Focus stacking allows the return of the mirror lens.

[Fine_Art]

The poor cousin mirror lens has been forgotten mostly due to donut bokeh and partly due to low contrast. With RAW the contrast issue is gone. Now with focus stacking the OOF problems can be gone. Lets keep in mind the fundamental physics that the lens manufacturers avoid talking about when they sell supertelephoto glass lenses for the price of cars:

Angular resolution is a function of the area of the lens. In supertelephoto we want detail from small angular objects like birds or other wildlife. The only way to get it is with a big aperture lens. The only reasonably priced big apertures are on mirror telescopes.

I was reviewing the PMPix numbers at DXO on the telephotos. They are aweful. 12MP resolution on a $8000 lens? Are you kidding? Of course that made me think back to the physics.

Something that would interest me is a review of a big premium mirror for supertelephoto. I am getting decent images with my 10" dob. It is a basic Chinese mirror. I would say it is producing 8MP of resolution. My 16MP APSC camera creates washed out detail at the pixel level. You have to downsample. Of course for under $1000 it is a deal especially with stitching.

What about a Zambuto or Waite mirror based scope? Could we have a review? Do they have the quality in aperture to fill 24 or 36MP sensors with detail?

[NancyP]

What are you shooting? Does the subject need responsive autofocus? Most supertelephoto users are shooting action, at various distances. I can't place a lot of faith in DXO stats unless the statistic applies to the exact lens and camera combination you are proposing to use. Even then, there are biases in the formulas toward faster lenses. The MTF of the Canon superteles look pretty promising, to me. I haven't handled one of the new generation, but others say that they are really sharp. The needs of a telescope user are for a set-up that need only work at infinity range, does not need autofocus, and has sufficiently flat field and sufficiently little coma for the intended sensor (eye + objective, CCD, dSLR CMOS of whatever size).

[Fine_Art]

I am shooting wildlife in MF with one hand on the focusser, one hand holding the remote cable while pointing the dobsonian. The A55 has a tilting LCD that lets me look at the live view while keeping the scene in peripheral view around the camera. It sounds a lot more awkward than it is after a few tries. An upside down view camera is probably similar in that sense. I like the fact I am far enough away that the animals don't spook. They can look at you curiously while looking calm.

[NancyP]

Do you mean focus PEAKING on the Sony, Magic Lantern-equiped Canons, etc? Stacking is for increasing total exposure time or for obtaining optimal focus from several focal points.

That gives new meaning to the slang term "Big White"!    Are you going to "Lens Coat" it? That's a lot of rolls of camo tape, or a custom stitching job on some tarp material. A 10" dob is sufficiently large, more on the order of a car than of a gun, that the ducks just might ignore you. Seriously, that is a lot of telescope - are you shooting from a roadside hide with a wooden floor or your porch or what? For the non astronomy-minded (which included me until a few months ago), a "dob" = altitude-azimuth Dobsonian mount, a weight-balanced tilting axis mount placed on top of a lazy susan rotating base. It is the preferred relatively lightweight portable inexpensive easy-peasy hand-guideable mount for larger reflector telescopes. One not all that fit person can easily wrangle scope and mount (separately) from the back of the car and set up on any level surface. The Dobs aren't good for astrophotography, an equatorial mount is needed for that to get adequate exposures.

FWIW, an Italian amateur astronomer and birder manual-focuses his refractors to get bird-in-flight shots. Interesting article in Nature Photographer way back.

 I am learning about scopes to buy my first scope (probably used from some club member who can show me how to use and care for it).

[Fine_Art]

There is a group on birdforum that does digiscoping with telescopes. Most use a 600mm refractor the ED80. One of them, a guy from Portugal, has inspired shots, some in flight, using focus trap with his scope. They actually made me decide to go with a scope from their results. I decided to go 8" first with a cheap dented display model to see if it works. For about $300 no big deal. Yes it does work. So I got a collapsible 10" for the resolution and a better mirror. After replacing the secondary with a smaller 15th wave secondary the image was as good as a typical kit zoom 70-300. At 1200mm. With the big aperture dof is thin. I believe a premium mirror would give images as good as a 300 or 400 f4. Theoretically my scope should give me detail at .5 seconds of arc. The mirror is not that good.

I'm talking about focus stacking just to eliminate donuts in the background if they are in the shot.

[Fine_Art]

And yes, it is an involved process to setup. Take the base out of the car to the spot. Take the tube out to the spot. use a normal 300f4 on a tripod while the tube temp acclimatizes if you didn't drive with the windows open. Collimate the optics (about 3-5 minutes). Wait as usual for wildlife to come in sight.

I have done it following herds. Collimation holds as long as the temp holds.

[Fine_Art]

Here is Fernando's website. I think all his bird shots are with a 600mm telescope.

http://fernandobatista.pt.vu/

[Fine_Art]

Here is a full frame of the 1200mm dob with D600.

http://farm4.staticflickr.com/3726/10621375476_d8ba8d3aa2_o.jpg

You can shoot in strong light with no fear of a fringed mess. Once you have the shot you want of the subject you can focus stack the small donuts out.

[Fine_Art]

This other gull in poor light at higher ISO is a little bit closer allowing the feather detail on the head to be resolved to some degree (no pun intended). Given this is 1200mm I would approximate it to a 600 prime in resolving ability. This is a $800 dob not a $5000 prime.

[BernardLanguillier]

Here is a full frame of the 1200mm dob with D600.

http://farm4.staticflickr.com/3726/10621375476_d8ba8d3aa2_o.jpg

You can shoot in strong light with no fear of a fringed mess. Once you have the shot you want of the subject you can focus stack the small donuts out.

Hum... not too sure I like the rendering of the oof areas in this one. Especially the yellowish circle on the right that seems to reveal the internal structure of the lens?

Cheers,
Bernard

[Fine_Art]

That is why you need focus stacking with a mirror lens.

[Fine_Art]

What is interesting about that specular reflection off the lake is you can see the diffraction rings generated by the spider (secondary mirror holder) and the edge of the scope. The dark ripples around the edge are also created by normal lenses.