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Tommohawk

Canon 200mm F2.8L vs. Fast Refractor

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My main interest is imaging DSOs currently with DSLR but probably moving over to cooled CCD/CMOS when funds allow.

I've recently been looking at options for a wider field fast lens/scope, partly to be airline portable also to be superfast to make the most of any imaging time on holidays.

My first choice is an F4 130mm newt - thread elsewhere on this - but given this doesnt seem to be available, was looking at a TS photoline 80mm APO. However, the Canon 200mm F2.8L is immensely appealing too, and quite a good price.

F2.8 sounds great - would knock spots of an F6 APO  (F4.8 with reducer) for fast imaging. But reading up on this most folk seem to advise stopping it down to F4. Seems a shame to have a superfast lens and then stop down - the lens is very highly rated, so whats the point of getting an F2.8 and using it at F4? 

I think the answer is that working with superfast lenses is likely to be problematic for fosusing at least so maybe better to prioritise the switch to cooled CCD.

I guess the question is, would an 80mm APO outperform the Canon 200mm at F4 in terms of overall image quality?

Thanks as ever for any advice.

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The Canon lens can be used wide open if you use thirds focussing but it does give better results at F4 which is still faster than any APO I know of, the star 71 would probably out perform it though but is F4.9 ish.

Alan

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The Canon lens can be used wide open if you use thirds focussing but it does give better results at F4 which is still faster than any APO I know of, the star 71 would probably out perform it though but is F4.9 ish.

Alan

Thanks for that, and am hurriedly reading up on "thirds" focussing! As you say, F4 still better than F6 or F4.4 with redducer. Is there a mid stop - say F3.5?

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I've used mine wide open and at f3.5, here are a couple zoomable example images on Flickr.

Andromeda at f3.5.

The Pleiades at f2.8.

These are unguided shots, so that could also have some bearing on the star shapes as well. It's certainly useable wide open but I'm still experimenting, the bright stars are a little messy so I suspect the aperture ring is intruding into the light path. I'm going to try fitting a step-down ring as a front aperture mask. Another thing I haven't tried yet is the lens tool in StarTools, to see if I can improve the corner stars.

I believe the author of the 'Astronomers Do it in the Dark' site uses his at f3.5, from my tests I don't see the need to stop it down any more than that. Hope that's some help. 

Edited by Knight of Clear Skies
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I've recently been looking at options for a wider field fast lens/scope, partly to be airline portable also to be superfast to make the most of any imaging time on holidays.

My first choice is an F4 130mm newt - thread elsewhere on this - but given this doesnt seem to be available,

If you want a portable F2.8 astrograph consider the 6" Boren-Simon Powernewt .

Similar to my home cooked version but with most of the mods done for you.

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If you want to even consider using it wide open then it has to be well collimated (check for centred vignetting).  If it's out, then because it's a professional lens, you can send it back to Canon (UK branch is Enfield I think) free in the first year for adjustment.

It's very good at f/2.8 with an 8300 sensor equipped camera but I found that f/3.2 produced outstanding images (in narrow band) if you can get it focussed accurately enough; the edge stars will be squiffy if you don't !

https://www.flickr.com/photos/29673723@N04/5007278823/in/album-72157624216353110/

The next is full size unbinned showing how good it is at the edges:

https://www.flickr.com/photos/29673723@N04/4238477498/sizes/o/

There are other examples there but it really is a top lens !

Hope that helps

Robert

Edited by robertm
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As above. I have the 300 f4 L. At f4 focusing is extremely tricky. Compared to my frac at f5.59 then the field is not flat either on my crop sensor.

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If you want a portable F2.8 astrograph consider the 6" Boren-Simon Powernewt .

Similar to my home cooked version but with most of the mods done for you.

Hi and yes I really like the idea of the ASA reducer used in multiple F4 newts as you have, either home made or Boren Simon. Trouble is the 150mm is too large for hand luggage which is key to this.

A 130PDS F4 with ASA option would be fabulous - but I cant source an F4 130, or even a mirror for a DIY job.

I've used mine wide open and at f3.5, here are a couple zoomable example images on Flickr.

Andromeda at f3.5.

The Pleiades at f2.8.

These are unguided shots, so that could also have some bearing on the star shapes as well. It's certainly useable wide open but I'm still experimenting, the bright stars are a little messy so I suspect the aperture ring is intruding into the light path. I'm going to try fitting a step-down ring as a front aperture mask. Another thing I haven't tried yet is the lens tool in StarTools, to see if I can improve the corner stars.

I believe the author of the 'Astronomers Do it in the Dark' site uses his at f3.5, from my tests I don't see the need to stop it down any more than that. Hope that's some help. 

Well that looks pretty convincing!! Didnt realise the Pleiades had so much going on!!

If you want to even consider using it wide open then it has to be well collimated (check for centred vignetting).  If it's out, then because it's a professional lens, you can send it back to Canon (UK branch is Enfield I think) free in the first year for adjustment.

It's very good at f/2.8 with an 8300 sensor equipped camera but I found that f/3.2 produced outstanding images (in narrow band) if you can get it focussed accurately enough; the edge stars will be squiffy if you don't !

https://www.flickr.com/photos/29673723@N04/5007278823/in/album-72157624216353110/

The next is full size unbinned showing how good it is at the edges:

https://www.flickr.com/photos/29673723@N04/4238477498/sizes/o/

There are other examples there but it really is a top lens !

Hope that helps

Robert

Yes it does help thanks - more great images!

As above. I have the 300 f4 L. At f4 focusing is extremely tricky. Compared to my frac at f5.59 then the field is not flat either on my crop sensor.

Well, funnily enough I wondered about the F4 300mm - but I'm surprised the field isnt flat given its meant specifially for imaging. Is it really that different to the frac?

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No, the corners are not flat on a big sensor. Probably OK on smaller sensors. Now, my 60mm macro is as flat as a pancake.

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Hi and yes I really like the idea of the ASA reducer used in multiple F4 newts as you have, either home made or Boren Simon. Trouble is the 150mm is too large for hand luggage which is key to this.

A 130PDS F4 with ASA option would be fabulous - but I cant source an F4 130, or even a mirror for a DIY job.

The 130PDS used with the ASA 0.7x corrector/reducer would give a respectable F3.5 / 455mm FL instrument. I have tried it with the 150PDS and it seems to work okay. Never going to be as portable as a camera lens though!

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The Canon 2.8 is fantastic. It was my entry to astrophotgraphy although i have not used it in a couple years since buying my frac. 

I never got the full potential out of the lens as back then i didnt have guiding. I have also since added a TS microfocuser and aperture rings. The rings are great as you can use the lens wide open and not be plagued by horrible star spikes!

Callum 

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No, the corners are not flat on a big sensor. Probably OK on smaller sensors. Now, my 60mm macro is as flat as a pancake.

Well I intend to move to a mid-size cooled CCD/CMOS so that should minimise the problem.

Talking of which, I now realise that getting filter wheel between the Canon lens and a ZWO or other camera will be tight or maybe impossible? It looks like Geoptik do a filter draw type option, but any chance of a filter wheel does anyone know??

The 130PDS used with the ASA 0.7x corrector/reducer would give a respectable F3.5 / 455mm FL instrument. I have tried it with the 150PDS and it seems to work okay. Never going to be as portable as a camera lens though!

Agreed that would be fast - but too long for cabin bag.

BTW - could you do me a big favour when you have a moment? I'd really like to know some measurements from the 130PDS to help with my F4 conversion raytrace. Can you tell me the tube diameter, and also the stock focuser height at mimimum - measured from tube to top of black 54mm adapter. That would be really helpful.

The Canon 2.8 is fantastic. It was my entry to astrophotgraphy although i have not used it in a couple years since buying my frac. 

I never got the full potential out of the lens as back then i didnt have guiding. I have also since added a TS microfocuser and aperture rings. The rings are great as you can use the lens wide open and not be plagued by horrible star spikes!

Callum 

Yes everyone seem to recommend the TS microfocuser rings - yet another bit of kit! My Fuji HS30EXR camera has "soft" manual focussing and its horrible - the Canon must be better than that. thanks for the tip about the aperture rings - youd think the diaphram would be clear without that when set wide open.

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The Canon lens can be used wide open if you use thirds focussing but it does give better results at F4 which is still faster than any APO I know of, the star 71 would probably out perform it though but is F4.9 ish.

Alan

Tak Baby Q, reduced, F3.9 and Tak 106, reduced, F3.6...  :grin:  ... but a whiff more pricey than a 200L!!! I've used the Canon 2.8. It was very good but my CCD pixels were too big for it. TS do a micro-focuser. 

IMG_1193-M.jpg

Olly

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Tak Baby Q, reduced, F3.9 and Tak 106, reduced, F3.6...  :grin:  ... but a whiff more pricey than a 200L!!! I've used the Canon 2.8. It was very good but my CCD pixels were too big for it. TS do a micro-focuser. 

Olly

Hi Olly yes that looks just the job - the TS microfocuser I mean, not the Tak!

Whats your thoughts on the Canon lens with filter wheel for CCD ? Is there space for a wheel?

BTW I just tried to check the depth of the ZWO filter wheel and noitced they now have cooled ZWO ASI 174 on sale for $799. Bit off topicHas anyone ordered direct from ZWO - do we get hit for VAT etc?

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Hi Olly yes that looks just the job - the TS microfocuser I mean, not the Tak!

Whats your thoughts on the Canon lens with filter wheel for CCD ? Is there space for a wheel?

BTW I just tried to check the depth of the ZWO filter wheel and noitced they now have cooled ZWO ASI 174 on sale for $799. Bit off topicHas anyone ordered direct from ZWO - do we get hit for VAT etc?

I don't know. It might be tight to get a F/W in there but I think someone (Gerd Neumann, maybe?) made a slide drawer for CCD to camera lens. This is only going to work sweetly for CCDs with very small pixels. I used a Geoptik CCD-Lens adapter and it can take filters, but involves a full disassembly to change filters. I know someone who's done this but it would be a step too far for me. Cameras with integrated filterwheels (QSI and some Atiks) might have a short enough backfocus requirement. In fact I think they do but this would need checking.

Olly

Edited by ollypenrice

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I don't know. It might be tight to get a F/W in there but I think someone (Gerd Neumann, maybe?) made a slide drawer for CCD to camera lens. This is only going to work sweetly for CCDs with very small pixels. I used a Geoptik CCD-Lens adapter and it can take filters, but involves a full disassembly to change filters. I know someone who's done this but it would be a step too far for me. Cameras with integrated filterwheels (QSI and some Atiks) might have a short enough backfocus requirement. In fact I think they do but this would need checking.

Olly

Had a look at this a while back (before I sold my Canon  EF200L) and Moravian and QSI definitely do dedicated adapters for canon EOS lenses that fit their cameras with integrated filter wheels. I think the only thing was you couldn't use it on the versions with integrated guide ports as well since they added too much length to the optical path. For shorter back focus CCDs without integrated filterwheels TS do a 15mm thickness filter drawer which is the thinnest I know of.

Paul

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An alternative to using a filter wheel would be to attach a 2" filter to the front of the lens, using a step-down ring.

But that would stop the 200mm down to less than F/4, which defeats the purpose

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I don't know. It might be tight to get a F/W in there but I think someone (Gerd Neumann, maybe?) made a slide drawer for CCD to camera lens. This is only going to work sweetly for CCDs with very small pixels. I used a Geoptik CCD-Lens adapter and it can take filters, but involves a full disassembly to change filters. I know someone who's done this but it would be a step too far for me. Cameras with integrated filterwheels (QSI and some Atiks) might have a short enough backfocus requirement. In fact I think they do but this would need checking.

Olly

Thanks, looks like filters are an option but maybe only with a draw.

Re the ideall CCD pixel size, I think I sort of follow the conventional thinking on this -  but received wisdom seems to suggest an ideal FL of approx 200x the pixel size, or in the case of a 300mm lens, thats 1.5 microns. Well the Canon has a pixel size of 4.3 microns and of course is typically used at much shorter FL. Would the ideal Pixel size for a 50mm lens really be 50/200 = 0.25 microns? I must be missing something!

Had a look at this a while back (before I sold my Canon  EF200L) and Moravian and QSI definitely do dedicated adapters for canon EOS lenses that fit their cameras with integrated filter wheels. I think the only thing was you couldn't use it on the versions with integrated guide ports as well since they added too much length to the optical path. For shorter back focus CCDs without integrated filterwheels TS do a 15mm thickness filter drawer which is the thinnest I know of.

Paul

OK thanks, thats useful to know.

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An alternative to using a filter wheel would be to attach a 2" filter to the front of the lens, using a step-down ring.

see below!

But that would stop the 200mm down to less than F/4, which defeats the purpose

And youd have to dismantle to change filters I guess.

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But that would stop the 200mm down to less than F/4, which defeats the purpose

f3.9 I think, which is still fairly quick. Thought I'd mention it as an option in case it was useful, most narrowband filters have issues at very fast focal ratios due to the steep light cone I believe. I don't know if the OP intends to use existing filters, is looking to buy new ones or is just shooting RGB.

And youd have to dismantle to change filters I guess.

Yes. Switching a filter only takes a moment but the lens hood gets in the way, and I find the one on my lens quite stiff and a pain to remove.

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As ever, we video astronomers do things a "bit differently"... ;)

The tube rings also provide a way of locking the focus.

But I'm not totally convinced about the idea. :p

post-539-0-25591700-1448827247.jpg

But I did manage to focus with a (15mm) TS Filter Drawer System

using the thinnest possible (10mm) T2 to Canon bayonet adapter:

http://www.teleskop-express.de/shop/product_info.php/info/p6991_TS-CCD-Adapter-for-Canon-EOS-lenses-to-T2---only-10-mm-length.html

I sense I should probably shim the system so the lens still focuses at "infinity". 

But I am probably not looking for the same perfection as classical imagers? :)

I can always get plenty of brightness (increase camera gain!) so envisage a

"mask"  to aid focussing. (I did find a source of mini Bahtinovs somewhere).

Or maybe something DIY will suffice. The RAIN isn't helping with progress. :o

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 The RAIN isn't helping with progress. :o

You can say that again - last time out I had goto and autoguiding problems which I have yet to sort before I can get back to any imaging. Forecast is for more rain and wind all week... and that's kind of the point of all this. Trying to find a way of taking compact set-up off the Canaries or wherever.

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f3.9 I think, which is still fairly quick. Thought I'd mention it as an option in case it was useful, most narrowband filters have issues at very fast focal ratios due to the steep light cone I believe. I don't know if the OP intends to use existing filters, is looking to buy new ones or is just shooting RGB.

Yes. Switching a filter only takes a moment but the lens hood gets in the way, and I find the one on my lens quite stiff and a pain to remove.

Starting from scratch with filters, but hoping for NB. Interesting what you say about problems using NB at fast F's. Will have to bear that in mind.

Through the course of this I'm wondering if the F4 300mm might suit better, especially given the point about matching to CCD camera pixels. Would still be faster than a fast frac.

Is the general view that the image from say a 300mm F4 Canon lens is as good as a dedicated frac, say an F6 with 0.8 reducer?

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