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Old 03-01-2020, 04:28 PM
Xeteth (David)
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Ideal Sub Length & Gain/Offset

I'm having trouble trying to determine the ideal sub length to use with my QHY8-Pro. Previously with the DSLR it was fairly easy to determine - I'd simply expose until the mountain on the histogram was about a third of the way along, making sure there was no clipping.

But now that I've moved to a 16 bit camera, things are vastly different. Often I can barely see the mountain in the histogram in Astrophotography Tool, although I have analysed in PI to make sure there is no clipping in my subs. I'm currently shooting 5 min subs, but really don't have enough experience in AP to know whether I'm at optimal sub length or not.

Furthermore, I've had various issues attempting to setup the camera gain/offset, but after using various guides I've landed on 10 gain and 125 offset... would this sound about right?

If anyone could offer any advice into determining sub length and/or gain/offset values that would be much appreciated. Many of the guides I have found online are either poorly documented (especially when it comes to QHY8-Pro specific guides) or are seemingly out-of-date. I shoot primarily from Melbourne bortle 8 skies which isn't optimal, but I am using LPS-P2 filter which has helped a great deal in cutting through the LP.
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Old 03-01-2020, 06:11 PM
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Nikolas (Nik)
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125 gain and 10 offset not the way you have it.
It's all dependant on the target, something like Orion only needs short subs, anything dimmer longer subs and plenty of them. Shoot darks same temp and exposure as the lights.
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Old 04-01-2020, 12:14 AM
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luka
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To find the right value of offset:
1. Take bias and get the average value (use Statistics in PI). It should be about 500 (anything between 100 and 1000 is ok).
2. If it is not then adjust the offset and repeat.

Once you have worked out the offset:
3. Turn camera to a light source so that the sensor is well lit and take an exposure that is a few seconds long. You want to saturate the sensor.
4. Use PI to measure the maximum value. It should be less but close to 65535.

5. If you changed the gain a lot in steps 3&4 maybe go back to step 1 and check the offset again and possibly repeat the whole procedure if needed.

6. Done.

Make sure you write down these values and never change them (unless you decide to repeat the procedure).

Although the sensor temperature does not affect the gain/offset much I like doing the measurements at the temperature that you will mainly be using your sensor.

Obligatory reading.



Regarding the exposure time:
1. The maximum exposure time will depend on the light pollution for your location. See this, this and this.
2. For dim targets use your maximum exposure time.
3. For clusters use much shorter time or the stars will be saturated. Often even 30s is too long.
4. For very bright targets (Orion for example) you may want to combine short subs (so you don't overexpose the bright sections) and long subs (to get the faint details).
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Old 04-01-2020, 04:19 PM
Xeteth (David)
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Thanks for the information Nik & luca. I managed to get out last night and take some shots, but really having trouble with the processing. I'm somewhat at a loss at this point, these issues have been plaguing me since getting the camera. I know I'm close, I can see the data there but there's something I haven't quite got right yet. If anyone can provide insight or suggestions that would be great.

Camera settings -
Gain 10
Offset 125
All images taken at -15C

Integration - (have uploaded an image of the masters... are these the issue?)
Using a Master Dark (40 Subs for the Master)
Using a Super Bias (60 Subs for the Master)
Using Master Flats (30 Subs for the Master)

First image I've attached is integration of 48x2 min subs with flats at 7k ADU (using APT's CCD Flat Aid tool) and the result after doing a simple ABE. Second image I've attached is integration of 48x2 min subs with flats at 25k ADU again with a result after doing a simple ABE.

There are a number of issues I can see, but can't fully diagnose -

1. There's image tilt evident by the halo around Alnitak, I've tried fixing this but everything is screwed/clasped down as tight as it will go...

2. Vignetting is evident, especially in the 25k ADU flat image. It almost seems like the flat frame has 'added' to the issue rather than fixing it...

3. Doing a simple ABE (as seen on the right of each image) leaves a horrendous mess, I have absolutely no clue as to why this happens.

Could issues 2 & 3 be caused by improper gain/offset settings in the camera? Any other ideas/suggestions?
Attached Thumbnails
Click for full-size image (7k ADU Flat.jpg)
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Click for full-size image (25k ADU Flat.jpg)
119.2 KB18 views
Click for full-size image (Dark Bias Masters.jpg)
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Old 04-01-2020, 05:11 PM
Xeteth (David)
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The same weird results can be seen on the integration of the subs I got of Eta Carina last night as well.
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Click for full-size image (EtaCarina.jpg)
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