standard. They are a challenge to use if you're not used to manual focus, but the results are well worth it, imo.

And no, I wouldn't go to an auto race with a telescope - the AF and IS are much needed in that application. Same with birds in flight - unless you're really really good with a rack and pinion focuser. For the slower objects, I'd have a telescope with me anyday! And their color correction is INCREDIBLE. :) Pretty much ZERO color casts and flaring.

These are for refractor type (lens) telescopes. I haven't shot with a Maksutov, though I am told that they have exceptionally good qualities. Schmidt Cassegrain types - I"ve been thoroughly disappointed in the amount of light scatter. Our observatory has many models, many types of reflectors and I've shot through pretty much all of them, only to go back to the refractor.

[Bernoulli]

I've never found a Schmidt-Cassegrain that was useful for lunar photography. Like you said, they scatter light very badly.

With a 6 inch scope, I feel like I'm limited in my resolution by the atmosphere and not by the diffraction limitation of the scope, taking as a rule of thumb Dawes criteria of 4.56 inches of clear aperture giving one arc second (or something like that). I'm guessing very few lenses can reach arc second resolution, right? At least not one costing less than a small car?

[PM01]

It depends on the car..if it's a Lotus Elise... ;)

Not too many scopes are highly figured (precision ground, pitch polished, 1/50th wave RMS) enough to meet the full theoretical resolution of the lens. The ones from Astro Phyics, Takahashi and Telescope Engineering company come extremely close though. A 6 inch clear aperture refractor should give you a 0.7 to 0.8 arc seconds of real resolution. With a central obstructed design as found in Mak Cass, the resolution drops a bit. I do have lots of respect for the Mak Cass though - one of the smallest central obstruction designs out there, but I've never tried one for terrestrial photography.

The telescope companies mentioned routinely have strehl ratios well above the .95 mark. For a triplet design, and knowing the companies as well as I do, it's more like the .995 mark. A 1.0 is considered an absolute perfect optic in figure. A commercially made SCT is lucky to have a 0.8 - but even with a good figure the SCT will just scatter light like crazy due to the materials and design.

If you're located in the southern areas of the US, and if the seeing conditions are really good, you WILL reach the theoretical limit of the telescope. If memory serves me correctly, Florida and Texas skies are among the most steady at certain times of the year. I've heard of people getting down to 0.5 arc seconds with no problems and they routinely push 700, 800, 1000 power and over to see planetary detail on Jupiter, Mars, Saturn, etc. They use 6, 7, 8 and even 10 inch (holy smokes!) refractors.

The Canon 1200 f/5.6L EF has a front diameter of about 200mm, 10 inches, but it wouldn't be a very good "high power hog" due to the figuring of the lenses - they're not pitch polished nor are they figured down to 1/50th of a wave rms.

Camera lenses - forget about it altogether. Just WAY too many errors that are compounded with 17 and more elements combined, and given the fact that the commercially made camera lenses are high speed polished with no real regard to their final figure. You won't see them publishing a strehl ratio. The best comment that I've heard from Canon ... "they're not designed for that application".

A few people on the board seem to have a problem with using a telescope (high end refractor) for terrestrial based photography. Strip away the AF, IS and aperture control on a Canon (or even a regular camera lens) and you basically have a telescope on the long focal lengths. But they just can't seem to get past the idea that a telescope is just a very long telephoto lens. Does that mean that you can't point a 600/4L EF IS at the sky to take pictures of the stars? People have done it, but their results aren't nearly the precision of a good scope.

Now if you take the precision (optics wise) of a good scope, and use it for terrestrial photography, the results are definitely eye opening. Depending on your "scale", they can either be "hmmm...there's a good idea here" to "holy smokes". Most of the a/b comparison shots have been of the "holy smokes" caliber, especially when printed up to larger sizes. A commercially made camera lens will tend to "mush out" at higher print sizes, native resolution. A telescope will take much higher magnification and print sizes before it will "mush out".

Do you have lots of atmospheric turbulence where you live?

[Bernoulli]

Very informative, thanks.

I'm in the NW corner or Arkansas and the seeing is quite good sometimes. Back when I had my beloved Celestron C8, I would test resolution at the Apollo 11 landing site. At low sun I could glimpse the crater Armstrong which is 4.6 km = 2.4 arc seconds but I never could get Aldrin (3.4 km) or Collins (2.4 km) even though I knew exactly where they were.

When it warms up outside, I may do some resolution testing on my Mak. Using my 40d at prime focus, I get almost exactly 1 km/pixel = 0.5 arc second.

I do my daytime testing by looking at a house that is 6190 feet away from my front deck (we live on hill). At that distance 1 arc second is 0.36 inches. I can see and photograph candles in their window so I'm not far from it. I'd love to go to their house and ask to measure their candles but they's probably call the police!

In the spring and summer we have a pervasive high pressure system over this part of the country and it stabilizes the atmosphere a bit. It usually gets blown out by the first hurricaine of the season in August or October and seeing goes downhill a bit then.

So you'll accept a bit of color abberation in a refractor in exchange for less scattering? Actually, based on the telescopes I've used, that makes sense. I just don't want to spend the bucks on a big refractor!

I agree that many folks have trouble thinking of a telescope as a big prime focus lens, which is what it is.

Do you think there is any benefit to stacking DSLR frames? It seems like this might not be a benefit until you get to the really high eyepiece projection magnifications, but I might try it.

I saw a study on the absolute best resolution on the moon from an Earthbound scope. There were stacked images from 18" dobs (carefully collimated, I'm sure) that clearly showed 600 m craters and hinted at 400 meter craters. That's about 0.25 arc second.

[PM01]

There is LOTS of benefits to stacking frames from DSLR RAW files. I do believe that Maxim DL + DSLR can do this. Plus the various methods of combining will often turn out interesting results. Signal to noise ratio increases and you need a minimum of 3 frames. I sometimes shoot 10 frames, and if you compare the single frame versus the 10 frame version, there is quite a difference.

.25 arc seconds?!?! WOW!!!! The seeing conditions around here are limited to about .9 to .8 arc seconds on a very calm hazy night. For those that are near the chicago area, the best nights aren't in the winter, they're in the summer. You'd think that winter skies have less junk in them, but with all the buildings and exhaust vents for heating, the stars are always twinkling. But, on a hot and muggy summer night, the skies are steady at times. I've been able to pick out the Encke ring in my 5.1 inch Astro physics at 700x. That was a treat!!!!

Interestingly enough, crystalline fluorite has pretty much ZERO scatter. ED glass has scatter and so does regular glass, but not nearly as much as a mirror based optic. I've never looked through a Mak Cass, but am told that the image will almost rival an equivalent sized refractor. That's mighty impressive! The dall-kirkham designs seem fairly good also.

What's your mag limit by your area? We're LUCKY to pick off mag 9.7 with a 14 inch scope. And unaided eye is right around 4.3 mag. Get into chicago and it's just nothing but soup of light pollution. :(

[Bernoulli]

That 0.25 arc seconds wasn't here, but I did manage to find the study where I saw it:

http://ltvt.wikispaces.com/Apollo+11+Area+Resolution+Examples

Can you recommend stacking program for large files on a Mac? I've heard of Registack, but I think it's PC only. Can Photoshop do this for just a few frames?

One of the great advantages of living in an area that looks like where they filmed "Deliverance" is that it's dark, very dark. I own 70 acres on the side of a mountain in very rural Madison County and the skies are very black at night. What we lack in culture we make up for in nature! Maybe I should try some deep sky imaging sometime.

[PM01]

Hmmm...for Mac...maybe Images Plus? mlunsold.com I don't know if he has a mac version. Not too much is supported in mac... :(

Yes, you should do some deep sky imaging! I'm guessing that you're probably at mag 6 or better! And the images should turn out fantastic. :)

[FarmerDave8N]

Can you recommend stacking program for large files on a Mac? I've heard of Registack, but I think it's PC only. Can Photoshop do this for just a few frames?
Craig Stark's Nebulosity is written on a mac, it's a great product for stacking deep space images - but I think you're talking about stacking lunar/planetary images, however. That's another animal, and I'm not sure what (if anything) is available for the mac. I own 70 acres on the side of a mountain in very rural Madison County and the skies are very black at night. Funny - my family used to own (back in the 70s/80s) 80 acres in southern Washington County (down near Devil's Den) - I graduated from FHS - it's a beautiful area. David

[Bernoulli]

I'm a prof at U of A. Jokes about "Deliverance" aside, we moved here in '87 intending to look for a position at a bigger school, but just fell in love with the area and now we plan to retire here. We came very close to buying acreage near Devil's Den ourselves, but ended up in Madison.

I've got a PC also, so I'll probably get RegiStack and use it on that.

[FarmerDave8N]

Sister and mom graduated from UofA, dad was a prof there as well (before your time), I played with the b-ball band several times while still in HS - woo pig sooie.

Here's a good tutorial on Registax - it has a link to the site for download as well:
http://starizona.com/acb/ccd/software/soft_proc_registax.aspx

HTH,
David