Ags

Hard to compare the images - Orion was binned and stretched much more and shot at different gain... My step-down rings arrived today - I can now shoot at F3.8 and F3.3. Looking at getting a Yongnuo 100/2.0. Apparently optics are good but autofocus which I don't need is bad. Also focusing at infinity may be easier with a non-macro lens (macro lenses are geared for fast focus travel over a very large range). If I can coax good performance out of my 100 Macro, then i can start dreaming of a dual camera setup....

When I saw them it looked to me like the Imperial battlefleet was entering orbit 😀

Don't forget the Hubble Space Telescope is an obstructed instrument. It does pretty good on deep space!

Not sure that's the reason - I have reduced the range of wavelengths so there should be a point of focus that is at least as good as without the filter, but with the (very cheap) Meade IR filter I get irreparably soft stars. @happy-kat the lens is still a F2.8, but I have some step-down rings coming.

I had a go at M42 with an IR block filter added. From my garden I can only collect 20 minutes of data and the gradient is from a near-by street light. The stars might be slightly rounder, but I don't seem to get good focus with the IR block in place?? Next clear night I will have another try with my Wratten 29 filter. It should reduce the gradient and remove all the shorter wavelengths. I think in the ye olde film days Wratten 29 were used for photographing Ha regions.

The M13 image is 100 ms frames.

For this one I shot a series of 1 minute SER files, then processed each in Autostakkert and stacked the results in DSS. More recently I have had similar results with collecting short FITS and stacking in DSS.

That was taken with my C6 and 6.3 focal reducer on the AZ-GTi. THe combination works well!

....or get a big hard drive and do lots of short subs 😀 My best effort so far (945 mm focal length, sub 1-second subs). Still a lot to learn but I think the approach has promise.

AZ-GTi, C6, 6.3 reducer, berlebach tripod - works for me 😀

I like sharing my images with family and friends, it works better than trying to convey in words what can be seen (people's eyes glaze over as I drone on...). So AP feels less of a solitary activity. With my bad memory, AP gives me tangible mementoes of the sessions, while visual sessions are soon forgotten. I also enjoy putting together the equipment and gathering the data - in an imaging session i just sit and tweak the scope and focus so it is relaxing, but visual sessions are quite manic involving weird contortions to get at the eyepiece. But visual can be very rewarding too.

....sounds like I got quite close with a figure of 0.8ish for an unobstructed telescope! Might revisit this now i have a much more powerful computer.

I am afraid that telescop-optics.net is coming at this from a completely different paradigm, so I can't help you - they are using Fourier transforms for a mathematical solution, while I am modeling the waves travelling though the telescope. Because there are a lot of waves to sample, I can only approach the answer but never actually reach it. But Fourier stuff makes my brain hurt - so my approach was good enough for me.

I think I was more focused at the time on the relative performance rather than on the individual absolute values. Due to the numerical and simulation-based approach I was not able to calculate the contrast precisely (would have required hundreds of times more processing time) but I could calculate the _relative_ levels of contrast.

Even in a refractor there is contrast loss due to diffraction. I can't recall how I got a number near 0.8 for no obstruction though - that looks too low.

The glass reduces the optical path due to refraction, just like a prism diagonal has a shorter path than a mirror diagonal.

On the subject of spacing - my spacing is 44mm, but it won't be after I mount a 1.25" IR filter in-between lens and camera. The same applie to everyone who mounts a clip filter in their DSLR. So how critical is the spacing in reality?

Yes, confused about the tilt though - if there was tilt, wouldn't there be a softness gradient across the chip, especially if the tilt is sufficient to distort individual stars? I had forgotten my IR block filter, so adding it back in should help a bit! Looking at getting some step down rings to stop the lens down F3.3. Also thinking of making a polygonal aperture mask as a last resort - I would be happier with diffraction spikes than with triangular stars!

You mean 31.5, right? On the subject of lenses, I have been thinking of replacing the macro with a 100/f2.0 portrait lens. These have a very good reputation, but never seen one used for astronomy.

This looks pretty good when zoomed in... https://www.astrobin.com/331725/?nc=all

I think my mono camera has an advantage over you here - I can focus for each channel. The reputation of the Canon 100/2.8 is that it is very sharp for astronomy. But individual samples vary. My stars look to me like pinched optics.

I have got the right spacing I think. I don't mind binning but the asymmetry extends far out into the halo of the stars so I don't think that is a solution. Maybe rotate the camera 120 degrees for each third of exposures, assuming it is the lens? I can try stopping down, but that is an anathema to me, as I am limited in capturing time.

These stars are 100 x 1250 ms subs stacked in DSS. Non-L Canon 100 Macro lens af F2.8 connected to an ASI 178 MM. The picture is a center crop. The misshapen stars can be seen on individual stbs so it is not a stacking issue. Bad lens, or something more fixable?

It was a full moon but I had a test run tonight on M31 and the double cluster. Now that I have a finder, the setup (ASI178MM, 100/f2.8 macro lens and AZ-GTi) is a real pleasure to use!

I think S and N are a counterweight for the objective lens.