Info long exposure

Discussion of using SharpCap for Deep Sky Imaging
nettid
Posts: 16
Joined: Fri May 19, 2017 12:53 pm

Info long exposure

Post by nettid »

Dear Guys,
For my basic steps in DSO capture I use this guide:
viewtopic.php?t=2552
When i set my gain, exposure and offset I do so that
in the image statistic fits liberator the min value is max of 0
and the max value is less then 65.535
My question is this:
I see that many user with my cam (AA 294 C) and my OTA (200/800 newton) with the same gain
take picitures with more long exposure than me... (240 sec/300 sec)
does that mean they deliberately overexpose?
User avatar
turfpit
Posts: 1393
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Info long exposure

Post by turfpit »

Thanks for using the guide.
in the image statistic fits liberator the min value is max of 0
If you mean your Min value is 0 then you will lose faint data. The Min value neds to be greater than 0 - this can be achieved by increasing the offset/brightness/black_level (QHY/ZWO/Altair) and/or increasing exposure.
take picitures with more long exposure than me... (240 sec/300 sec)
does that mean they deliberately overexpose?
Other factors can come into play such as light pollution and Bortle scale. Also using filters such as IDAS and dual band will mean that exposure will need to be increased. Brightness of the object being imaged will also affect the exposure required.

Aim for a (log) histogram which looks like this
deep-sky-histogram.JPG
deep-sky-histogram.JPG (12.95 KiB) Viewed 119 times


There are no hard and fast figures in this hobby - only guidelines.

Dave
nettid
Posts: 16
Joined: Fri May 19, 2017 12:53 pm

Re: Info long exposure

Post by nettid »

Thanks a lot, Turfpit,
Sorry for my bad English....
Your guide was been very important for me!!!!
I attached the screen of my single light
captured with my newton 200/800, and L-pro filter at 120 sec.
Whit this image statistic, doesn't it make sense to increase exposure?
I ask this, because i seen many pictures with my strumentations but with a very long exposure...
Image
User avatar
turfpit
Posts: 1393
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Info long exposure

Post by turfpit »

Your Min value is 2100 - this is OK because it means the histogram is not hitting the left hand side. The fact the gap cannot be seen is down to trying to display the horizontal scale from 0 .. 65535 on a monitor.

The LPro filter will have an effect on the light allowed through to the camera sensor. The easiest way is to take some single frames at various increasing exposures and see what happens to the histogram. You will hit other limits such as mount tracking capability and sky brightness. What is your Bortle reading? When you look at other images (eg on astrobin) note what Bortle scale the imager has at their site.

Another approach would be to use SharpCap's Smart Histogram.

I am glad you found the document helpful.

Dave
nettid
Posts: 16
Joined: Fri May 19, 2017 12:53 pm

Re: Info long exposure

Post by nettid »

Your document make me the first steps in DSO imager
and I am grateful to you!!!
I can try to increase the exposure but my max value must not exceed 65.535.
Is it correct?
User avatar
turfpit
Posts: 1393
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Info long exposure

Post by turfpit »

You cannot exceed 65535 as this is the largest number that can be stored in 16-bits. What does happen is that when the exposure is large enough the histogram hits the right hand side and pixels become saturated (white). Increasing the exposure more will saturate (burn out) more pixels.

These were tests I carried out against IC434 using an Ha filter which is an extreme example of blocking light. Here viewtopic.php?f=9&t=3867 is the result.

capture-histograms.JPG
capture-histograms.JPG (50.01 KiB) Viewed 101 times
The exposures were 2m, 5m, 10m, 15m, 20m, 25m. It was only at 15m that the histogram moved away from the left hand side. As the exposure increases, the histogram gets fatter (= more data). Notice also that the histogram starts to move towards the right hand side as exposure is increased. From the histograms, I decided that for that particular object and filter, at least 15m exposure was needed.

In this example, the histogram has a large gap at the left hand side and is hitting the right hand side. This suggests that exposure needs to be decreased.
too-much-exposure.JPG
too-much-exposure.JPG (12.75 KiB) Viewed 101 times

A useful thing for you to try would be to select a bright object (a globular cluster would be a good example). Capture single frames at various exposures with and without the L-Pro filter. Repeat the exercise for a dim object. That will help you better understand the interaction between exposure, the filter and your skies.

Dave
nettid
Posts: 16
Joined: Fri May 19, 2017 12:53 pm

Re: Info long exposure

Post by nettid »

turfpit wrote: Wed Apr 07, 2021 7:35 pm What is your Bortle reading?
My Bortle reading is 6
User avatar
turfpit
Posts: 1393
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Info long exposure

Post by turfpit »

Same as me. I am able to image from a Bortle 4 sight - there is a big difference. At Bortle 6 I never see the Milky Way, at Bortle 4 it is clearly visible. At the Bortle 4 site I get much more detail for the same expsoure.

I don't use any kind of light pollution filter. I remove light gradients using the tool provided in Astro Pixel Processor.

Dave
nettid
Posts: 16
Joined: Fri May 19, 2017 12:53 pm

Re: Info long exposure

Post by nettid »

@turfpit
yesterday I tried with 180 sec and gain 900 (HGC)
What do you think about it?
Image
User avatar
turfpit
Posts: 1393
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Info long exposure

Post by turfpit »

Looks decent to me and you are certainly moving in the right direction. What really matters is what you think. How many exposures to capture that?
M106 is not the easiest of objects to acquire detail. We are in the 'season of galaxies' at the moment and the targets are small for your optics. There will be some larger objects along soon once the Summer Triangle has risen properly.

This is the annotated version - processed at astrometry.net - looks like you captured a few more fainter objects.

annotated.JPG
annotated.JPG (32.3 KiB) Viewed 54 times

Dave
Post Reply