Hello and a little help (Integration time and cooling)

[bomberbaz]

Hello everyone and so nice to find another useful forum in addition the other I frequent. So I am not yet familiar with the forum or if this is post in the right place, the little help bit is more relevant to deep sky imaging so here goes.

I have run sensor analysis of my ZWO 183MC and then did the smart histogram thing outdoors in my heavily light polluted back garden and it came back with a unity of 115 (ZWO say it's 119 but I am not that bothered over such a small variation) but that optimum exposure time is 2.2 seconds.

Prior to this I had been doing 2 minute subs in the said garden. Oh and the other things I am unsure about is cooling.

Q1. Previously on the wizard I grabbed 5 hours of data at subs of 2 minutes (150), so do I have to gather 5 hours of data at 2.2 seconds (8,180 subs sounds crazy) to match this or will I need far less integration time of data as I am optimising my gathering. (hope this makes sense)

Q2. Big one here is how do I calculate my optimum cooling temp for my chip, can't seem to get a straight answer.

Thanks everyone for help, I am sure this or similar has been asked before but if your like me you enjoy repeating yourself

Steve

[Menno555]

Hi Steve and welcome

The smart histogram is only a suggestion. This suggestion depends on many things like focal length of your telescope, aperture, camera, sky quality, and so on.
For instance, a telescope with a short focal length (a fast telescope) will gather more light than a telescope with a long focal length (a slow telescope). This will reflect in the calculation done by the Smart Histogram. I have a slow telescope with a small FOV for example and the Smart Histogram can give me a suggestion of 4 minutes. But with a fast telescope indeed it can give very short exposure times.
Just experiment with it. If in the end you are happy with the results you get with 2 minutes, then stick to 2 minutes

The cooling of the camera is best set to -15 deg Celsius. On the Zwo product page is a Dark current vs Temperature scale and this is showing it useful till -15 degrees Celsius.
On the Zwo page it also says "Delta T: 40°C - 45°C below ambient". Delta T means difference. Simply put it is the outside temperature of that moment minus 40 to 45 degrees.
So if the temperature outside is 15 degrees for example, you could set the camera to 15 - 45 = -30. This of course is of no use because -15 degrees is the lowest that stil effective.
On a hot summer night is can be different but it must be over 30 degrees Celsius before you have to worry.
So in general it's save to say to set it on -15 degrees.

Hope this helps a bit.
Menno

[admin]

Hi Steve,

Menno is right to say that the value calculated is the *minimum* that you should use - using a longer exposure will not give you significantly better images, but it also won't make them worse (at least not until you take long enough exposures that you run into issues with guiding/tracking). 2s is extreme, but it reflects the combination of light pollution and probably quite a fast telescope. I personally wouldn't use 2s even if recommended, as it will give you too many images - 15 to 30s would be a nice compromise.

The real point is that lots of people take 2 minute exposures like you have, then start thinking that they will get better images by taking 5 minute exposures, so upgrade their mount or invest in guiding cameras, etc... In reality, a lot of that money is wasted, since in a situation like yours there will be no significant improvement in the final stacked image beyond the 2.2s subs recommended by SharpCap.

cheers,

Robin

[bomberbaz]

Great information from menno and robin there.

Happy to receive a facts based reply on cooling, I shall park that issue now as resolved.

Regarding exposure time, I had kinda settled on around 12-15s to be honest and given what you said robin, I shall settle on that too and mark resolved.

I just need to recheck my findings on a clear night as the first test was not actually that clear, also do it with a filter but so far so good.

Very impressed with results for what is for the end user, a relatively simple process.

cheers guys.

Steve

[bomberbaz]

Just thought I would update this thread.

Since this initial enquiry I have got back out under clear skies, the other night when I first did smart histogram it was overcast so the reading was obviously affected by light reflecting from the cloud.

Anyway a significant change from 2.2 unfiltered to 13.1 seconds & 71 seconds with the dual narrowband filter. It was actually about what I expected though from charts that Robin showed on a video I watched.

So I think I am settled for now on my garden settings at least. I just have to see what results are like.