QHY8L /Maxim - Advice needed

[A320Flyer]

Hi,

I have recently upgraded from a Canon 1000D to a QHY8L ccd camera. Reading through various forums etc., it appears that, because of the camera's 2-stage exposure, it will be necessary for my Bias and Flat frames to have a minimum exposure of 4secs.

Have I understood this correctly? How do you achieve this when using lightbox? A 4sec exposure saturates the frame.

Regarding Maxim, when you set the image type to Bias, it automatically sets the exposure to zero secs and you are not able to alter this. How do you get round this problem?

Also, in colour converting in Maxim, what are the correct RGB percentages and x/y offset values to achieve the correct colour balance?

Thanks for any assistance.

Bill

[Zakalwe]

Do you actually need Bias frame? I thought that the Dark frame will contain the Bias information. AFAIK you will only need Bias if you are doing Bad Pixel Map subtraction.

If you are taking short exposures with it what happens is that the camera reads the data out of the electronics as the next shot is being taken. This alters the output of the camera as the amp is running. The read-out time is a little under 4 seconds, so if you are taking flats you need to allow time for the data to download.

You can do this in two ways:

1) If you are using EZCap, then set the flat up in the Planner as a single exposure. Take the shot, wait for it to download, then take the second shot, and so on. As flats are short duration, this isn't really that much fuss.

2) Use a different software package. I use Nebulosity, which allows you to program a delay between exposures. I set a plan up with an exposure lengths of, say, 0.7 seconds, a delay of 4 seconds, then the next shot, and so on. Will Maxim do similar?

[steppenwolf]

Not sure where you are getting this information from as a Bias frame should be taken at the shortest possible exposure length (zero if you can) irrespective of the CCD or its cooling method. Flat frames should be whatever length is required to saturate the pixels 30% to 50%, again, irrespective of the cooling method.

Also, in colour converting in Maxim, what are the correct RGB percentages and x/y offset values to achieve the correct colour balance?

These figures are dependent on the actual sensor in your CCD camera and any LP filter that you may have in use for your imaging. What I do with one shot colour cameras like yours is to take a daytime image of, say, a garden scene with either my LP filter or an IR filter in place. I then set the percentages to 100% each and If the colours are seriously way off, I try each permutation of X and Y offset until I get colours that are approximately right and then I fine tune the percentages to get a realistically coloured daytime image.

[steppenwolf]

In my post above, I am referring to the OP's information as I doubled with Zakalwe!!

[A320Flyer]

Thanks Zakalwe and Steppenwolf.

My understandingt of DARKS, FLATS and BIAS is exactly as described by Steppenwolf and this is the method I have been using with my Canon 1000D. However, the following thread:

mentions that the BAIS and FLATS should be at least 4secs long if they are to properly match the DARKS. This is where I am a bit confused.

Bill

[Zakalwe]

You will not need BIAS frames, as the noise information is contained in the darks. According to Craig Stark in his tutorial (link) "is that that dark frame has the bias error in it already" and "If you are using normal dark subtraction and not Bad Pixel Mapping to address the hot pixels, your darks already have the bias error in them. Do not collect extra bias frames and do not use any bias frames during preprocessing. Just use the darks and both the dark current and the bias error will be removed."

HTH

[steppenwolf]

Having done a little research, It would appear that the QHY8L uses a Sony ICX413AQ sensor unlike the QHY8 Pro that I am conversant with that uses the ICX453AQ sensor. This is indeed significant so I stand corrected! The ICX413AQ is an interlaced design whereas the ICX453AQ is a progressive scan type. Well, ya lives and ya learns .........

[A320Flyer]

Thanks guys.

Zakalwe,

I thought Bias frames were needed to calibrate the Flats?

Steppenwolf,

Does this mean that a minimum of 4secs is required for Bias and Flats to ensure compatibility with the Darks?

Bill

[ncjunk]

Having done a little research, It would appear that the QHY8L uses a Sony ICX413AQ sensor unlike the QHY8 Pro that I am conversant with that uses the ICX453AQ sensor. This is indeed significant so I stand corrected! The ICX413AQ is an interlaced design whereas the ICX453AQ is a progressive scan type. Well, ya lives and ya learns .........

yep there is indeed a minimum bias time. not sure if it's 4s, on the qhy forum it's mentioned to make sure bias is not set to 0 seconds.

I havent used bias on mine but then i have never been one for processing....its the devils work!

[A320Flyer]

Neil,

Yes, thats what I understood.

For Bias, its easy to get a 4sec exposure. But a 4sec exposure for Flats is almost certainly going to cause saturation.

I have a homemade lightbox thats fitted with 12V lamps. If I reduce the brightness by reducing the voltage to about 6V, I can get a reasonable Flat. But I'm just not sure if this is the correct way to do it.

I just wonder what other QHY8L owners do regarding their Flats and Bias frames.

Bill

[steppenwolf]

For bias to work on images taken with a sensor like the Sony ICX413AQ, they must be of sufficient duration to cover the integration time of the two parts of the interlace. I can't confirm if this is 4 seconds or not but the absolute maximum time would be the total time it takes to download an image from the camera.

If you are taking dark frames then you will not require bias frames as the bias is incorporated in the dark image which will of necessity have an exposure length of more than enough.

The situation with flats is more complicated as they should be bias subtracted before being applied to your light images. This means that the flats must be at least as long as the bias frames to calibrate properly so to avoid over-saturating the photosites the incoming light will require attenuation - this could be done with layers of white paper but dimming by reducing the voltage to the light source could introduce an unwanted colour cast.

[A320Flyer]

Thanks Steve,

A single sheet of A4 paper in front of the lightbox gives me a 4sec Flat with a max saturation of around 28,000. Sorted

The simplest solution is often the best.

Cheers

Bill