Hi fellow Astronuts,

Had a chance to test the 110mm lens today to see what kind of resolution it can achieve under near perfect seeing conditions. These conditions are not common here during a typical night when astrophotography is normally done. But there is about a 1 hour time window when the skies switch from daytime scintillation to nighttime seeing conditions. That happens just when the sun begins to set at dusk, and the air becomes very still and there is not yet a nighttime radiation of earth's heat into space.

Today we are in the middle of a high pressure dome with the jet stream far away to the north. Good time to test optics on real stars. I chose Arcturus since it is quite high near the zenith right now. I wanted to see what kind of resolution can be achieved at various wavelengths. Theoretically the red wavelengths have less resolution than green or blue. I chose the 3 narrowband filters because normal wide band filters were completely saturated while the sky was still bright at dusk. I added an original Traveler field flattener which brought the 110mm lens up to F6.3. This flattener is not ideally matched, but close enough to use with this scope for now until we finish the proper mating flattener/reducer for testing. I used fast exposures to minimize seeing effects and to prevent saturation. At F6.3 the pixel scale is 1.116 arc sec per pixel (at F7 it would have been exactly 1 arc sec/pixel).

It turns out that the theoretical FWHM resolution of a 110mm aperture is right around 1.1 arc seconds at the green visual peak wavelength, which happens to be quite near the OIII filter passband. The Ha red wavelength has theoretically lower resolution of approximately 1.2 arc sec because red waves have longer wavelength. SII is farther into the deep red, basically at the edge of the infrared and has close to 1.3 arc sec FWHM theoretical resolution. As the images below show, the 110 lens produces almost exactly that theoretical resolution for these 3 wavelengths.

The interesting thing for me is that at best focus the 3.76 micron pixel size produces pixelated stars if the seeing is absolutely perfect. It appears that a lens of around F6 focal ratio could actually use smaller pixel cameras to produce non-pixelated round stars under perfect seeing conditions. Since we don't have such a thing around these parts, I guess that I can do with the larger pixels of the 6200MM camera.

Tonight the seeing is supposed to be average, and I plan on doing some narrowband imaging of a couple of objects, just to see what a wide field scope can produce as far as high res imaging of small objects.

Rolando

I have not yet settled on a flattener F-ratio. Something between F6 and F7 for larger format coverage and better resolution for small objects. depends somewhat on what users are looking for.

Roland

Heck, my CCD cameras are always running at -25 to -30. I thought these CMOS cameras should be able to get down to at least -15.

Rolando

This is what the seeing looks like right now from Clear Sky Clock: https://www.cleardarksky.com/c/AstrPhyILkey.html

Seeing is marked as 3 out of 5, average. Stars in a 60 sec exposure measure between 1.5 and 1.7 arc sec FWHM in Ha. So, I'm not getting the theoretical resolution now. But I think the image will be quite good anyhow.

Rolando