From: Milton Aupperle <milton@outcastsoft.com>

Date: July 15, 2010 9:50:37 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Impromptu ccd shootout

Mark;

On 15-Jul-10, at 7:52 AM, Mark Gaffney wrote:

Hi Alan, Jim & others,

I`d like to experiment again with my Scorpion on DSO`s.

I finally have a C-T2 Mogg adaptor for it to do away with the nosepiece & possible slipping of the camera.

It`d be good to see how it stacks up against my Nikon DSLR..I recently did some practise images with it of NGC 3372.

I`ll attach an example below @ ISO 3200 6 x 20 second exposures + 9 darks. (I`m not guiding yet because of difficulties parfocalising my CRG OAG with only 100 metres distance for land objects)..I`m at a new site over in one of our paddocks which gives clear views of the south.

My Bahtinov mask could be used to focus the Scorpion as I`ve done with the DSLR but what still perplexes me is how to determine the right gain setting for DSO`s with the Scorpion..?

The gain settings for a Scorpion should be kept fairly low to keep the noise levels low when doing long exposures.

I realise you guys (Alan & Jim) seldom venture to image DSO`s but maybe someone (Milton?) can give me a clue to the right gain setting for a relatively bright object like the Eta Carinae nebula while it`s still around? When I tried M 42 earlier this year, doing this was trial & error..

It IS trial and error. You have to find a balance between what is acceptable noise for the gain level. And I don't vary the gains at all for targets. Once I find the best setting for noise versus exposures, I leave it there. That is also what SBIG and other Astro cameras manufacturers do too - but it's "hardware wired" in.

I spent about half a day on benchmark measuring this for the Grasshopper to build up curves that show gain versus brightness of a gray card and then gain versus noise too to see what difference increased gain makes. I used the "Measure Brightness" feature in Astro IIDC (page 27) to monitor a small 4x4 pixel area on the card. The average of about 1 minute of samples (at 15 fps) gave me the brightness and the variance between each samples gave me the noise estimate.

My sweet spot for gain versus noise works out to be 251 on the Grasshoppers EXHAD Mono camera. That boosted my brightness by 2.77 times and the noise level only went up by 138%, however it's still a very low. Basically at lowest gain of 64, the brightness was changing by +/- 25 (out of 65535) and at a gain of 251 the brightness was changing by +/- 64 (out of 65535).  So percentage wise it's 0.038% change for gain 64 and 0.097% at 251.

The gain setting is not transferrable between camera models, so you can't just plunk this down and expect it to work.

HTH..

Milton Aupperle