Objectives: Test Hyperstar image correctness at a focal length of 67.8mm. Test Hyperstar vignetting after opening optical path from 42mm to 48mm.
Background: The circa 2007 Hyperstar M14 was first resurrected on Jan 2nd, and all attempts to use it since have been problematic. The primary problem is that the backfocus needs to be precise to ±0.5mm and neither the 49.9mm distance given me by Starizona nor the 61.9mm distance I’d used 10 years ago nor distances of 55.1mm and 45.1mm provided acceptable images. The 61.9mm setup provided the sharpest image but had enormous vignetting.
Procedure: After a couple of weeks and several emails Starizona provided new backfocus specs of either 67.4 or 70.4mm Due to the unworkable vignetting at the longest distance tested a new mount was cobbled up using a Canon adapter and 48mm T-thread rings. The camera was remounted early in the afternoon and NGC2174 was chosen as the test target not only because it was optimally placed but because there are comparative images. The moon’s up and initial focusing is greatly simplified by focusing on Luna before aligning on Aldebaran.
Results:
Spikes have been a problem. A dozen spikes are present when fine focusing on Aldebaran but they disappear when best focus is achieved.
There’s been an ASI Camera forum argument that 2 second or longer exposures for flats are necessary for CMOS cameras because many of these cameras used different read configurations for long and short exposures. CMOS sensors aren’t specifically designed for astro imaging. The sensor is designed to do high resolution video as well as long exposures. Supposedly when the camera is set to short exposures it uses read circuitry to optimize data transfer rates that aren’t used for longer exposures. Therefore longer flat exposures will be a better match to the long light exposures. BUT, knowledgeable source has also told me that if I want to run SharpCap bias frames along with the flat capture gain must be set for whatever one images at.
Since there’s no luminescent panel I’m out at 8AM the following AM taking advantage of a gray winter sky. I want bias frames so gain is my standard setting of 120. And with that gain, exposure is set to 1/10 second so as to center the ADU histogram. 50 frames averaged.
- 1 flat balance.jpg (294.91 KiB) Viewed 737 times
- 2 100ms 120g -20c flat.jpg (59.47 KiB) Viewed 737 times
Initiated imaging at 18:30 – 3 minute exposures. But SoftCap isn’t aligning. Changed to 5 minutes, which is ridiculous with a 14” at f/2. And the alignment isn’t working. So frames were stacked without alignment and with the calm air the guiding is so good that alignment’s not needed.
Here’s what a single frame looks like, after flats have been added, with IP sine stretch at 1 to bring out vignetting.
- single frame stretched to 1 for vignetting eval.jpg (149.45 KiB) Viewed 737 times
Ignoring the auto stack, the Monkey Head was run thru IP, stacking 32 5-minute frames that had dark subtract at the time of exposure.
- NGC2174 stretched to 1 with flats.jpg (280.61 KiB) Viewed 737 times
- try2.jpg (293.8 KiB) Viewed 737 times
The moon is 52° from the Monkey Head. Haven’t had this particular problem previously and I think what’s going on is that the moon is shining into the east side of the dew shield / light shield.
Cropping, the final processed image:
- NGC2174 8X10.jpg (336.63 KiB) Viewed 737 times
32 X 5 minutes, gain at 120, -20°C cooling, 3 second PHD2 guiding
19:00 – 21:30 hrs, 17 Feb 2021
Analysis & Conclusions:
The 48mm mount has pretty much minimized vignetting. Additional work is needed to prove that moonlight interfered with tonight’s work.
This image is head and shoulders above the image thru the f/11 80mm refractor. It could have been stretched a bit further if the left side of the frame didn’t bloom. Using Roger M’s image as an achievement target, it’s almost there.
Image quality is the best yet, probably associated with the new 67.4mm backfocus setting. No color or other aberration on stars.
I’d somewhat given up on the Hyperstar, but if moonlight was the problem the Hyperstar is viable.
What next? Run a couple of more nights with tonight’s setup. Experiment with shifting to the alternative 70.5mm backfocus once this setup is understood. Design, machine and anodize a custom camera adapter mount with a 60mm optical path to possibly reduce what appears to be minor vignetting even further.
Overall, things are looking up and as far as the Hyperstar is concerned I'm optimistic at this time
hgp 18 Feb 2021
