The next step after last night’s brief work at focusing and running a test image is to capture appropriate flats. 30 X 2 sec and then 20 X 4 sec flats were taken in early twilight with multiple layers of t shirts. Perhaps significantly, the second and longer exposure flats were much darker in the corners and the vignetting extended much further into the frame.
A few throwaway frames were taken to compare file size of 8 bit vs. 16 bit images. Last night’ unsatisfactory test image was indeed taken at 8 bit.
Last night’s image of M42 very disappointing due to the 8 bit problem. My initial M42 taken back on Jan 2nd looked good but cropping was necessary to eliminate bad edges. Let’s take one more M42 as a comparison image before it disappears.
- M42 240X15s 0g -20c 1x annotated.jpg (1.45 MiB) Viewed 653 times
The plan was to that at a short exposure with gain set at 0 so that the trapezium wouldn’t be saturated. And so that having full well depth I’d be able to tease some further detail from the midrange. In the raw frames the central stars were discernable but my processing wasn’t up to the task of stretching the midrange without blowing out the bright sections. With 16 bit FITS the depth is there though.
IC434 and NGC 2024 were also previously imaged with poor results. Again, another comparison before they disappear.
- IC434 & NGC 2024 annotated.jpg (1.6 MiB) Viewed 653 times
Much better this time around
In at 21:36 hrs. Temperature’s down to 24F. Note that the wind has died from 20mph at the start of the session to around 10mph and the sky is stable at this time.
Conclusions and Lessons learned:
My flats were OK but not great. Still a work in progress, but minimum is 2 second exposure.
These 16 bit images have the well depth I was expecting. But my processing is weak and I wasn’t able to stretch M42 to best advantage.
Scott Tucker from Starizona got back to me pretty quickly. “Glad it is working. If the spacing is wrong you get spherical aberration which shows up as what we call "space bubbles," sort of halos around bright stars. Even though you can makes the stars small, they have these artifacts at best focus. That's what I would look for to see which spacing is better. But good to know both are at least working pretty well.” After close comparison tonight’s stars were tiny but even the dim ones exhibited mini halos. The 67.4mm spacing produced stars with no halos. Will change scope over from the current 70.5 mm spacing back to 67.4mm
