Comparing reflector vs Refractor digiscope image quality (1 Viewer)

[Rodney99]

I've been looking to compare the image quality which I will be able to accomplish with a refractor telescope as compared to a refractor spotting scope. So I'm hoping people here can help with this.

I currently do my wildlife photography now with my Canon digital SLR and a Canon 500mm f4 telephoto when I need long reach..

The goal is to get substantially longer reach than I currently can get with my 500mm lens and a doubler. As an example I am watching a Bald Eagle nest which is just over 1000 feet from the allowed observation point. With the camera crop factor , the TC and the 500mm lens, I can get 1600mm focal length at f-stop=8. Adding another TC (1.4x) gets me out at 2240mm but decreases image quality a bit more than is acceptable.

I still want more "pixels per bird". A female mature Bald Eagle here is going to be around 32 to 36 inches high. It would be great to get bird to fill 70 to 80% of my image ( in portrait mode).

So that's where the search starts. I have looked into a Questar reflector but can't afford one. Reflectors seem attractive due to their greater aperture and resulting light gathering capability. Weight is not too much of a concern in that I am currently taking a lot of weight out into the field.

I have seen some very excellent images taken with refractor spotting scopes, but most often at distances much shorter than what I want to work with. But I have not seen equivalent images take with reflectors.

How will a 5, 6, or 8 inch reflector image compare to those taken with the more standard refractor spotting scopes, in general? And how will digiscope images at these distances compare to the quality my Canon 500 gives me?

thanks for reading all this. I currently have found a Celestron C5 for sale nearby. But not sure it's big enough or that the 1/4 wave length accuracy mirror is precise enough.

Rodney

 

[mooreorless]

Hi Rodney, Welcome to Birdforum. There is a fellow on Cloudy Nights that did a comparison of the Canon 500 f4 and a TeleVue 127mm refractor. If my memory serves me right. I will try to find it. I would think at that distance heat waves etc. would limit the QC.
Regards,Steve

 

[fugl]

Like mooreorless, I would have thought that atmospheric distortion rather than optical quality would be the principal limitation for the kind of extreme long distance photography you have in mind. But I’ve had no personal experience with digiscoping & I’ll be interested in hearing what others have to say about this.

 

[mooreorless]

I found the comparison, but it is a Canon 600mm and a TeleVue 140mm APO.

http://www.samirkharusi.net/televue_canon.html

 

[mooreorless]

You can buy Celestron C8 2232 focal length "Cat" OTA off Astromart for anywhere from $300 to $450 as an experiment and there are C6 "Cats" on there that go for $250-$300. I couldn't find any posts on Cloudy Nights of "Cat" scopes with pictures of daytime use.

 

[dannat]

Rodney, when you are talking about refractor i presume you are going to use prime focus, and not ep projection..if the former the f/l is not going to be much longer than your canon 500mm unless you go big - the length of a big refractor is unweildy. A reflector is going to give you the length you need, but they re not WP -and would be more prone to losing collimation if given hard bumps in the field (the refractor has adv here).
What refractors have you considered? the questar does look the goods doesn't it

 

[etudiant]

Hi Rodney,
A big reflecting scope, say 8 inch or more aperture, is a formidable bit of kit to lug around.
It may give you the kind of pixels per bird you seek, but also involves a weight of 50 plus pounds when adding in the tripod and mount. So it really is a fixed site solution. For a raptor with preferred sites, that might be adequate.
There are small Questar size and up Maksutov scopes on the market, but they are not as good optically as your big Canon lens, at least imo.
The Questar 7 inch OTA is occasionally offered on Ebay, presumably from government surplus. That might be the basis for a solution, but it would need custom fitted adapters, so it would be a $$ project.

 

[dahyon]

Hello Rodney

Your wish list of requirements can be plugged into this calculator:
http://www.jayandwanda.com/digiscope/digiscope_calc.html

I have had go and it seems you would need 8000+mm 35mm equivalent focal length in order to achieve an image filling the proportion of the frame you have described, that is 160x magnification. This causes enormous difficulties some of which have been described by Yao et al in a number of papers, see for example:
http://www.computer.org/portal/web/csdl/doi?doc=doi/10.1109/ICVS.2006.21

This paper illustrates the difficulties of imaging at extreme magnification, essentially they end up trying to recover from severe blurring effects.
Food for thought.
David

 

[Kevin Purcell]

I have had go and it seems you would need 8000+mm 35mm equivalent focal length in order to achieve an image filling the proportion of the frame you have described, that is 160x magnification. This causes enormous difficulties some of which have been described by Yao et al in a number of papers, see for example:
http://www.computer.org/portal/web/csdl/doi?doc=doi/10.1109/ICVS.2006.21

This paper illustrates the difficulties of imaging at extreme magnification, essentially they end up trying to recover from severe blurring effects.
Food for thought.
David

Thanks for that reference, David.

Of course that's behind the IEEE paywall but the curious can find a copy pointed to by citeseer

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.105.919

or at his UTK site

http://imaging.utk.edu/publications/publications.htm

Seems like work that's relevant to says drone surveillance e.g. these guys also do work on face recognition and autofocus at extreme magnification too. Perhaps a short hop to automatic bird ID ("Is that really a flamingo out there in the heat haze?").

 

[Rodney99]

Thanks for these replies.

dahyon, Yep you ran the calcs right. While that goal may not be reasonably attainable, I still am wanting more reach. And the decision to get something will depend on how much it "increases the bird size" and the net image quality. I'm not naive, just an engineer who is looking to push the reach at least somewhat significantly.

etudiant, The weight is not that much of an issue. My photo-cart can carry 200#'s and has pneumatic tires. The scope would travel in a well padded glass taxi lens case in the cart or in its own case. And only a simple mount would be needed for the tripod. I estimated weight for a Celestron 8" reflector system would be 20 to 30 #'s.

Vulnerability and the need for realignment is a concern of course. And I expect that it would be really only useful for relatively stationary subjects like nesting birds.

Kevin Purcell, Thanks for those links. Will wade thru them in more detail but it does look like there are real issues regarding sharpness. Though I have to say that the 1800x magnification sounds impressive. For my intended application even at its most optimistic level, 160x would "suffice" at 1000 feet. Actually a clear sharp 1500 to 2500 mm focal length into the slr camera would be nice.

mooreorless, Thanks for that comparison article. What do you conclude from it?

dannat, Thanks for your post. I had been looking at Questars, but have dropped their 3.5" ones due to the f14. Need more light gathering. The Q's seem to go for 1500 to 2600 on Ebay. I can only guess what their 7" models go for.

Next moved on to the Celestron line their 90, 5", 6" and 8" reflectors. I can get f10 but can not find anything on their Depth of Field for terrestrial photography. With the dark feathered Bald Eagles, even on a bright day one needs to increase the exposure to get the feather detail. And shutter speeds need to be at 1/1250th to 1/2000th of a second to handle bird & feather motion. So that along with trying to keep a reasonable ISO, generally says I would rather lose the small dimenesions or design focal length in order to get more light (read a lower f-stop.

At this point I am still exploring this and the nestlings are all going to fly soon. So immediate need is reduced. But the interest is still here.

I appreciate this help and information. The camera would be one of my DSLR's connected directly with a T-adapter and T-ring. At this time there are a lot of questions still. But the main factors seem to be the effective F-stop and the actual depth of field.

thanks
Rodney

 

[mooreorless]

Hi Rodney, A slight advantage goes to the TeleVue scope on imagining the planets. I think the TeleVue scope should of been sent back to the maker to have a look see to be fair.
Regards,Steve

 

[dahyon]

Hello Rodney
Have you seen George Raiche's site:
http://www.digibird.com/primerdir/primer0.htm
He has an excel spreadsheet which is excellent for theoretical/practical digiscoping rigs. It is in fact the originator of Jay Turberville's calculator
Some of his added information is particularly insightful including "maximum detail digiscoping magnification" and "object width spanned by one pixel".

Good luck
David

 

[Rodney99]

Thanks for these followup replies. This has become a frustrating project. And help is certainly needed.

mooreorless, Yes I would agree with the Televue "winning". In general my other research at this point is not showing the reflectors to be very suitable for photography. It seems that providing an image to the human eye is an easier task that providing a crisp high quality image to the camera at these distances.

dahyon, That's a very interesting article. Lots of good items covered. But unfortunately it seems to be put together only for digiscoping with a Point & Shoot camera. And does not look to apply for connecting a dSLR directly to the telescope. Did I miss something there?

And if I interpreted his DOF calcs, he's saying there is less than 1 inch of dof at 25mm in his example (154mm at 25 meters). Wow. My birds are "thicker" than that by about an order of magnitude even for the chicks. But yes I am at a lot longer distance to the object. Though with that shallow of dof, even my longer distances of say 200 to 333 meters, the telescope dof would seem to be a lot less than that of most super telescopic lenses, like my Canon 500 f4.

Continuing In general
Wondering where to go with this next? This does not look very good right now. A friend of mine shot a couple of pics thru a Meade 8" reflector over the weekend. The image quality was simply put junk. And the camera settings (shutter speed & iso) to get the image on a very bright day were way below what we need to "freeze" the birds. And the image lacked any hint of accurate sharpness.

Is this a lost cause? Or am I just not looking in the right places yet? Where are the scopes with an f8 or f5.6 and 1000 to 2000 mm FL? Or is this a case where the best I will do is to put a P&S on a refractor and just document nests instead of photographing them?

I wonder what a 7 inch Questar can do?

Rodney

 

[etudiant]

Hi Rodney,
The DoF provided by the 7" Questar may not be a what you hope. The application I saw documented was to monitor instruments in an experimental setup. DoF was not a concern.
The Questar web site gives links to them, maybe they have helpful insights.

 

[dannat]

you are going to have difficulty finding a telescope which is f5.6 & over 1000mm f/l - those things aren't small either how are you going to mount it & then take shots of a flying bird???
maybe the canon 500mm & 2.0x tc is the way to go..i have used 6" reflectors for moon shots and they are unweildy on a big mount -they are not designed to be moved quickly to take action shots
Have you thought about a good quality movie camera & pulling stills off the movie?

 

[Rodney99]

Thanks for these replies.

etudiant, Thanks for the warning on the 7" Questar. I haven't looked into that one previously. I had figured it was just too expensive even if I found one used. But now I am interested in what the 15k $ item can do.

At least it looks like I am identifying some of the critical factors, dof, light gathering (f-stop), image quality for still photos.

dannat, Yep it would be heavy. But a couple of things may still make it reasonable, (or reasonable at some extreme :-C). The rig won't be used for tracking birds in flight. But used for critters in more stationary locations examples include nests or where they revisit a location somewhat repeatedly, like skimmers over a lake where they feed along the same path again and again. I'm not looking for fast reaction times for the system, just a system that is fast light-wise, exceptionally long reach and decent depth of field.

Weight is not a real issue. I can trek over 100 #s somewhat easily in my pneumatic wheeled photo cart. The Celestron 8" weighs in at 13 pounds and if the tripod weighs another 13 pounds, that is not a problem. I'd be willing to sacrifice part of the 8"'s 2000mm focal length for a better f-stop than f10. I'd also be willing to take a shorter or longer tube. But I am thinking that this is not where Celestron or others see a market and do not make the faster reflectors.

On Canon lenses: Yes I do use my Canon 500 f4 with TC's and I have it on a crop body (1.6 factor). Each TC costs me light (f-stops). I shoot now often with either a 1.4 Canon TC or a 2x TC. Stacking both reduces image quality and costs 3 f-stops versus 1 f-stop for the 1.4 TC and 2 stops for the 2xTC. Occasionally I take some shots with the TC's stacked at 2240mm effective FL, but the extra glass with that setup does not really give me what I'm wanting quality-wise..

The speed is needed for just incidental activity of my critters, wind motion of the feathers, etc. Even in the nest, they are still fairly active. Plus we are always shooting up at a nest it seems, so we only get to see them when the are awake and moving around. Also I greedily want as many photons as possible to deal with the anti-aliasing filter inherent in DSLR's

On using video and taking out stills: Unfortunately stills taken from most video systems do not make for really good photographs. While the video results can look good, they are taking advantage of the human eye and how we perceive images with motion. Our eye is a lot more forgiving and fills in details that serve to enhance our perception of a video image. So while I have seen even the video from Canon rebels that looked good on the camera LCD and would look pretty decent on a TV, the data is just not there to get a good to great 10x14 still image.

So I'm still looking into this, but I'm wondering if I do have to back off and just put a cheap P&S on something for documentation images and not be trying to get these really long distance quality shots that I want?

btw owners of even the Canon 800 tell me they consider these nest distances to be too long for their equipment. But I think maybe they are just spoiled by seeing in flight Eagle close up where they can read their "name badges".

So the target that now looks like it may be unattainable, I've reduced to 1000, 1500, 2000 mm FL on my 1.6x body with an f-stop at 8 or wider. Giving 1600, 2400, 3200 effective FL.

I have found that Questar made a 700mm f8 lens back in teh 60's and 70's. That would give me 1120 FL at f8. One is on Ebay for about $500, but the seller says it shows the edge of its mirror either corroding or flaking off. And the DOF question is still unanswered. Has anyone used this Questar 700mm lens?

Agian thanks for the help with this.