Soft images over time just atmospheric issues?

[geoflewis]

On 02/04/2020 at 13:47, sploo said:

A motor focuser is one of the things on the cards. I've just started putting together a prototype for a tracking mount for the 300P scope (stepper motor driven - from your advice on a post of mine some weeks back).

It did occur to me that it should be possible to control focus with another stepper; certainly at high magnifications it's so easy to lose a target simply by bumping the focuser (and very hard to focus when the target is moving around).

A focus motor will certainly help keep tight focus over time, but as your earlier analysis shows, the biggest change in in the first hour when the camera body and lens is rapidly change temperature to acclimatise to ambient conditions outside, so just putting your gear out earlier before focusing will probably make a huge difference. Good luck.

Edited April 3, 2020 by geoflewis

[sploo]

Cross fingers I think the main issue did turn out to be temperature. I left the camera and lens outside last night for a good hour before I started imaging. A test of M42 did show a drop off in image quality over time, but much more gradual - and entirely expected as it moved closer to the horizon (I could see it getting worse even just looking through the viewfinder). While I deleted a number of images (approx IMG_6350 to IMG_6420) due to an obstruction (hence the gap in the graph below) you can see that the reported quality varied much more within a small time window, and only dropped off slowly. Each of those exposures were only 10s, so from IMG_6190 to IMG_6549 is almost exactly 1 hour):

After those image were taken, I pointed the camera due west, and probably about 45 degrees above the horizon. A run of 38 exposures (each 30s, so about 19 minutes) showed a pretty consistent quality level:

I think the widely varying quality of the M42 images over a short space of time is due to my Star Adventurer struggling with the load - a gripped DSLR with two batteries, a large ball head and a 100-400 lens comes in at around 4kg; so I'm probably pushing my luck with that little tracking mount.

[Erling G-P]

Good to hear that you're apparently zeroing in on the reason

You mention being able to spot the deteriorating quality through the viewfinder.  Does that mean you image with the viewfinder uncovered ?   It's generally considered a good idea to cover it up (most cameras comes with a small plastic cover to slide on), to prevent stray light entering through the finder and potentially mess up your image.  Can't say how much of an issue it is, but there must be a reason for those covers being supplied with cameras.

[geoflewis]

Good to see that you are making progress and I agree with @Erling G-P you should put a cover over the viewfinder to eliminate, or at least reduce the risk of light leaking in through there. What did the images from this latest session look like?

[sploo]

10 hours ago, Erling G-P said:

Good to hear that you're apparently zeroing in on the reason

You mention being able to spot the deteriorating quality through the viewfinder.  Does that mean you image with the viewfinder uncovered ?   It's generally considered a good idea to cover it up (most cameras comes with a small plastic cover to slide on), to prevent stray light entering through the finder and potentially mess up your image.  Can't say how much of an issue it is, but there must be a reason for those covers being supplied with cameras.

I paused the shooting a few times to check the view through the viewfinder and refocus (basically checking the last shot before I paused, refocused, and confirmed there was no difference - i.e. no drift in focus). That said, I do usually cover the viewfinder, but forgot on that shoot - and the full moon was pretty much behind the camera and shining into the viewfinder 

I do need to sort out some remote control/capture for the camera, as being able to remotely check images would be quite helpful.

[sploo]

2 hours ago, geoflewis said:

What did the images from this latest session look like?

About as mediocre as my usual images

It's about 44 minutes total data, on an old (unmodified) Canon 7D, though with a CLS filter. Pretty much everything else "wrong"; in that it's a very poor sensor (lots of read noise, combined with very short exposures [10s] as I was having some tracking issues). Seeing conditions were OK but not great (and obviously a full moon).