Getting The Samyang Setup Ready For Imaging

[This is Part 2 of 2 about my new Samyang 135mm f/2 lens imaging system]

What is the Samyang Setup? 

It's the same setup I use for imaging with the FSQ-106 -- with a few changes. Obviously the imaging scope is now the Samyang lens and the Pegasus FocusCube 3 is swapped out for a ZWO EAFN. The connecting hardware between the lens and my imaging camera (ASI 2600MM) is different as well because of backfocus needs.

Questions to Answer

Is the lens optically sound?  Can it provide focus at infinity? Does it have significant aberration? Will it work well at f/2, or does it need to be stopped down to f/2.4, f/2.8, or f/4? Do the lens adapters introduce significant tilt?

Does autofocus work well?

How much can I reduce the time it takes to make a single dither?

Given the cloudy nights typical at this time of year it will take a while to get things sorted out. Because it only requires stars to do this I can stay in my back yard; dark sky is not necessary.

Night One (10 September)

The lens would not focus at infinity.  This meant autofocusing and image quality assessment were off the agenda. 

What did work was tracking. Plate solving was 100% despite the stars being somewhat out of focus.  I was able to slew and center without any issues. 

Clouds came in before I could look into dithering -- or anything else, for that matter.

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A letter to the M42 adapter person  at Thinkable Creations got a fast reply that pointed me to this video that shows how to remove the focus travel stop. This was an easy fix and with the stop removed the lens should be able to focus stars. 

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Night Two (22 September; summer is over!)

Really, it was almost two weeks between clear nights that I could use! Worst Summer Ever: clouds, smoke aloft, smoke at ground level with air quality alerts, rain, and the abundance of mosquitoes that the rains produced. Onward to Autumn!

First business: star focus. I set the EAF zero position at the full out focus, and infinity focus is near  position 750. Park position will be a little larger than the backlash.

I used a standard methodology* for getting autofocus configured.  

1. Manually** find a very good focus. 
2. Change position** gradually until you see greater than 50% growth in star size. Set step size to the amount of position change.
3. Run autofocus and see if the ratio of defocused:focused HFR is about 3:1 to 4:1; estimate how much backlash is in the system and enter that in the OUT field of NINA's autofocuser settings. Backlash will appear as unchanging HFR in the first few measurements. The change in position from the first measurement to the last one at the same HFR is the amount of backlash.
4. Run autofocus again and adjust step size and backlash accordingly until a decent hyperbola emerges
5. Repeat Step 4 until HFR ratio is about 3:1 to 4:1 and hyperbolic quality is close to 1.00
6. (optional) Reduce number of autofocus points and run autofocus to confirm it still works well 

*This is described by another fine Patriot Astro video starting at the 16:29 point, where the process is used with a ZWO EAF.

**My suggestion is to start at focuser position zero (the new "infinity" stop, or close to it) and move to best focus. Stop at a good focus and don't try for perfection; don't decrease the focus position at any time while hunting for focus. Then continue increasing the position while determining the step size. If you happen to pass through a better focus, note its position and measure step size from it. This insures that backlash does not factor into step size.

I did get AF to work reasonably well, with a focus step of 100 and backlash also at 100. However, this was with NINA's built-in AF, not Hocus Focus, so for Night 3 I'm going back with Hocus Focus

Sample frames were also collected at f/2.0, f2.4, f/2.8, f/3.3, and f/4.0. 

This gives me hope that I can image at f/2.0. Night 3 will be tuning Hocus Focus for better focusing and seeing if I need to adjust backfocus. If this works out Night 4 might be trying to create an actual RGB image!

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Loading the sample frames into ASTAP suggests dreadfully large tilt: 42% at f/2.0, and 16% (barely tolerable) at f/4.0. Here is are the diagrams of interest at f/2.0:

f/2.0 Tilt Original

This indicates a strong bottom to top tilt.

f/2.0 Aberration Inspector Original

The bottom row has badly elongated stars, but the top row isn't too bad at all. I think the tilt adds elongation in the bottom row while essentially nulling it out the top row. If I could selectively remove the tilt I'd probably have a better idea of the aberration due to backfocus error and  could possibly fix it mechanically.  

Toward that end I've ordered some very thin 3D-printed tilt shims. (Hardware doesn't permit me to use the tilt plate that came with the camera). Correcting some of the tilt might help with focus and other star-diameter calculations. An alternative it to use software to correct both tilt and any other aberrations simultaneously. The software of choice for doing this is BlurXTerminator (BXT).

Applying BXT (using its default settings) gives me this:

f/2.0 Tilt after BXT

f/2.0 Aberration Inspector after BXT

Quite an amazing improvement, isn't it? Tilt has essentially vanished and corner stars are much rounder.

Night Three (23 September)

Hocus focus worked well with the existing values of backlash and step size. I did bump backlash upward a little to 150 after looking at a few runs. With HF running the hyperbolic fits were much better and the luminance focal position seemed more consistent.

I ran the filter compensation calculator with mixed results. Red and green were basically parfocal with luminance, but blue was quite offset. This might be because I need to adjust exposure times? I'll repeat this.

Night Four (25 September)

This time the best focus (smallest NINA HFR) determined manually was at focuser position 735. Blue best focus came at 835, so the offset was +100. This is essentially the same as the software-determined +93. 

I took a baseline R-G-B-Dither 10 times; the target was M52. My main goal is to get a baseline for how long it takes to gather this data. It appears that a simple 60 s frame consumes about 70.4 s; a frame followed by a dither uses 101.4 s. Ignoring autofocusing, this means a single RGBD(ither) sequences uses about 242.2 s to collect 180 s of data. Roughly speaking, multiply the total exposure time by 4/3 to get the actual acquisition time. It's pretty much the same as if I was shooting LRGB.

An "adequate" data set of 40 frames per channel, suitable for drizzling, means 2 hours of data. This means acquisition time will be about 2.7 hours, plus some for refocusing. This isn't half bad, and might be bettered by adjusting settling times and optimizing the filter order.

Anyway, here is the first light image for the lens:

M52 at center

This is surprisingly good, at least to me. I'm under a Bortle 7 sky and not using filters of any kind. The ability of PixInsight to remove light pollution boggles me, and how well BlurXTerminator reduces aberrations is equally amazing. At f/2.0 the lens has considerable chromatic aberration:

CA in corner star

This star elongation is almost all chromatic aberration, and amounts to about 4 pixels between blue and red. Because BXT is able to correct this I'm going to go with f/2.0 for my first "real" image. If that doesn't turn out well I may move to f/2.8.

Questions Answered?

Is the lens optically sound?  Can it provide focus at infinity? Yes, after a little surgery. Does it have significant aberration? Yes, but it appears to be correctable using BlurXTerminator. Will it work well at f/2, or does it need to be stopped down to f/2.4, f/2.8, or f/4? It's adequate at f/2.0, but might be better at f/2.8. Do the lens adapters introduce significant tilt? I suspect this is the source of the tilt I'm seeing, but I need to look closer at this issue. Maybe the shims I ordered will be the remedy, or I may revert to using a Canon to M42 adapter to see how that works.

Does autofocus work well? It seems to work well enough.

How much can I reduce the time it takes to make a single dither? I still need to play with the dither settings and find out.

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That's the last of the prep nights!  Next I'm going to try to resolve an issue I've had while using two ZWO cameras (one for imaging, one for guiding) at the same time; this was a problem that first popped up while at a remote dark-sky site in which the two cameras switched roles. Using the ASI2600 as the guide camera really does not work.

Another issue I need to explore is why it takes NINA so long to connect to my Losmandy Gemini 2, why it throws an error at first and then makes a good connection. Strange!