In preparation for the Iowa Star Party I'm doing a some imaging housekeeping. The guider needs a fresh calibration and the filter offsets need a good redoing. More complicated are the changes needed for a basic imaging sequence using the Samyang lens. Because rotating the camera/lens requires the EAF belt to be disengaged and then re-engaged, one needs to put some breaks into the sequence to make opportunities for working with the belt. Here's what I'm setting up for the Iowa party.

The following instructions are for the NINA advanced sequencer. If you're not using that, you really should be! Also note that this is specific to a short focal length system for which you intend shoot RGB suitable for drizzle processing.

• NINA's Advanced Sequencer (If you are not, you should be)
• Your EAF position is zero when the lens focus is at the infinity stop

All good? Great. Here's what you need to do if you plan to orient the imagining camera while it sits in an Astrodymium cradle. It's a little complicated by the need to disengage the EAF belt while turning the lens.

Rotating the lens requires that you add two Message Box (MB) instructions to the sequence. The MB instruction is found in the Utility section. Obviously, if you don't care about the camera orientation, you can just skip the following.

If you're doing a Slew, Center, and Rotate (SCR) and have connected the Manual Rotator to NINA, place one MB just before the SCR command and another just after it. When the sequence reaches the first one, it will stop. You should then disengage the belt from the lens, being careful to not disturb the focus. Then click away the Message Box. NINA will slew, center, and tell you the amount and direction to rotate the lens. Do this until NINA is happy. It may also do some additional centering, but eventually it will reach the second MB and stop. Re-engage the focus belt. Verify that focus is good enough for autofocusing, and click away the second MB. The sequence will now resume normal operation.

If you're using Slew and Center (SC) and simply eyeballing the orientation, just use one MB immediately after the SC instruction and a second SC right after that*. When sequencer get to the MB, Disengage the EAF belt and manually orient the camera using snapshots. When the orientation is correct, re-engage the belt, check focus, and click away the MB.

*The second SC is needed to insure the target is at frame center; it acts as the post-rotation recentering performed by SCR but not by SC.

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I still wasn't happy with the tilt I was seeing, and I wondered if maybe it was coming from the way I had the camera attached to the Samyang -- that partially attached 5 mm spacer seemed iffy. I sent a note to Nic at ThinkableCreations asking if he had some advice about how to remove about 2 mm from the M42 threads on the adapter he sells. He didn't, but he expressed some concerns about the shorter thread length not providing a solid connection. I thought that would not be a problem and went ahead with my plan to sand off the end of the threads.

I used 600 Ultrafine "Wetordry" sandpaper from 3M for a while and made little progress, so I switched to 400 and it went nicely. Eventually I was able to attach my 7.5 mm spacer to the Thinkable adapter, giving me 45.5 mm of backfocus, close enough to the 45.0 desired.

Sounds good, but the reality was that this change once again made the lens unable to focus infinity.

Swapping off the 7.5 mm spacer for a 5 mm, back focus was now 43 mm and focus was restored at a EAF position of about 3200. Still bad tilt, possibly because I didn't sand the Thinkable adapter down far enough. I added two spacer shims to bring the back focus up to 44 mm. The focus position was now around 2300. One last tilt tilt-correcting shim took it up to 44.2 mm and an EAF position of 1400. Possibly I'll add another thin spacer in Iowa.

Evidently the ability of a lens to reach focus depends strongly on backfocus, something I didn't know. 

EAF position (vertical axis) vs. Backfocus (horizontal axis, mm)

(The zero position in the diagram corresponds to the maximum lens movement in the direction of infinity focus. The negative position at 45.5 mm is a guesstimate.)

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Anyway, here I am a few days away from the Iowa Star Party and I'm going to shove my perfectionist nature to the curb and settle for "good enough." I'm going to rely on BlurXTerminator to handle aberration and tilt issues.

Here's an idea of what the lens can do. This is about a 16th of the full image area, unscaled. M31 straddles the right edge of the image. This is a single 20 second frame of luminance; click to see it at 1:1 scale.

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!

Finished: Integrated Flux Nebula Image

Here's the image at quarter-scale:

1/4 Scale Image

Full-Scale image at AstroBin.

Where to even start with this? How about the data?

Originally there were 13.2 hours of data, but I came across a video in which someone explained how they use PixInsight's SubframeSelector process to cull bad frames. My approach to data culling has always been to keep all that aren't terribly bad, but for this project I thought I'd get tough. Using SFS led me to reject 3.6 hours of data!  To be fair, about a third of that was because of my penchant for starting data collection before the end of twilight. There were very few visibly bad frames as viewed in Blink, so I'm going to call this approach "2 sigma" aggressive, in that it basically culls any frame that has  FWHM, eccentricity, or median values more than two standard deviations above the mean. Note that those rejected frames might be perfectly fine in and of themselves, but relative to their cohort they are of significantly lesser quality. Frames with anomalously low star counts are also culled. An example of this is the set collected during the session that a smoke layer moved in and began obscuring stars in the late morning. Star count fell markedly and I removed frames.

Worth mentioning was the need to use WBPP's Grouping Keywords to make sure that light frames and their appropriate flats were processed together. This was the first time I used it, and it worked perfectly. 

Also, I no longer use dark flats, or "flat darks," if you prefer. Only dark, flat, and bias frames are used for calibration. (Flat and dark frames are now taken for granted at Astrobin, it seems; it no longer asks if you use them.)

Now about the calibration frames, specifically the flats. It seems that most of the time my flat illumination was asymmetric for reasons I don't understand, and this gave the background modelization processing some problems. That big bright Polaris didn't help, either, nor did the fact that most of the image was nebulosity. My first pass used GradientCorrection and that left the right side with a green cast. After playing with that for a while I moved on to DynamicBackgroundExtraction. That didn't clear it up, either. After thinking about it for a while I reverted to AutomaticBackgroundExtractor with a 5th-order function and that did the job. 

Next, those darn satellites. The first processing pass got most of them, but a few stuck around in weakened form. They should have been removed during light frame integration, so I looked at what WBPP was using for rejection and it was Generalized Extreme Studentized Deviate (ESD). Some hunting around took me to a PixInsight forum where it was noted that ESD (using its default settings) wasn't doing a great job with satellites. So I told WBPP to instead use Linear Fit Clipping and that seemed to work better. Not perfect, just better. I will need to find out what ESD settings work best since overall it's probably the scheme to use. It may be that satellites and an image full of nebulosity are always going to be a problem.

I also learned that my usual haphazard application of the XTerminator family has been wrong. It's a processing sin to use NoiseXT before BlurXT and NoiseXT before SPCC. For this image I only applied NXT after taking the image nonlinear.

Here's my workflow for this project with the ">" symbol meaning "creates":

WBPP  >  Cropped channel masters

ABE (color channels) > Backgrounded color channel masters

ChannelCombination > RGB master

ImageSolve > RGB master with astrometry 

SPCC > color-calibrated RGB master

ABE (luminance) > Backgrounded luminance master

BXT (luminance master and RGB master) > enhanced masters

STF and HT > nonlinear masters

NXT > de-noised masters

CurveTransformation (with gentle "S" curve) > enhanced masters

LRGBCombination > LRGB master

assorted tweaks (saturation, sharpness, contrast, etc.) > Finished image

Not shown is an additional DynamicCrop after the ABE of luminance because ABE was a little overaggressive at the left edge. Even with two crops, the final image lost only 4.2% off the short axis and 5.5% off the long axis for a 10% areal loss. The reproducibility of the image framing was impressive. Thank you, NINA. 

Another lesson learned was that the XTerminators could be sped up quite a bit. Normally the necessary files are installed by XTs, but on my old computer the install did not engage the GPU. My graphics card is an NVIDEA GeForce GTX 1050 Ti circa 2018. This post explains how to upgrade a computer to use the GPU for faster XT performance. In my case it sped up the XTs by a factor of 4. I may need to repeat this every time XT does an upgrade.

So how did the processing work out? Mostly I was concerned that the area around Polaris was darkened by background extraction and didn't represent reality. I searched AstroBin for an image I could use as a sort of "ground truth" for what I had done. I found just what I wanted in an image by captured_mom8nts (which I'm guessing is not their real name). It appears to have been taken at a much shorter focal length and so should have suffered much less Polaris bloom, keeping the area around the star reasonably pristine. A little crop/rotate/scale/stretch and it matched my image's scale and orientation:

Comparison: Mine (top), captured_mom8nts (bottom)

I think it fairly obvious that the dark areas on either side of  Polaris in my image match those in captured_mom8nt's image, even though mine is much deeper. I'm happy!

I'm also happy with the star color. Shooting only 90 s exposures may have been the key to that in that it kept stars from saturating. Next time I'll be shooting at f/2, but with a smaller objective so I may keep the exposure time as is. 

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All the components of my new Samyang 135 mm f/2 imaging system have arrived or are on their way. Next time I'll have a picture of it all assembled and possibly already taken on its first test drive!

IFN Setup Passes Its First Tests (With Help From BlurXTerminator)

Last night was good for running some tests on my integrated flux nebula (IFN) imaging setup. I was able to take test frames at different exposures (60, 90, 120, 180 and 240 seconds) and test go-to and tracking. Everything came out well, or as well as could be expected and many questions were answered.

Did Polaris throw an internal reflection?

I stretched a 240 s exposure using PixInsight's Boosted Autostretch and there was no evidence of any internal reflections. It's possible something will show up in an image stack, but this suggests even if it does it will be faint.

Was the field suitably free of aberration? 

Here's a visual comparison of the corners and center in one of the 2 minute exposures (click for full scale). This is basically a raw frame; it's only been brute-force flatted and then delinearized with PI's default stretch.

Pretty awful, isn't it? My guess is that the aberration is coming from backfocus being significantly off. How much off, and in what sense? I'd use NINA's aberration inspector to find out, but this is a manual-focus lens. Sadly, my ability to change the backfocus is next to zero. Skip the following paragraph if talk about backfocus bores you :)

My required backfocus should be about 45 mm (44 for the lens plus1 for the filter), and I'm currently at 42.5 mm. The best I can do to increase this is by adding a 0.8 mm thin spacer or whatever the thinnest M42 spacer ring is (probably 5 mm?). Decreasing the backfocus would require a Canon-M42 adapter that's thinner than 10 mm or a filter drawer that's thinner than 20 mm and can be adapted to use 36 mm filters. Increasing the fun is that this lens is actually at heart a Tamron Adaptall lens circa 1980, fitted with a ring to make it compatible with OM-1 cameras. Both date back to the 1970s, so good luck finding things to take advantage of any of that. And yes, I looked into having a custom adapter built, both Canon to M42 and Olympus OM to M42, and they can't do it in a way that works for me (not to mention it would be $$$ if they could).

Fortunately the aberration can be adequately dealt with using BlurXTerminator (BXT). The corner diagram below shows how well BXT fixes things using the very non-aggressive settings of zero for both its "Sharpen Stars" and "Adjust Star Halos" parameters:

The improvement is almost miraculous. All the corners look sharp. So the lens passes this test thanks to processing with BXT.

Can the lens reach a decent focus?

The focus you see is the result of a few minutes of shooting test frames and making very tiny manual adjustments (just like in the old days before I had an electronic focusing motor). I think it's quite good. Yes, I'd love to have NINA do the focusing for me, but that's not going to happen.

I will need to stabilize the focus wheel to avoid accidental movement; a piece of tape should work.

Was there tilt?

A first look at tilt as calculated by ASTAP gave this.

It's not so much tilt around one axis as it is a sign of the aberration. ASTAP considers the tilt severe (see the small text along bottom of image). After BXT has been run the results confirm the improvement seen in the second corner diagram above: 

Star size outside the center is dramatically reduced and ASTAP now considers the tilt to be "almost none." This means I won't need to add the tilt plate or shims.

What about the mount and Go-To?

The mount slewed to Polaris and plate solved without fault. Manually correcting the rotation was simple and fast. The required rotation was only eight degrees, so the riser wasn't necessary and will be removed.

How was the tracking?

The center stars in the pre-BXTcorner diagram were nice and round so I'll assume tracking is close to perfect. The corner stars in the post-BXT are fine, too, so there's no appreciable field rotation in a 2 minute span.  I use PoleMaster for polar alignment, and it provides almost no visible rotation even over a multi-hour session, so I think that tracking should be more than adequate.

In other words, there's no need to add autoguiding or fiddle with PEC.

Were there any composition issues?

The amount of camera rotation required was only eight degrees from the filter wheel's long-axis up position, meaning that the riser wasn't needed. I'll remove it and the system will be a little sturdier.

Oddly NGC 188 was not quite where I wanted it. It's a touch close to an edge than I expected. I'll have to look at the instructions I gave NINA.

Summary

Everything worked better than expected with help from BXT! Basically, the system is ready for field use, although a little more work remains to get it set up for dew control. Because BXT removed the aberration using very nonaggressive settings, I don't think it will damage the IFN.

A major concern was the mount: would it behave itself for a target so close to the celestial pole? It slewed and centered on the target without difficulty. Manual rotation was easy and the riser won't be needed.

I think the ASI-2600 will do fine without dithering, but I may try that anyway with the PixInsight manual dithering tool.

It looks like almost any exposure will serve well; even a 4-minute exposure had nice round stars; I think I'll probably use 2 minutes for all four channels.

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My reading of Hugo Award winning novels is winding down. I skipped back to the 2000 winner, A Deepness in the Sky, by Vernor Vinge, and it was slog to get through. A few times it was almost a DNF (did not finish). I just couldn't connect with the author's writing style, his use of many side characters that were of little consequence, and the too-happy ending that seemed rather forced. For some reason he felt compelled to add a bloated epilog that served mainly to punish readers. Most of the Hugos have been worth reading, but not this one.

Up next is the 1939 Retro Hugo Award winner, The Sword in the Stone by T. H. White. Yes, it's that sword in that stone; the story has spawned a number of adaptations (the 1963 animated version by Disney is probably still the best known.

This will be the end of the Hugos for me, at least for a while. Maybe I'll start another reading marathon when my wife has her other knee replaced.

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It's lithium battery maintenance time: all the batteries have been fully recharged and then discharged to about 50% of capacity. Just before Northern Nights Star Fest I'll be bringing them up to full charge and selling/swapping them at 80% below list price. I'll provide a full description of what I'm bringing to NNSF in an August post.