Celestron 500mm F3.6 ("Comet Catcher")

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

Today arrived an T2 adapter so I was finally able to try this lens correctly Smile

Lens is very sharp and has zero CAs.
DOF is razor thin, even with a 100m distance Very Happy

Very hard to focus.

These pic was made while it was snowing very strong, so there's a lot obstruction.
I think the results are good anyway for an 500mm F3.6 (!) Smile

memento · Joined: 05 Nov 2012 Posts: 79

Amazing!

simbon4o · Joined: 19 Dec 2011 Posts: 390 Location: Bulgaria

Try it on daylight and post samples Smile

looks promising!

David · Expire: 2013-01-25

That almost looks like a well-made miniature set.

memento · Joined: 05 Nov 2012 Posts: 79

500 f/3.6 is rather well for extreme low depth of field, obviously.... I agree with the others, it will be interesting to see how it performs at daylight. Probably it will show somewhat low contrast because stray light can enter the inside of the telescope very easily, as with any Newtonian designs....?

martyn_bannister · Joined: 23 May 2010 Posts: 1151

Is this used Prime Focus, or Eyepiece Projection????? Can we see shot of set up?

INTERESTING THREAD

/\/\

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

It's straight focal projection - no eyepiece between camera and lens.
My copy has a normal T-Mount.
Except the front-plate the lens doen't contain any glas (It's Schmidt-Newton with an parabolized mirror).

I haven't used it on daylight yet. I guess contrast will suffer slightly as the front element is very unprotected from sidelight but I'm pretty sure it will be usable anyway.

hifisapi · Joined: 25 Sep 2012 Posts: 941 Location: USA

I have a celestron telescope very similar to that one. Never tried using it for photography though.

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

Nice result, this newton looks very handy for "wider field" astrophotography. Smile

This thread gave me motivation Smile

I have to try and take some photos with my 1200mm (focal length) f4 Newton. I didn't use it much for photography even if it was meant for. The coma corrector was also included, but I never used it as it caused some spherical abberation in the center of the image.

hifisapi · Joined: 25 Sep 2012 Posts: 941 Location: USA

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

I've just tried my 500/3.6 at daylight. Results are still very contrasty Smile

LucisPictor · Expire: 2013-12-03

500/3.6?? Wow! Shocked

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

Her comes a daylight test pic:

Resolution is limited by seeing/heat flickering in center (distance 332 meter).
As you can see it's still contrasty and center is very sharp but corners are not perfect. I think fullformat might get critical.

I wonder how good it might work for macro shots Smile

memento · Joined: 05 Nov 2012 Posts: 79

Wow, very sharp in the image center!

It would need a field flattener, just like other fast telescopes do.

My brother has a Comet Catcher, a black one just like yours. They do rarely show up on the market so we were happy when he found his one locally. We have so far only used it for viewing stars, though, never for photography. We surely have to try it with my Olympus E-M5, it seems.

There are a lot of field flatteners available on the market but the question would be if they match the Comet Catcher's optics.

Did you ever try other astronomical telescopes with your NEX, such as the small INED or William Optics Megrez or something?

Thomas

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

I'm not sure if the weaker corner are caused by an curved field of sharpness or (what I think) more of coma.
Most coma correctors are also focal reducers and the Comet Catcher tends to vignett slightly, so that would be only an option for MFT cameras.
Somewhere I read that stopping down the lens to F5 with an custom aperture, placed on the front-plate gives a big quality boost for photography. That should be the way to go. And it's for free Wink

memento · Joined: 05 Nov 2012 Posts: 79

I mainly thought about one of the cheaper "Chinese" APOs, they are probably in a normal person's budget Smile

Teleskop-Service (biggest German astro dealer) sells e.g. a 50mm f/6.6 and a INED 70mm f/6, both less than 400 Euro new, plus about 150-200 Euro for the corrector. I'm sure they are also sold in the US and elsewhere, but have no idea under which brand / name.

But I have no ideal how good these are for talking photos, probably they would be nice for birding....?

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

400� is still very expensive imho.
200� for a used one and I would give it a try Smile

As far is I know they are not real APOs bye the way, they are "only" two-element ED lenses with very decent CA control and they need a field flatener to get usable corners. For birding on 1.5x or 2x crop the should work very well though also without corrector as noone cares about the corners in a bird-potrait.

s58y · Expire: 2013-09-10

memento · Joined: 05 Nov 2012 Posts: 79

The short answer Smile

Parabolic mirrors introduce optical errors that get more and more disturbing, the bigger the aperture is.

The Schmidt plate corrects these optical errors. So you can design an optic with very fast aperture and build it with a spherical mirror which is very easy to manufacture. On the downside, you have to include that Schmidt plate with an optical surface that has a complex curvature.

A big downside of Schmidt designs is field curvarture, that's why former "Schmidt cameras" had also curved film holders for analogue film. Apart from that, in theory they perform really well and have no coma. They really have no coma if the Schmidt plate is positioned at the right distance from the mirror. In most real telescopes and mirror lenses, this is NOT the case, because then the whole lens would be much longer and heavier.

Standard Newton telescopes with that fast an aperture have harder-to-make parabolic main mirrors.

And for digital cameras, they all also need additional field flatteners (lens systems next to the eyepiece / camera) to really work well.

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

Yup, Schmidt-Newtons have a correcting front element.

But this particular Schmidt-Newton still has a little coma in the corners! Not much but it's there!
Somwhere I read that it was necessary compromise to keep the scope that compact and that the schmidt-plate only elimniates 75% of the coma.
The whole scope is only ~15x48cm large and it's sharp enough in center to reach pretty high magnifications.

SXR_Mark · Joined: 12 Jun 2010 Posts: 506 Location: England

memento · Joined: 05 Nov 2012 Posts: 79

You are absolutely right.

I confused parabolic mirrors � which are hard to manufacture � with the cheaper spherical ones, as used in Schmidt-Newton and Schmidt-Cassegrain lenses and telescopes. I edited my post above, now it should be correct Wink

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

This lens uses both a schmidt-plate and a parabolic mirror as far as I know.

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

There is a spherical primary mirror if the telescope has a schmidt corrector plate. If you have a parabolic mirror it's much easier to use a comma corrector only, doesn't make sense to have a big heavy expensive corrector in the front.
Parabolic mirrors (if high quality) have excellent performance in the center of the field (can be diffraction limited at f/4), performing better than a 5x-10x more expensive refractor, but suffer bad from coma away from the center of the field.