Louis D

Thank you for the confirmation. All you have to do is look at the geometry to see that camera lens will be looking well past the edges of the eye lens unless it's a long telephoto lens, and therefore it can't possibly image anything other than the very center of the field of view.

Good idea. I picked up my 24mm APM UFF second hand for $130 3 years ago.

It's got to be killing your business if you have no stock to sell.

For the same money, the various 24mm APM UFF class eyepieces have a similar performance level to the 24mm ES-68 at the same price, at least here in the US. They are also more readily available. Look for the 24mm APM Ultra Flat Field, 24mm Altair Ultraflat, 24mm Meade 5000 UHD, 24mm Celestron Ultima Edge Flat Field, and 24mm Orion Ultra Flat Field. Below is a comparison image of my various ~24mm eyepieces, including the 24mm APM UFF, in an f/6 field flattened 72ED refractor: I don't have either the 24mm ES-68 or Panoptic for comparison due to their tight eye relief. The Panoptic, at least, would be similarly sharp to the 27mm Panoptic shown near the bottom.

If the focuser can accept a 3" diagonal, the 30mm ES-100 comes into play along with custom Siebert 3" observatory eyepieces. Many refractors intended for astrophotography come with 3" or larger focusers, so there might be some options there if there is enough back focus, which there should be since they're intended to accept filter wheels, OAGs, and other gear in between the focuser and camera.

Folks were taking well guided astro images using ordinary tracking mounts long before goto mounts came to be, so they are not a prerequisite.

If using an image intensifier, the 40mm will allow for a larger field of view than will the 32mm if it has an angle of view of less than 43 degrees. Obscure reason, but it might apply someday.

Eyepiece projection might be possible with that camera, an eyepiece, and an eyepiece projection adapter of the proper type now that think about it.

The Mak will need a mount and tripod, so you'll need to budget for that. The mount is built into the Dob. I find Dobs easier to track with than alt-az mounts on tripods and are better at resisting diving when changing out heavy eyepieces.

Maybe it will go on sale in the interim?

Next chance I get, I'll dig out my Sigma 50mm f/1.4 lens (with 1.6x DSLR crop factor on my Canon Rebel T3i) and try to image a typical eyepiece's AFOV just to illustrate the result for you.

Sometime try looking at a bright star at the edge of a low power Erfle, Kellner, or other poorly corrected eyepiece, and then swing the scope over to the full moon and observe for a while. Now, quickly swing the scope back over to the bright star. Does it look sharper at the edge now?

Only for eyepieces with sharply defined field stops. For eyepieces that have fuzzy field stops due to being poorly placed or non-existent (barrel defined or field lens defined field stops), you'll be lucky to nail down the AFOV to within a degree.

In your experience, does an astigmatic eyepiece look sharper when your iris is smaller? Not in my experience. The astigmatism in my own eye becomes less noticeable since I'm only viewing through the central part of my cornea, but the eyepiece image does not improve.

All I can say is go out and try afocal projection with that setup. You'll find out really quickly that you'll only be able to image the very center of the AFOV because you can't get the camera lens in close enough to match exit/entrance pupils. The objective lens will bump into the eyepiece unless you're using an eyepiece with several inches of eye relief to project that AFOV well behind the eyepiece. I've been doing afocal projection photography for well over 20 years, and until the advent of small objective lenses with early digital cameras using small sensors in the early 2000s, the method was not very usable except with low power, long eye relief eyepieces. It's literally no different than trying to see the entire AFOV of an eyepiece from anywhere except with the eye's entrance pupil positioned at the eyepiece's exit pupil. The cell phone camera replaces the human eye. The software will have to get rid of keystone distortion as you point it off axis. As someone who doesn't do image stitching at all, I have no desire to go down that route. Another problem can be getting the camera tipped correctly when eye relief is very limited since you want to keep the lens's entrance pupil at the eyepiece's exit pupil as you collect images. If the camera has any significant width, and if the eyepiece is not a volcano top, the two will bump into each other. You can back up the camera in this case, but you will capture an even narrower sliver of the AFOV. I think this only comes into play if the camera's f-ratio is slower than that of the telescope's f-ratio. Since cell phone cameras are very fast, this is never an issue. Michael Covington over on CN would be the best to clarify this question.

Put the eyepiece in a telescope, point a flashlight (UK torch) into the telescope, put an eyepiece in the focuser (a diagonal is handy here), and aim the eyepiece at a wall. Move the light around until you get a nice, sharp circle showing the entire AFOV all the way out to the field stop. Now, measure the diameter of the projected circle and the distance from the top of the eyepiece to the wall. Make sure to keep the top of the eyepiece parallel to the wall. You want a nice circle to measure. Next, using a white box (or card stock), determine the position of the exit pupil by inserting it between the eyepiece and the wall. Move the box/card toward the eyepiece until the image circle is minimized. If there is gross CAEP, this can be difficult to determine. For most eyepieces, it's fairly obvious, though. Now measure the distance from the top of the eyepiece to the box/card. This is the usable eye relief. Lastly, use the following formula to calculate the AFOV: 2*arctan[(Circle diameter/2)/(Eyepiece to wall distance - usable eye relief)] You're just calculating the half angle of the projected circle as seen from the exit pupil with trigonometry (tangent(angle) = opposite/adjacent) and they multiplying it by 2. The drift timing method is a good way of measuring the true field of view (TFOV) of an eyepiece, and thus its field stop, but it tells you very little about its AFOV due to variable magnification distortion across the FOV.

Unless you can find a DSLR lens with a tiny lens depth wise or are using an eyepiece with a large amount of eye relief, you'll never be able to get the exit pupil of the eyepiece to coincide with the entrance pupil of the lens (generally where the iris resides). As such, you won't be able to image the full AFOV of the eyepiece. That's why today's cell phone cameras are so fantastic for this application. High resolution, wide angle field of view, well corrected, and insanely short distance to the entrance pupil. This is completely irrelevant for imaging the AFOV to determine eyepiece characteristics. The image will be dimmer than if it could have accepted all the photons available in the exit pupil, but the entire AFOV will be visible. It's no different than looking at the full moon with the human eye through a low power eyepiece. The eye's entrance pupil will be narrower than 5mm because the full moon is too bright to allow it to fully dilate. However, a low power eyepiece could very well be producing a 5mm or larger exit pupil at the same time. This has no effect on seeing the entire AFOV which is what we're trying to achieve here.

It's not in the insertion barrel? It must take a lot of in-focus to reach focus with that eyepiece.

You should be able to directly measure the AFOV using the projection method to within a degree of accuracy. If you have some eyepieces with known good FS diameter values (such as TV ones), you should be able to extrapolate a conversion factor from the number of millimeters you see on your ruler through the camera lens to the FS diameter in millimeters. That's how I've been able to determine FS values for all of my eyepieces based on known good values for some of my eyepieces. I'm usually within 0.2mm of the value from other sources such as Ernest of Russia's measurements. From the FS and FL values, you can then calculate the eAFOV (effective AFOV) using mathematics (FS/FL*57.3, IIRC).

I've measured my 40mm Pentax XW-R to have a 46.2mm field stop, a 70 AFOV (via projection), and a 66 degree eAFOV (effective AFOV) based on the FS and FL. I think you are referring to eAFOV rather than projected (perceived) AFOV. Go back and remeasure your AFOV using the projection method and see if yours isn't also 70 degrees.

Don't feel too bad, it took the Hubble folks a few tries as well to get things sorted:

Tom Dey over on CN put this graph together to show where each barrel size maxes out: There is some fudging allowed if you put the field stop above the insertion barrel and accept some vignetting and lots of required in-focus. It's sort of analogous to how you can use 2" eyepieces with 127mm Maks despite their 27mm diameter rear port.

Without tracking, you'll be having to keep pushing the scope, letting it settle, and then allowing the planet image to drift across the imager. Doable, but you'll tire of the process fairly quickly. An equatorial platform will allow tracking for up to an hour before resetting it.

The shorter the Barlow, the larger the effect of placing it in front of the diagonal (that whole 1/FL deal). If you can screw the optical element directly to the front of the diagonal, you can greatly reduce the magnifying effect.

Sure. You might need to refocus the reticle due to exit pupil issues. Give it a try if you've got both.