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Home Forums Observing and Imaging Group My week in dark skies, Brecon Beacon.

This topic contains 61 replies, has 7 voices, and was last updated by  Astrograph 2 years, 9 months ago.

Viewing 2 posts - 61 through 62 (of 62 total)
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  • #8522

    Andy Sawers
    Moderator
    Topics: 113
    Replies: 588

    I didnt realise they split the astrophotography courses into non telescopic and telescopic

    Here’s the course details for the telescope course starting in January.

    #8524

    Astrograph
    Participant
    Topics: 27
    Replies: 101

    Hi Tej

    Sorry for the waffle earlier!

    Calibration frames are basically there to do two things.

    1) They correct for uneven illumination caused by the optical system. When you use your SW flattener / reducer your going to experience this because a reducer on an F6 scope will usually cause vignetting.

    2) They remove (well, help remove) noise and hot pixels generated by the camera. Stretching and processing are such an integral part of astrophotography that you have to start with the lowest possible noise, or more to the point, highest signal to noise ratio otherwise how much you can process will be limited because you will begin to enhance noise.

    So its always best to use them. Even a pure Astrograph running a Finger Lakes camera needs calibration frames.

    When stacking, the theoretical best number of frames to stack is 30 as after this there is no more improvement to be had. If you are only taking 60 sec exposures, then 30 frames is easy to achieve. In town though there ain’t no way you are going to use ISO6400 so you are going to have to expose longer.

    I would definitely use the CLS filter. I always use that. What you should do though is point your system to the zenith and do a custom white balance or your images will have a blue cast. Its easier to do this in camera than adjust the frames later. A CLS will add about 2 stops to the exposure. Its a pain but worth it. Use ISO 1250, aim at M31 and image with for 120 secs and see what the histogram says. If its a quarter to one third from the left thats fine. For mega M31 images you have to expose for the core and the spiral separately but we don’t need to go there just yet!

    My experience with CLS filters (at F8 / 800mm) is that I can get to at least 7 and sometimes 9 minutes before light pollution overwhelms the filter. On your system at sub F6 /  500mm it will be less.

    You may still get some banding from light pollution with the CLS filter and this can be removed to an extent. Like all processing though (and with respect to Tom Kerrs view), its rubbish in, rubbish out, you can’t polish a t*** etc. Start with low noise, flat image and the more you can do.

    Re your mount and guiding. When I used an HEQ5 I could get nearly 3 minutes @ 700mm focal length without guiding. At under 500mm you should be able to do that easily. Some frames might show tracking error but others won’t. Guiding absorbs a lot of time calibrating when you could be imaging. The image below was taken a few years back on that HEQ5 with a 700mm scope and flattener. It was a 180 second exposure and is a single frame, i.e. no stacking taken in North Norfolk. Its pretty much as is because if you look closely you can see there is a north / south stretch to the stars which is poor polar alignment so I did not try to process it further. The key point though is you can go a long way before guiding.

    107 M31 test by astrograph ltd[/url], on Flickr

     

    • This reply was modified 2 years, 9 months ago by  Astrograph.
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