British Astronomical Association
Supporting amateur astronomers since 1890

Secondary menu

Main menu

Home Forums Imaging
Terms of use

Options for high-resolution imaging?

2 posts / 0 new
Last post
Xilman's picture
Last seen: 25 min 50 sec ago
Joined: 24/03/2018 - 15:17
Options for high-resolution imaging?

My SBIG-8 is a fine though elderly device but has its limitations, principal of which is that the minimum exposure time is 0.11 seconds.  I'm wondering whether I should replace it and would welcome comments and advice.  First some technical specs.

The 9 micron pixels of the SBIG-8 corresponds to 0.72 arcsec on my 0.4m f/6.5 scope, whereas the diffraction limit is about half that.  Although the long term (>= 0.11s) resolution is seeing-limited to perhaps 2as, it would be nice to be able to use lucky imaging and deconvolution to attain effectively diffraction-limited performance.  The Nyquist criterion suggests that a resolution of 0.2as/pixel (ie. diffraction / 2) should be optimal. To achieve this either the focal length needs to be increased (a 4x Barlow, for instance, with 9 micron pixels) or the pixel size reduced to 2.5 micron or so.  That figure is uncomfortably close to the wavelength of light and so a compromise is very likely to be necessary. There are severe trade-offs when small pixel sizes are used.  Either the FOV is seriously curtailed or the detector costs serious money or both.  In either case, the SNR may be severely compromised.

My principle interest at present is VS photometry where a FOV of, say 15 arcmin, enables BAA and AAVSO comparison sequences to be used and where detector dynamic range, noise and (especially) linearity is critical.  A field of 15 arcmin at 0.2as/pixel corresponds to 4500 pixels --- a seriously expensive detector with (very probably) lengthy download times.  However, I also want to branch out into high resolution imaging of objects beyond our immediate neighbourhood, here defined as anything further away than Saturn.  A number of people have imaging the inner solar system down to a fine art and I see little purpose in competing with them.  That said, their skills and experience are very relevant to my ambitions.

Finally, and unless you can convince me otherwise, I've no use for colour cameras. On the odd occasion that I want to take a pretty picture I'll combine several images taken through several filters.  A colour camera is just a waste of resolution and/or sensitivity as far as I am concerned.

Any suggestions on what cameras may be an acceptable compromise  (and compromise is clearly necessary)?  I've no hang-ups on whether CCD or CMOS detectors are used now that CMOS detectors appear to be getting their noise issues under control.

Perhaps it's not possible to do both VS observations and high-resolution imaging with the same gear and I'll have to resign myself to swapping Barlows and/or cameras in and out according to what is to be imaged.  It's just that retaking flats after each exchange is so tedious...

Thanks in advance.

andrew.j.smith1905's picture
Last seen: 40 min 41 sec ago
Joined: 02/12/2016 - 12:22
New camera

Hi Paul, I would advise you look at what the best images use in the categories you mention.

I am not an expert but a few points.

Modern CMOS cameras can quickly download large numbers of pixels with USB 3

Nyquist does not really apply to area detectors like CCD and CMOS cameras and greater oversampling is needed.

If I remember correctly planetary imagers go for about 0.1as.

Regards Andrew