Pre-Meridian Flip subs came out with crazy high ADU's. No idea why!

[Xiga]

Hi guys

So i haven't been able to do any imaging at all in recent times (and in all likelihood that won't change anytime soon), but as luck would have it, i did manage to get out last weekend, so i tried my luck on NGC 1333. 

I captured 120 x 2min subs (4 hrs), and 70 of the subs were pre-Meridian flip, with the last 50 being post-flip. I used a Qhy163c camera and SGP for capture. 

I shot everything at Gain 80, Offset 35, and both SGP and APP both show a Gain of 80 in the fits header of both sets of exposure (they don't show the Offset, but i know it was set to 35, as i have presets saved, so i don't manually input anything). At the very end of the session, i shot my Flats and Dark Flats. 

But when i later looked at the subs, something clearly wasn't right with the pre-flip subs. SGP reports a Mean ADU level of about 9,600 which is insanely high. I was aiming for an ADU of about 1,400, and curiously, the post-flip subs came out very close to this. And sure enough, when i tried calibrating the subs, the post-flip subs calibrate fine (at least i think they do, this is actually my 1st light for the camera proper), but the pre-flip subs do not calibrate at all. The problem is definitely not with the calibration subs, it is 100% down to the pre-flip Lights. 

The thing is, i never changed any settings pre and post flip. I set my alarm for 10 mins before the flip, as i like to make sure no cables get caught, but i awoke to see that the sequence had aborted, due to passing clouds. So i performed the flip, and continued with the sequence as normal, but i didn't change any camera setting at all. 

I've attached screen grabs below of the histogram in SGP, for both a pre-flip and post-flip sub. You can clearly see the issue, the pre-flip sub levels are just way too high. I've also attached links below to a raw pre and post flip light, as well as the Master Flat, Master Dark, and BPM that i was using. 

As it stands, the 50 post-flip subs are nowhere near enough to even render an image worthy of posting. If the pre-flip subs could be salvaged in any way, i might stand a chance of making a passable image, but at present i can't see any way of making them useable (these cooled CMOS cameras really need Darks badly). 

Have to say, i was pretty miffed that this happened. Unless i get to the bottom of what happened, i'm always going to worry that some subs could randomly come out like this and the night will be wasted. 

TIA for any help anyone can offer!

Pre-Flip:

Post-Flip:

 

Pre-Flip Light:  https://1drv.ms/u/s!AhhWC3D3zU7BpUdI2EAf9HEbbdIs?e=ZCYMa3

Post-Flip Light:  https://1drv.ms/u/s!AhhWC3D3zU7BpUjcA1iG5vKY6RrM?e=dqHkgx

Master Dark:  https://1drv.ms/u/s!AhhWC3D3zU7BpUqIjyKs2qaO40Gy?e=mEYaBr

Master Flat:  https://1drv.ms/u/s!AhhWC3D3zU7BpUqIjyKs2qaO40Gy?e=mEYaBr

BPM:  https://1drv.ms/u/s!AhhWC3D3zU7BpUupNcSdWttMhYnf?e=LiR2g8

 

 

[Xiga]

I should also point out, i've noticed that the Master Flat looks odd, in that it has banding. I haven't been too worried about this, as it seems to calibrate out ok, but thought it was worth mentioning all the same.

Note, i created the Master Flat from 50 Flats of 1s duration. I'm always wary of shooting flats that are too short, so i usually alter the brightness of my ipad so that i can shoot 1s flats. This has always worked perfectly for me with my D5300, but i wonder if the banding is due to an electronic shutter rather than a mechanical one, in which case, do i need to shoot much longer flats? In any case, they still seem to calibrate fine on the post-flip subs, so i don't think it's the cause of the issues i'm having. 

FWIW, here's a couple of jpg's of what the pre-flip and post-flip subs look like after calibration:

Pre-flip:

Post-flip:

[Miguel1983]

High clouds that dissipated later on ?

If every setting remains the same, you would think it's an external factor ?

[Whirlwind]

You should be able to test this simply indoors by just taking exposures of the same length/temp and see what the darks look like and then undertake a meridian flip.  That should at least determine whether it is something electronic before and after.  

I would agree though that it is probably some form of external factor that you are looking at.  Were there any lights in the vicinity that might be impacted that might get into the telescope at certain angles?

[Xiga]

16 hours ago, Miguel1983 said:

High clouds that dissipated later on ?

If every setting remains the same, you would think it's an external factor ?

I too thought it must have been clouds, as there were definitely intermittent clouds about, and enough to lose the guide star shortly before the flip. But most of the time it was clear skies and no moon. I've also checked a good sample of the subs, from the beginning, middle, and end and every one shows the same weirdly boosted signal, so it's not down to clouds. 

[Xiga]

14 hours ago, Whirlwind said:

You should be able to test this simply indoors by just taking exposures of the same length/temp and see what the darks look like and then undertake a meridian flip.  That should at least determine whether it is something electronic before and after.  

I would agree though that it is probably some form of external factor that you are looking at.  Were there any lights in the vicinity that might be impacted that might get into the telescope at certain angles?

I can't actually check this atm as I keep my gear at the old family home where I do all my imaging. But I don't think the meridian flip had anything to do with it tbh. I suspect that the camera was just misbehaving in some weird way, and for some reason, when the sequence aborted and then got restarted, it suddenly started working correctly. It just so happens that this was punctuated with a meridian flip. 

I don't think it was due to an external light either, although that's a good shout. The scope was setup in the normal place, in the driveway pointing South. No passing cars or lights could interfere. Just the usual streetlights, which are  all shielded from direct view by roofs, and I use an IDAS-D1 anyways and it's always worked very well for me.

[Xiga]

I have a spreadsheet where I calculated the optimum Median ADU level at each Gain and Offset preset that i have saved on the camera. That's how i knew to aim for an ADU level of 1400ish, and after the post-flip subs (which came out ok), I now know that 2 min subs are pretty much perfect for my skies. Now, what's even weirder,  is that, even at the Max Gain preset I have saved (300), with 2 min subs I should only be seeing a Median ADU of ~6k, still way short of the ~10k I was getting.

I wonder did the Offset get jacked somehow on it's own? At Gain 80, by my calculations it would take an Offset in the region of 300ish to get the level of ADU I was getting. I can't do anything about the Flats, but I might try shooting a few Darks with Gain 80 Offset 300 and see how they calibrate.

This was my 1st light with a cooled Cmos. Are they known to be buggy from time to time?

Edited November 3, 2019 by Xiga