How do people get long exposures on QHY183C cameras? - Forums

Topic starter

May 4, 2021

Hi Everyone!
I've been recently browsing astrobin and specifically searching for equipment that I own and see what I could possibly image next.
But I'm running into a problem.
The QHY183C camera + Williams Optics with F5.7 at 360mm, I can't do what others do.
Specificially they able to image a galaxy or a nebula for like 240 seconds per sub?
I'm at Bortle 5 and my images are over excposed at 100 seconds?
This camera has a well depth of 15k, and 2.4 um size sensors.
I figured they must be using a light pollution filter and I am not but I don't see that along the listed equipment.
Also on the specs, it lists that it has a built in IR cut glass front of the sensor, but people are putting another IR/CUT lens in their imaging train?
I tried Sharpcap Analysis and the longest exposure time (based on my sky darkness) was like 45 seconds?
I also asked others on other forums and they all recommended me to use short exposures with this camera, because it can't handle a lot of time and I'm gonna end up with bloated stars and over saturated subs.
So I am stumped and I have no clue how some people are able to take 240 or even 300 seconds with this camera.
Could someone please explain to me what's going on?

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Cedric Raguenaud

May 4, 2021 · Edited: May 4, 2021

Hi there,

I can't answer all your questions, but I can answer about the long exposures: I have a QHY183C with a Newton (300/1500) and I do 600 second exposures nearly exclusively unless I do photometry or comets.

In fact I was a very early user of that camera and they happen to have kind of lied a bit in its description: they said when I bought it (I haven't checked again recently) that it could do 30 minute exposures. But in reality the USB window limits it to 20 minutes. Testing it, I also noticed that the noise isn't linear and beyond 15 minutes becomes a bit iffy (could just be my camera).

For information I image from a Bortle 2-3 site, so I don't have light pollution issues.

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dkamen

May 4, 2021

Hi,

You do not mention the gain you are using. Same camera and sky yield very different results depending on gain.

Also what do you mean overexposed? It's one thing if the image has a relatively right-leaning histogram with perhaps a few stars clipped and quite another if it is pure white.

One thing you can try is taking a 200 second dark. If that too is overexposed you are looking at either a light leakage close to the sensor or perhaps a thermal/electronic problem.

But it could just be that your sky is more light polluted. In that case you should use short exposures (regardless of camera FW).

Cheers,

Dimitris

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Björn Arnold

May 4, 2021

Hi,

I don’t own this camera but have a model with the same sensor. I assume that when you say overexposed, you mean the whole image has a very high gray level.

Certainly for 240 seconds you‘re likely to saturate brighter stars but not the whole image. My guess is that your gain and/offset settings are way too high.

For your settings, even without light pollution, you should be around unity gain or below.

Cheers,
Björn

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Ferenc Szabo

Topic starter

May 4, 2021

I use Gain 11 (it's gain 110 for ZWO with the same sensor) and my offset is 30. I can go as far as 120 seconds. The image I had stacked which used those settings was lacking dynamic range (Rosette Nebula) it was very posterised looking. Then people I asked, they suggested, with this camera I should use max 60 seconds top for single subs with my W.O. 61 refractor.
I would be even better off using gain 0 and try to do 120 or even 180 seconds?

Björn Arnold

May 4, 2021

Assuming that gain 11 is unity gain for this QHY camera and that the offset is at the lower end, you should be able to expose for 300 seconds and don't saturate your image.
Just for testing: capture a 600 second dark at gain 0 and offset 0. This should be a dark image. Otherwise I would assume that another camera setting somehow got screwed.
Regarding the posterization: verify that you capture at 12bit depth and not by accident at 8bit.

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HR_Maurer

IOTD Staff

May 4, 2021

Hi,
this camera is a 12 bit camera, so getting saturated stars is rather easy.
I had it, too. Gain 11 is close to unity gain, i used it for narrowband, if i'm right, and the FWC is already reduced. My suggestion is to lower your gain, to get more dynamic range. I think for RGB it's rather high.
Or, just go for the shorter subs, and apply something like a HDR technique.

CS Horst

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