Frost

[dx_ron]

I have 8 total hours of subs on NGC 2903 with a 6" RC. One night's worth - 3 hours of the 8 - I am pretty sure I had frost on the secondary mirror, because no set of flats produce what looks like a properly calibrated image. I have been playing around with various ways to take flats with this scope so I have flats from the prior night, that night and the next night and none of them calibrate properly (and, yes, there was frost on the secondary spiders in the morning) (and yes, I now have an ugly makeshift foam dew shield).

I'm still new enough at all this, and we get few enough clear nights, that the thought of throwing away 7 hours of data (these 3 plus the 4 hours on another target that night) is quite annoying.

What would you do? Accept that the night was lost to frost, or stack them with the rest and hope Wipe can sort it all out?
Here is just the first hour of the frosty galaxy (was hoping things didn't get frosty until later).

[admin]

As long as you make sure you get rid of any and all stacking artefacts at the borders, this appears to be a dataset where you can go quite aggressive with Wipe's "Aggressiveness" settings. That's because a small-ish object on an otherwise "uninteresting" background greatly helps Wipe separate large-scale gradient from small-scale detail.

It's no replacement for proper flats, but you should be able to get quite good results without materially affecting the galaxy. Let us know how you go!

[dx_ron]

Thanks, Ivo. I obviously should have shot a special set of flats with the frost in place

I have learned that "RCs don't dew up" is a myth.

[dx_ron]

Here's what I got as a first go. Looks like cranked-up aggressive wipe is doing its thing on the part of the stack with frost. Another 8 hours would help bring out the outer arms better, but I suppose that's why some people refuse to do broadband imaging from Bortle 7.

A bit of walking noise, too, apparently. A bit disappointing, because I have dithering set at 10 pixels, and that is the max setting Ekos allows. There are 90 different dither positions in the data set.

NGC_2903_02_20-26-27_2022_2nd-pass_ST.jpg (499.99 KiB) Viewed 3761 times

Guess I should add the usual info, what with posting the image.
iOptron RC6 reduced to ~880mm with the AP CCDT67
QHY183C (original image scale ~0.58"/px)
OAG + QHY5III462C guiding
CEM40
480x60s

[admin]

That background looks nice and flat!
All-in-all a respectable image. I have to admit, I cannot spot any walking noise (StarTools' walking noise reduction doing its thing?).

[dx_ron]

I'll take "all in all a respectable image", and be happy.
I did use a bit of walking noise reduction. I still see some patterning at a 45° angle in the image, but maybe that's just over-active human pattern recognition.

[admin]

I'll take "all in all a respectable image", and be happy.
I did use a bit of walking noise reduction. I still see some patterning at a 45° angle in the image, but maybe that's just over-active human pattern recognition.

Ha! Sure enough, now that you actually told me angle I can see it (and I looked for it on three different monitors before!). the WN reduction is disguising it quite well.