Saturday, 19 Sep 2020
Objective: My image stars are blobs. And Roger M gets pinpoints. Looking to determine whether my problem is with the old 80mm Orion long tube (with a 903mm FL) telescope, the tracking, or both?
Took Roger up on his invitation and brought the new ASI2600MC camera over to his observatory. He wanted to assess this new color version of his monochrome ASI071M. And I was looking for comparative images from his high end sharply focused refractor.
Roger took images to compare against his highly processed NGC 7380 images, and while there were differences I actually preferred the nebula from the ASI2600MC. Rog was impressed as well, only if because there was minimal effort to process. Guess it’s one of those diminishing returns things.
For my test Rog centered on M57, using it as a landmark. And turned off the drive, exposing star trails for 5 and 10 seconds. His scope has a 907mm focal length and mine is 903mm. By comparing the thickness of his star trails against mine it should be apparent if the quality of my optics is significantly inferior.
Got home around 11. Tired, but the sky’s clear and conditions haven’t changed. Best opportunity to finish test. After sizing and aligning in PS here’s the result with our two star trails at exactly the same scale:
Roger’s 5 second star trail image:
My 8 second star trail:
Conclusion: When tracking/guiding is removed from the equation image, image sharpness between the scopes (line thickness) is comparable. Probably due to the $119 Orion’s long focal ration (f/11.3) (Later learned that this 17 year old discontinued scope got some pretty good reviews back then.)
So my mushy stars are associated with tracking/guiding/seeing.
It’s midnight now. Attempted to duplicate Roger’s imaging of NGC 7380, but the scope is jumping all over the place. To the point where the tracking star falls out of the imaging frame and tracking is gone. Tired brain didn’t solve the problem. Shut down at 01:00 hrs Sunday morning.
Monday, 21 Sep 2020
Installed a powered 16’ USB3 cable from the pier to the desk. The cable was a gift from Roger. Ran it tonight connecting the laptop and ASI2600 and there were zero drop-outs. Thank you Rog.
Downloaded ImagesPlus 6.5 image processing freeware as Roger suggested. Haven’t got it figured out so for purposes of analysis tonight’s frames are unprocessed images that would undoubtedly benefit from stretching.
Discovered that the scope was horribly out of balance after having substituted the SBIG for the little ASI 120. That had been causing Sunday night’s problems. Carefully rebalanced and balance is excellent on all axis.
For the record, tonight’s setup is imaging with the ASI2600 on the 80mm Orion long tube and guiding with the SBIG ST-8 relay connections. The long heavy dew shield is mounted as that’s how I normally balance, and it’s helpful with figuring out where the scope’s pointing in relation to the shutter.
NGC 7023 – The Iris Nebula is mag 6.8 with an8.6 mag central star. The 1st 2 minute frame was discarded. Bad start. Tuned off the FWHM filter and all is well. Stacked 12 X 5 minutes with tack sharp stars and no discarded frames!!! I’d mistakenly left the scope binned 2X2. As for drop outs, perhaps the longer exposures result in more alignment stars? Tracking is phenomenal. +/- 0.7 Mostly +/- 0.15
What’s going on?
I’d just proved that the scope is good. And now the guiding, for some reason, is much improved over that from the past couple of weeks.
What has changed is that the air is still and I’m guiding at 1 second rather than at 2 seconds. The effective pixel size is much larger than normal. But the tracking numbers suggest the binning had nothing to do with achieving pinpoint stars.
NGC 7380 (The Wizard Nebula) (That Roger’s been using as an image standarad.) Unbinned this time.
Auto aligned & stacked the first 3 of the 5 minute exposures. Only 4 of the 9 frames taken aligned. The Y axis alignment is off and it has drifted +4.04 in 45 minutes. Perhaps the Y drift is a problem but it wasn’t with 7023. Perhaps the problem is with this image set I’d changed from a binned 3128 X 3128 (effective 1564 /x 1564) pixel image size to an unbinned 6248 X 4174 pixel image size? Ahh well, stars in all 9 frames are tack sharp, even if they didn't stack. And that’s half the battle.
NGC 6960 (the Veil Nebula) Tracking jumping +/- 1.5. 2nd image didn’t align/stack.
Calling it a night at 00:08 hrs. Air temp’s down to 44F. Seeing’s still good though.
Conclusions & lessons learned:
Achieved high precision tracking for most of the night. What changed?
Unstacked images may have been the result of using the FWHM filter. No idea of this filter function. But apparently its causing auto align problems.
Revelation! Since I’m not imaging thru the 14” don’t really need the huge heavy dew shield way out on the nose of the scope. Removing the huge dew shield will be like removing a sail that’s buffeted by the slightest breeze. And it will result in a shorter coupling making it much easier for the scope to respond to minute drive corrections.
Is there a correlation between image size and alignment/auto-stack problems?
hgp 9/23/20
