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.
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