image quality (sensor?) issues

[scpanish]

Hi Jo and Chris,
Here are some questions concerning image quality/sensor issues on my mono Infinity, which I bought last summer.  It is a great piece of kit, but I can’t always get performance where I want it to be.

The attached (compressed) .jpg is for illustration only, it isn’t meant to be a good or bad picture.  It was taken with Image Quality->autorange->low just so you have a standard to judge it by, software v1.3.  That setting always produces very light images, I always use  the stretch tool to optimize.   I haven’t found any of the autorange settings to be useful other than as a starting point to see what you are getting early in the stacking, and for that “low” is very good.  This was taken on a pretty cold night (well below freezing) so temperature should not be an issue. 

There are 3 issues.   

1) On the left is broad vertical banding, which decreases in contrast across the frame.  Most any stretch that brings out nebulosity will show this banding . If the low luminance cutoff is moved into the distribution sufficiently far to eliminate the bars, the image will be so dark that nebulosity is invisible, and dimmer stars.

2) Across the image are narrow, light vertical lines which look like read noise.  These go away easily with a normal stretch, so it isn’t much of an issue.

3) There is an overall graininess which sometimes is present and sometimes is not.  I have not been able to characterize when it will appear, and when present, how to get rid of it. 

Note:  Typical exposures are 3 to 5 sec, unbinned.  Scope is F4.7, 10”, 1200mmFL.  My ability to use long exposures is limited since I am using an AZ mount and will frequently get unacceptable image rotation with  exposures over 5 seconds (depending on where the object is). 

Is there some way to minimize these artifacts? 

Might a new feature help, for instance, manual gain /offset controls?  I have really played around a lot with the stretches and conclude there is some automatic functioning here that I cannot get around.  In practice, virtually all of the DAC range is unused.  Might some advanced user controls allow better optimizing the range, so more of those 16 bits are used?

Thanks,

Steve

[roelb]

I think that your exposure times are too short.
I also use a ALT/AZ and apply exposures up to 30 s and sometimes up to 45 s or 60 s (depending on field rotation).

[scpanish]

I wondered if my exposures were too short, but found that I typically got too much field rotation when I went over 10 sec., sometimes less.  And a lot of bloom on the brighter stars.  I'm  using a SkyWatcher goto Dob, F4.7, so maybe the precision of the drives is low.  I'll try longer exposures when the weather allows.  Thanks for the response.

Steve

[Chris]

If you are using a Dob in AltAz mode then field rotation is a fact of life, it's the physics, nothing to do with the mount, other than it being AltAz, not equatorial.

The amount depends on where you are looking, it's least when looking East or West.

Chris R