Dear Robin,
When I first started using a camera about a year ago the first step was to use Sharpcap livestacking with my 10 " Goto Dobsonian (after first fixing Dob focuser problem). The results were quite captivating and really hooked me into EAA. I further found that, even with field rotation, it was possible for SC to stack for up to half an hour or so, combine images from different sessions and, after cropping, produce really quite good images.
In fact - far from Dobs being useless for deepsky astrophotography - subjectively at least the Dob images of objects like planetary nebula, globular clusters and galaxies seem better than I have obtained from my far more complicated and expensive 4 " guided etc refractor set up. The F 4.2 (reduced) to 4.7 and f 1050-1200 mm makes for a brighter and more ideal image scale. I always just guessed appropriate values for exposure (usually about 10s) and gain to minimise rotational blurring while retaining adequate sensitivity of faint objects.
Maybe asking for far too much but it is perhaps a a notion consistent with the USP and EEA livestacking roots of Sharpcap? Plus I think that quite a few folks are interested in making the most of their dobs rather than invest in big equatorial mounts etc.
Could a version of Brain be specifically developed for Dob use ? So, for example, the user might input the focal length of their scope, the pixel uM of their camera and the RA, DEC of the object to be observed and the maximum tolerable FWHM 'blurring' --- and the Brain then calculate the maximum exposure and then say the minimum gain (for best dynamic range) while taking into account the measured sky brightness through the telescope (which should also account for F number and use of filters etc?) ?
https://onedrive.live.com/?cid=330584F8 ... 29&o=OneUp
best wishes
TimH
A version of Brain tailored for GoTo Dobsonian use?
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Re: A version of Brain tailored for GoTo Dobsonian use?
Hi Tim,
actually this sounds more like something that can be built directly into the live stacking. The stacking code knows how long the exposures are and it also knows how quickly rotation is happening based on the angle change between each successive frame. Given that it shouldn't be too hard to work out how many pixels of blurring you are getting at the sides or corners of the image (assuming the rotation centre is in the centre of the frame.). If the blurring amount exceeds a threshold then you can just have a message shown that says that field rotation is blurring the corners of your image – consider using a shorter exposure.
A nice idea!
Cheers, Robin
actually this sounds more like something that can be built directly into the live stacking. The stacking code knows how long the exposures are and it also knows how quickly rotation is happening based on the angle change between each successive frame. Given that it shouldn't be too hard to work out how many pixels of blurring you are getting at the sides or corners of the image (assuming the rotation centre is in the centre of the frame.). If the blurring amount exceeds a threshold then you can just have a message shown that says that field rotation is blurring the corners of your image – consider using a shorter exposure.
A nice idea!
Cheers, Robin
Re: A version of Brain tailored for GoTo Dobsonian use?
Thanks Robin,
Clever! That is a far more elegant solution and would be great if you could implement it in some way. For dob photography, the object of interest is very often confined to just the central 25% or so of the frame (with an expectation of later cropping the picture) so the exposure warning flag might be nuanced in some way to indicate the % radius from the centre where blurring is likely to be unacceptable?
Just as a final thought -- it would actually be quite useful to have a simple visible readout of the cumulative field rotation as each stack progresses. Then you can stop stacking - twist the camera by a similar amount - and then carry on with a new stack which will not be so far rotated relative to the first?
best wishes
Tim
Clever! That is a far more elegant solution and would be great if you could implement it in some way. For dob photography, the object of interest is very often confined to just the central 25% or so of the frame (with an expectation of later cropping the picture) so the exposure warning flag might be nuanced in some way to indicate the % radius from the centre where blurring is likely to be unacceptable?
Just as a final thought -- it would actually be quite useful to have a simple visible readout of the cumulative field rotation as each stack progresses. Then you can stop stacking - twist the camera by a similar amount - and then carry on with a new stack which will not be so far rotated relative to the first?
best wishes
Tim
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Re: A version of Brain tailored for GoTo Dobsonian use?
Hi Tim,
The data on total rotation that you are looking for is already there – in graph form no less
https://docs.sharpcap.co.uk/3.2/#Drift%20Graph%20Tab
Hope this helps, Robin
The data on total rotation that you are looking for is already there – in graph form no less
https://docs.sharpcap.co.uk/3.2/#Drift%20Graph%20Tab
Hope this helps, Robin
Re: A version of Brain tailored for GoTo Dobsonian use?
Many thanks Robin that does really help
Yet more hidden jewels in SC! So that better informs when to pause stacking in order to re-centre the dob and to twist the camera a further increment. With a bit of trigonometry should also be able to convert the field rotation angle into pixels traversed at different radii from the centre -- although maybe the new SC feature (if indeed a new feature is merited) could be to also add a graph that directly expresses rotation in terms of pixels traversed/ frame at some given (pixel) radius?
thanks again
TimH
Yet more hidden jewels in SC! So that better informs when to pause stacking in order to re-centre the dob and to twist the camera a further increment. With a bit of trigonometry should also be able to convert the field rotation angle into pixels traversed at different radii from the centre -- although maybe the new SC feature (if indeed a new feature is merited) could be to also add a graph that directly expresses rotation in terms of pixels traversed/ frame at some given (pixel) radius?
thanks again
TimH
Re: A version of Brain tailored for GoTo Dobsonian use?
Hi Robin,
Just to maybe help wrap this thread up.
I think that my original notion was flawed because I hadn't understood that, even with a GOTO dobsonian the translational movement is generally a bigger limitation even than field rotation in setting the maximum exposure time. Through paying better attention to both of the drift graphs and trying a few experiments I proved to myself that - far from being able to exceed 10s for compact objects at the centre of the field - it was better to actually cut down to about 5s.
But sometimes the dob is set up better than at other times - I guess some (and also other types of motorized AltAz telescopes) are better than others etc and the field rotates at different speeds according to where in the sky. So maybe, as you suggested, it would be worthwhile to add a simple warning message - based on the drift graphs - that comes up and warns folk whenever the exposure time is set such that translational and or rotational movement will result in (say) more than 1.5 pixel blurring?
best wishes
Tim
Just to maybe help wrap this thread up.
I think that my original notion was flawed because I hadn't understood that, even with a GOTO dobsonian the translational movement is generally a bigger limitation even than field rotation in setting the maximum exposure time. Through paying better attention to both of the drift graphs and trying a few experiments I proved to myself that - far from being able to exceed 10s for compact objects at the centre of the field - it was better to actually cut down to about 5s.
But sometimes the dob is set up better than at other times - I guess some (and also other types of motorized AltAz telescopes) are better than others etc and the field rotates at different speeds according to where in the sky. So maybe, as you suggested, it would be worthwhile to add a simple warning message - based on the drift graphs - that comes up and warns folk whenever the exposure time is set such that translational and or rotational movement will result in (say) more than 1.5 pixel blurring?
best wishes
Tim