Old 27-04-2008, 08:23 AM
Alchemy (Clive)
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How do you set the offset for CCD cameras

i have been looking at a new camera purchase QHY8 and am trying to sort out all the "issues" prior to purchase.

My question is this as per thread title , how do you set the offset..... i have done some reading and gathered the following.

Quote from WIN4AIP.

Although it is tempting to expect the signal to noise ratio to be (formula) that simply is not true. the reason is that although you have calculated the mean value of x in ADU (analog to digital ratio- amount of signal which includes noise) you dont actually know whether zero ADU means zero photons . in many types of measurement, and in all CD cameras , the zero point in ADUs is OFFSET from the true signal level. This zero offset is called the signal bias , or just plain bias. In other words when zero photons fall on the detector the camera reports a bias signal usually a small positive value such as 100

So do you simply shoot with the cap on and try different values til you get a clean frame , will it vary with exposure time, is it time dependant, or is this just being too simplistic?

Having only dealt with digital cameras till now its been point and shoot, the technical difficulty has ben ramped up anotch (not ready for the next level mono just yet)

im sure someone with CCD experience can make it simple to understand , and give a practical explanation of how to do it.
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Old 27-04-2008, 10:41 AM
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[1ponders] (Paul)
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My understanding is that you can't change the bias on most consumer grade CCD cameras, it is set in the factory.

To remove the bias all you need to do is subtract your dark frame, the bias will be included in that subtraction. If you want to scale a dark frame or remove the bias from a flat frame then you will need to take a dark image of zero time length (or as close as you can get). Your software with the camera should allow you to do a zero time exposure. You can then subtract the master bias (remember to take a few of them and median combine them ) from whichever frame you want to.

For general purposes though subtracting the dark from your light and subtracting a flat dark (if your flats are more than a couple of seconds long) from your flats prior to dividing them into the lights, will deal with any bias in your images.
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Old 27-04-2008, 04:11 PM
DeanoNZL (Adrian)
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Hi Guys,
Nebulosity, from Stark-Labs, has an auto-offset calculation feature.
Even in trial mode, it would give an idea of what an appropriate setting would be for the camera mentioned. It may help
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Old 27-04-2008, 07:41 PM
Alchemy (Clive)
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thanks for your input guys.

firstly deano ... i checked out seeker372011, s post and quote
gain was 20% and offset was 130-the auto offset didnt work in nebulosity for some reason..the shadows were clipped so next time I will need to fix those setting
thanks for your suggestion its possible he hadnt used it in the appropriate manner perhaps.

1ponders .... im familiar with the usual process of flats, darks, bias, flat/darks. and know how i would deal with these for a point and shoot camera. Again thanks for your ideas.

The problem that occurs to me is this
1. the qhy8 has an adjustment "OFFSET" , as best as i can guess/deduce, this is to do with the bias signal , as you suggest preset in the factory. the settingof the OFFSET counteracts this (so the WIN4AIP suggests). i guess is this deduction correct?
2. If i am correct then shooting bias frames with varying offset values will produce frames with varying amounts of bias noise, the frame that has the LEAST noise is the correct OFFSET value..... this requires some extreme stretching to detect.
3.As there is an OFFSET setting it HAS to be set at something.... is my deduction correct or even logical. I have actually PM ed a number of IIS QHY8 owners and so far no-one has a real idea of what its settings are , other than to google someone elses images and copy the settings.
4. If this fails then trial and error shooting of some exposures of normal length at an object of wide brightness scales with varying OFFSET values and comparing the results.

there has to be a logical and simple answer to this , i can tell you NO_oNe using the camera i know of has a clue how to set it. It should be in the camera documentaton , much as collimation comes with a newt telescope.

Again thanks for taking the time to answer and sugestions.

maybee JASE, SJASTRO, GAMMA, STROGMANMIKE, BASSNUT, or PETERWARD or anyone who has used this can advise.
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Old 27-04-2008, 08:17 PM
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seeker372011 (Narayan)

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Craig Stark -who wrote nebulosity-suggests that offsets ar camra dependent and vary from camera to camera.

This is why h has created an auto fuction in Nebulosity

basically you covr your camera, hav TEc running for a while and away you go

Problem is when i tried it it didnt't seem to have worked-I got an error message to that effect..but somehow a mysterious number appeared in the offset field

since I had nothing better to go by I used this number-the one time I used my QHY so far. My sense is it it is too high

but I am going to ask at the stark labs yahoo forum and someone should be able to help

will lt you know what I find out

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Old 27-04-2008, 08:29 PM
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This subject has been covered on the QH8 site I would have attached a pdf writen by on of the users on how to work out the offset for your camera and gain setting, but the PDF is too big. The procedure is easy to follow..
For those using Maxdl
1 Get your camera to working temp.
2 Set your gain say 50 and offset 125
3 Keep yout telescope covered.
4 Take a Bias frame.
5 Have a look at the hist it should be a nice bell curve with the max some where between 400 and 1000. Change your offset by 5 up or down to get this. you need to see a hint of grey in your image, ie not black.
What you are trying to do is set your black offset so that any signal your recieve is dispalyed. If you don't set this correctly then it is no big deal it just may mean you have to expose for longer. hope this helps.
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Old 27-04-2008, 09:17 PM
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Kenny is correct guys,

Try not to hassle your selves with calculations and numbers..
What happens is as you expose the CCD to light, the bell curve starts to shift to the right. The more data you collect (Photons) the more to the right it moves. You need to position your histogram window around this bell shape.

All you need to do is start with a reasonable setting, like gain 50 and offsett 135.
Take an exposure and see how your histogram looks. Like Kenny said, you should see the whole bell curve within the hstogram. If you see an all black image an no bell curve, then you need to change the offset to bring the curve within the histogram.
Once you have done this a few times, you automatically would know the values next time you do imaging or change exposure times.
For example, i shoot with 63 gain and 135 offset for between 1 sec to 3 minute exposures thru my 14" RCX400, but if im going to increase my time to 5 minutes, or im going to use my 22" f4 Newt, then i know that i need to offsett a little more to take the extra signal into account. So i set the gain to 63 but offsett to 120 or 125 and sometimes 115 depending on sky glow.

Where other cameras would have just saturated the image with white, the offsett ability with some ccd cameras allows you to avoid this and capture the image cleanly.
Basically spend 5 minutes getting used to offsett and gain one night to grasp the basics. I use the same settings for 90 % of my exposures.
Only when i bin the image do i use a lower offsett like 115 etc.

These values are not FIXED and so documenting it as such would cause problems to users that dont have the exact same optical train as the user who set the values. Offsett changes with you lens/mirror size and f ratio.

Oh yeah, here is the link to that pdf that Kenny mentioned http://www.lightstorm.at/pdfs/QHY8_settings.pdf .


Last edited by Gama; 27-04-2008 at 09:27 PM.
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Old 28-04-2008, 06:09 AM
Alchemy (Clive)
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Thanks for the reply guys. At last i can make sense of the how to use it.
this was the last issue with regards to the camera i had not covered.

i reckon this camera will be the break in point for all the ccd wannabees like me. priced right , good performance, reasonable pixel amount...... just dont buy the last one gama has before me
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Old 28-04-2008, 09:11 AM
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iceman (Mike)
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Interesting thread. I look forward to seeing what you can produce with it, Clive.

I'd definitely like to get one of these too, but the cost is out of my budget right now. Maybe next upgrade.
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Old 28-04-2008, 12:26 PM
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Mine should arrive this Tuesday!
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