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Old 29-04-2015, 09:41 AM
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exposure length qhy10

exposure length qhy10
hello guys i have been playing around with my new qhy10 and it seems that no matter how long the exposure the histogram on the left of the ezcap screen does not seem to move to the right very much?
how can i tell when the sensor is saturated?
pat
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Old 29-04-2015, 10:41 PM
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Hi Pat,

Did you take your telescope's cover off ? Just kidding

How long are your exposures? Are you aiming at a dim DSO?

It is unlikely you that you will saturate the entire sensor with 'normal' imaging, maybe except for the stars. Would be helpful if you attached a print-screen of your histogram.
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Old 30-04-2015, 09:57 AM
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hi slawomir and thanks for a reply
usually i cannot get more than a couple of minutes from my dslr at iso 800 or 1000 before sturation on histogram, however just testing my ccd the histo doesnt seem to move hardly at all to the right even after 5 or 6 minutes!
i have been reading a bit about the offset and gain but i do not know wether or not it will make any difference
pat
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Old 30-04-2015, 10:52 AM
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Quote:
Originally Posted by blink138 View Post
hi slawomir and thanks for a reply
usually i cannot get more than a couple of minutes from my dslr at iso 800 or 1000 before sturation on histogram, however just testing my ccd the histo doesnt seem to move hardly at all to the right even after 5 or 6 minutes!
i have been reading a bit about the offset and gain but i do not know wether or not it will make any difference
pat
Hi Pat,

5-6 minutes is a relatively short exposure for an astro CCD-based camera, unless you aim it at bright objects such as the Orion Nebula or Carina Nebula.

I am it sure whether you are aware of it or not, but astro images need careful stretching to bring up faint detail while at the same time keeping stars from reaching saturation point as much as possible. NASA's FITS Liberator (free software) is quite good at automatic stretching of your images. I prefer using PixInsight for entire processing of my images.

So usually, a histogram of an image straight from the camera will look underexposed on a computer screen and predominantly populated on the left side (majority of pixels having low ADU values).

EDIT: This might help- you can go straight to page 13: https://www.spacetelescope.org/stati.../userguide.pdf
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Old 30-04-2015, 05:30 PM
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hi slawomir, is 5 or 6 minutes short exposures even from 4km out from the city do you think?
that is a good link thanks mate, i will try a much longer exposure next time and see if it changes
pat
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Old 04-05-2015, 09:54 AM
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I have Qhy10 on an f4 8" newt. I run 10min sub's for most items, except things like m42, m8, eta where I run 5min sub's.

I run gain and offset at 0,130. If I push the gain higher, I lose color on dimmer objects once stretched.
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Old 04-05-2015, 10:08 AM
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thanks trevor i must obviously have to increase the exposure time, btw what does your hgisto look like after 10 mins exposure?
pat
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Old 04-05-2015, 12:25 PM
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thanks trevor i must obviously have to increase the exposure time, btw what does your hgisto look like after 10 mins exposure?
pat
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Old 05-05-2015, 12:42 PM
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Pat,

I run 10 minute LRGB exposures with my ST-2000XM (different completely to the QHY10) in town lighting. I find that sometimes isn't enough. H-a is a minimum 20 minute exposure.

When doing M42, I do 10 minute LRGB, followed by a 3 minute run just for the Trapezium, then combine the 2 sets.
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Old 05-05-2015, 06:41 PM
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thanks for chipping in lewis, they must be too short even in the city!
pat
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Old 05-05-2015, 07:26 PM
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Hi Pat,

Just checked the specs for your camera; full well 45k and quantum efficiency around 50-60%, it means you could go for longer exposures. Experimenting with various exposures on the same target will probably be of greatest value.

Here are a few examples of images taken with the same camera:

http://www.astrobin.com/157902/

http://www.astrobin.com/41687/
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Old 05-05-2015, 08:03 PM
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ok this has taken me completely unawares!
i am so used to only getting a maximum of three minute exposures with a dslr and it seems counter intuitive to think that i am going to need three times the exposure on a dedicated ccd
more testing for me as soon as the weather clears, thanks people at least i know now
any other tips?
pat
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Old 05-05-2015, 08:33 PM
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Hi Pat,

I like to think that dedicated CCD does not need longer exposures than a dslr, but rather allows for them. Astro CCD cameras usually/always? have a greater dynamic range than a dslr, so you can expose for longer and will possibly get much more detail in your photos.
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Old 05-05-2015, 10:10 PM
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slawomir i do understand the better dynamic range actually, it just seems to take a lot longer to collect the same information than a dslr, so, actually seems less sensitive?
pat
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Old 06-05-2015, 06:27 AM
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Quote:
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slawomir i do understand the better dynamic range actually, it just seems to take a lot longer to collect the same information than a dslr, so, actually seems less sensitive?
pat
I doubt it, but you would need to compare pixel sizes in your cameras as well as their quantum efficiencies.The way I understand it, and please correct me if I am wrong, with a dslr you can adjust gain by setting different ISO - it does not physically improve sensitivity of the chip, but amplifies (multiplies) already collected signal. So the image from a dslr appears brighter;DSLRs are designed to produce a ready for viewing image. However, the higher the ISO the lower the dynamic range. I think you can also adjust gain in your astro camera, but there would be an optimal gain value when you can take advantage of full dynamic range of your CCD.

EDIT: Have you tried using NASA's FITS Liberator on your images?

Last edited by Slawomir; 06-05-2015 at 06:38 AM.
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Old 06-05-2015, 07:52 AM
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I think you have hit the nail on the head here Slawomir. Pat, see how your dslr camera responds and the exposure you need if you run it at 50 or 100 iso.

I can push the gain on my qhy10, and it works OK on bright objects, but when I use it on dim stuff I lose a lot of definition and colour.
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Old 06-05-2015, 10:03 AM
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thanks a lot guys i think i can see a lot more clearly now
it is better for me having some dslr analogies as that is all i have ever used
i will see what the nasa liberator is all about
pat
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Old 14-05-2015, 11:51 PM
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When comparing to the DSLR are you talking to the histogram in the DSLR on the screen? If so remember that your DSLR will have a gamma curve applied to the image straight out of the camera.

The picture displayed in EZCAP is linear. Have a look at the picture of NGC5139 I have attached. This is a straight capture using EZCAP. It looks dark, but the statistics show that the stars are very VERY close to full saturation.

If you want to check what your saturation is in the image in EZCAP go to:
Menu->Image process->Noise Analyze

When you next take an image it will put 9 boxes on the image and give you statistics for RMS, Min, Avg, and Max values in those boxes as counts of 16bit (i.e. 65535 is the max) That will tell you if you've saturated the sensor with a star.
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Old 15-05-2015, 10:00 AM
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thanks for chipping in chris, the saturation point of the sensor is exactly what i am trying to ask
can you post the 5139 here with offset gain and exposure please?
pat
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Old 15-05-2015, 12:01 PM
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thanks for chipping in chris, the saturation point of the sensor is exactly what i am trying to ask
can you post the 5139 here with offset gain and exposure please?
pat
I'll post you an example frame with a 300second exposure when I get home.

Offset and gain however are sensor specific and it's up to you to calibrate your camera to suit. I have seen some wildly different settings all working well, and likewise when someone copied my settings at astrofest last year the rest was bright green bleeding all over the frame.

Instructions for setting the offset and bias are in the QHY10 manual http://qhyccd.com/file/repository/PD...r%20manual.pdf and relies on the Noise Analysis function of EZCAP. It involves taking a dark frame with lens cap on and a bright frame so you can do it during the day.
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