Shoot-outs: Camera Lenses vs Astro Scopes

[samirkharusi]

OK, we have all heard that astronomical imaging scopes are so superior to camera lenses that no self-respecting astrophotographer would bother with long camera lenses, even the ones that cost upwards of $5000. But if you already own some of these superteles, and are thinking of trying out astrophotography, you may feel a bit silly spending $8000 for a 6" aperture APO refractor with a dedicated image flattener when you already own that equally expensive 600mm/4.0 lens (same 6" aperture size). It is very rare that you see direct comparisons, simply because, let's face it, most of us do not own one of each, and very likely we do not know anybody who owns one that is anywhere comparable to the one we own, at a similar premium quality level. I have been lucky to be able to do a couple such comparisons, a Canon 400mm/5.6L (71mm aperture, $1100) against a TeleVue 60is (360mm focal length, f6.0, 60mm aperture, $1695):
http://www.samirkharusi.net/tv60_canon.html
and an earlier comparison between the Canon 600mm/4.0L IS (150mm aperture, $7500) and a TeleVue Custom 140 (700mm focal length, 140mm aperture, f5.0, modified Petzval design, price unknown but probably in similar range):
http://www.samirkharusi.net/televue_canon.html
Both the telescopes have been designed primarily for imaging, not just visual use, and their prices reflect that. Have a peruse and draw your own conclusions. Normally we dish out the $8000 for an astro imaging scope because it has received rave reviews in astronomy circles, but without anyone having compared it to a supertele we already own. I hope these two shootouts may help you judge things in a fashion that straddles your daytime and your night time desires.

[Misirlou]

Fascinating. Something I'd always wondered about, but had no way of exploring.

Clearly, a high-quality camera lens can compete with a dedicated telescope when imaging, at least at these focal lengths.

As you increase magnification, refractors get so expensive that reflectors eventually become the only practical option. It would be interesting to compare these OTAs against reflectors of similar power. Visually, refractors usually look crisper (to me anyway) than reflectors of the same objective size. I wonder how the Canon would compare to a really good reflector.

I suppose another way to think about it would be to determine what is the best image that can be captured by a DSLR given a fixed amount of scope/lens-purchasing cash. Or, considering that most reflectors sold now are folded designs, Cassegrain, Maksutov, etc., what is the best image one can get when one's optics are limited by tube length and weight? What is the best image we can produce using a lens or scope that will fit under the seat of an airliner?

[sojournerphoto]

I've used my MakCas (1800mm f12 150mm aperture) to photograph a lunar eclipse and am planning to have a go at some planets using my 5D and T ring sooner or later. There is some drop off at the frame edges, but not as much as I expected and I've yet to use it for daytime photography. One day I'll have a go, but tripods are a bit limiting (though my new gitzo/Markins combo might work!

there is a moon shot on my flikr page - http://www.flickr.com/photo_zoom.gne?id=12...22052073&size=o

Mike

OK, we have all heard that astronomical imaging scopes are so superior to camera lenses that no self-respecting astrophotographer would bother with long camera lenses, even the ones that cost upwards of $5000. But if you already own some of these superteles, and are thinking of trying out astrophotography, you may feel a bit silly spending $8000 for a 6" aperture APO refractor with a dedicated image flattener when you already own that equally expensive 600mm/4.0 lens (same 6" aperture size). It is very rare that you see direct comparisons, simply because, let's face it, most of us do not own one of each, and very likely we do not know anybody who owns one that is anywhere comparable to the one we own, at a similar premium quality level. I have been lucky to be able to do a couple such comparisons, a Canon 400mm/5.6L (71mm aperture, $1100) against a TeleVue 60is (360mm focal length, f6.0, 60mm aperture, $1695):
http://www.samirkharusi.net/tv60_canon.html
and an earlier comparison between the Canon 600mm/4.0L IS (150mm aperture, $7500) and a TeleVue Custom 140 (700mm focal length, 140mm aperture, f5.0, modified Petzval design, price unknown but probably in similar range):
http://www.samirkharusi.net/televue_canon.html
Both the telescopes have been designed primarily for imaging, not just visual use, and their prices reflect that. Have a peruse and draw your own conclusions. Normally we dish out the $8000 for an astro imaging scope because it has received rave reviews in astronomy circles, but without anyone having compared it to a supertele we already own. I hope these two shootouts may help you judge things in a fashion that straddles your daytime and your night time desires.
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[Dr. Gary]

OK, we have all heard that astronomical imaging scopes are so superior to camera lenses that no self-respecting astrophotographer would bother with long camera lenses, even the ones that cost upwards of $5000. But if you already own some of these superteles, and are thinking of trying out astrophotography, you may feel a bit silly spending $8000 for a 6" aperture APO refractor with a dedicated image flattener when you already own that equally expensive 600mm/4.0 lens (same 6" aperture size). It is very rare that you see direct comparisons, simply because, let's face it, most of us do not own one of each, and very likely we do not know anybody who owns one that is anywhere comparable to the one we own, at a similar premium quality level. I have been lucky to be able to do a couple such comparisons, a Canon 400mm/5.6L (71mm aperture, $1100) against a TeleVue 60is (360mm focal length, f6.0, 60mm aperture, $1695):
http://www.samirkharusi.net/tv60_canon.html
and an earlier comparison between the Canon 600mm/4.0L IS (150mm aperture, $7500) and a TeleVue Custom 140 (700mm focal length, 140mm aperture, f5.0, modified Petzval design, price unknown but probably in similar range):
http://www.samirkharusi.net/televue_canon.html
Both the telescopes have been designed primarily for imaging, not just visual use, and their prices reflect that. Have a peruse and draw your own conclusions. Normally we dish out the $8000 for an astro imaging scope because it has received rave reviews in astronomy circles, but without anyone having compared it to a supertele we already own. I hope these two shootouts may help you judge things in a fashion that straddles your daytime and your night time desires.
[a href=\"index.php?act=findpost&pid=156570\"][{POST_SNAPBACK}][/a]

Would it be possible to use more extension tubes instead of a barlow lens to extend back focus? That would give you perhaps a wider field of view.
drgary

[samirkharusi]

Would it be possible to use more extension tubes instead of a barlow lens to extend back focus? That would give you perhaps a wider field of view.
drgary
[a href=\"index.php?act=findpost&pid=157994\"][{POST_SNAPBACK}][/a]

Not possible. Extension tubes do not change the focus position. Only a lens element can do that.

[Dr. Gary]

Not possible. Extension tubes do not change the focus position. Only a lens element can do that.
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Could you design an optic to give you the needed extension without the doubling of the power? Would a 2X barlow would turn my 500 f/4 into a 1000 f/5.6, or would it just change the effective power of the optic?
drgary

[samirkharusi]

Yes. One could have a zero power relay lens. eg if you place a 50mm focal length lens at 100mm behind the focal plane of your main lens, then the new focal plane will be at 100mm behind the 50mm lens. I decided that this would be too long, adding 200mm behind the main lens, and then you have to worry about aberrations, etc. A Barlow does multiply the focal length by 2x, but is an off-the-shelf solution. The final telescope is still generally faster, eg at f5.6 from a 2.8 lens, than a normal astro scope anyway. You can also try locating a Kenko Scope Eyepiece. This is simply a 10x eyepiece with a prism inverter built-in. Very good for birding (straight through view) but a pain in the neck for astro. I bought mine some 15 years back from Adorama, but I have not seen them advertised recently. Zoom lens, you get a zoom birding scope. a 28-300mm lens gives you a 2.8x to 30x scope.