New to ccd imaging

[nightcasper]

Hi all having been in to astrophotography form about 18 months i have been using a canon 1100d dslr. Ihave purchased a qhy8l one shot colour ccd it seems a lot different to using a dslr ie with a dslr you adjust the iso setting but looking at the ezcap software do you adjust the gain and offset is this right any more help would be appreciated thanks dave.

[Spikey]

Welcome to SGL - Thats correct you adjust the gain and offset in the software. Try something like GAIN=50 OFFSET=120 exposure time = 500ms for testing.

[MartinB]

You don't use the gain and offset in the way you would use the ISO settings though. You need to set up the values for your camera and then stick with them.

To set the offset take a bias frame and check the adu values - they should be somewhere between 100 and 1000, if more reduce the offset, if less increase. This ensures you aren't converting low light values to black.

To set the gain take a day time pic that part saturates the chip. Look at the histogram and look at the white point where the histogram comes to an abrupt fall off. If this is well to left increase the gain, if it is hard up on the right side you need to reduce. You are aiming to have a saturation adu level of around 64 000. You can go up to a max of 65 535 but at this level the non linear response of an antiblooming chip may be creating odd effects.

Spikey's suggested values sound on the money

[nightcasper]

Thanks simon to get more detail do you change the gain and offset like iso or keep it the same and increase the exposure. Do you know of any good tutorials online

dave

[MartinB]

I think the forum software is having a double posting fit again!

Your CCD chip is 16 bit and you will discover the joys of this when you use levels and curves to stretch the histogram. Gain doesn't improve the signal to noise ratio, it just makes everything (signal and noise) brighter. You don't need or actually want this with a 16bit chip.

[nightcasper]

I think the forum software is having a double posting fit again!

Your CCD chip is 16 bit and you will discover the joys of this when you use levels and curves to stretch the histogram. Gain doesn't improve the signal to noise ratio, it just makes everything (signal and noise) brighter. You don't need or actually want this with a 16bit chip.

HI martin thanks for the info i'm a total newbie to ccd so please bear with me. I calibrated the qhy8l as the instructions said but the offset was 116 which i think is ok but the gain seems a bit low i got 5. So once you have set the gain and offset you dont need to touch again you just need to increase exposure time? sorry about double post i did it in error. thanks dave

[MartinB]

HI martin thanks for the info i'm a total newbie to ccd so please bear with me. I calibrated the qhy8l as the instructions said but the offset was 116 which i think is ok but the gain seems a bit low i got 5. So once you have set the gain and offset you dont need to touch again you just need to increase exposure time? sorry about double post i did it in error. thanks dave

That's right. Don't worry about the 5 figure for the gain if your histogram looks right. I believe some people suggest not using any gain at all but my only experience is with the QHY8.

10mins of exposure with the QHY8 will look much dimmer than 10mins with a DSLR at ISO 1600 but that is deceptive. A monitor displays at 8bit i.e. 256 brightness levels for each channel. To display the QHY image each level contains 256 16 bit levels, assuming you are using 16bit software e.g. photoshop. The histogram will be way over to the left. You need to know how to use levels and curves, especially curves, to stretch the histogram to release these hidden levels. It is a bit of processing magic when all the hidden data within the image comes out to play!

[nightcasper]

That's right. Don't worry about the 5 figure for the gain if your histogram looks right. I believe some people suggest not using any gain at all but my only experience is with the QHY8.

10mins of exposure with the QHY8 will look much dimmer than 10mins with a DSLR at ISO 1600 but that is deceptive. A monitor displays at 8bit i.e. 256 brightness levels for each channel. To display the QHY image each level contains 256 16 bit levels, assuming you are using 16bit software e.g. photoshop. The histogram will be way over to the left. You need to know how to use levels and curves, especially curves, to stretch the histogram to release these hidden levels. It is a bit of processing magic when all the hidden data within the image comes out to play!

by nightcasper » Thu May 23, 2013 5:34 pm

Hi all this my first go with my QHY8L http://www.flickr.co...per/8793172319/ Bubble Nebula taken on 22/5/2013 5x240 secs subs 50 darks 50 flats 50 bias cooled to -20. Not to sure about this looked at other images and they seem to be more red around the nebula area and mine is green is it to do with my setting in ezcap or Nebulosity 3. Stacked and processed in Nebulosity 3 and processed further in photoshop 5 any help would be appreciated

thanks dave

[ncjunk]

A bit late to this but my settings on the qhy8l are similar 117 and 7 i think.

Sounds like you're in the right ballpark.

Sent from my GT-I9003 using Tapatalk 2

[nightcasper]

An update after changing the demosaic setting in Nebulosity 3 and further processed in Photoshop 5 need more subs now as when I try and stretch it the image it gets more noisy and its going to be clear tonight and tomorrow night.