Challenging the resolution limits of my imaging rig – a technical exercise

[whitfieldp]

My rig is way better optimized for nebulae, so galaxy season is never the most satisfying time of year. For a change I did a bit of experimentation this year rather than cycling through the usual larger galaxy targets. Now I've pretty much given up for summer I can look back and take stock.

After capturing the extremely distant (12 billion light yr) QSO APM 08279+5255 for pure curiosity in April, I saw Galactic Hunter’s video on the Twin Quasar QSO 0957+561A&B. The gravitationally lensed QSOs have a separation of 6 arcseconds and located conveniently in Ursa Major so decided it was worth a shot.
Being a scientist, (possibly a bit sad that a professional chemist has a science-based hobby but there you have it!) I did a quick back-of-the-envelope first….

•    Dawes limit for 102mm aperture =  1.14”. Image scale of my QHY9 mono (5.4 micron pixel size) with my 102mm ES102CF triplet at F/7 = 1.56”/pixel. (Being generous we’ll make that 4 pixels separation)
•    The ultimate resolution of a 102mm aperture scope makes it feasible. Classical physics spoil the party a little as points are smeared out into Airey disks on the sensor, their radius given by the equation:
                              (1.22 x wavelength x focal length)/aperture = 4.4 microns for green at 520nm (up to 6 microns for limit of red light in my L filter of 700nm). That’s around a single 5.4 micron pixel for my QHY9.

Depending on where the centre points of the QSO are, the separation in the camera if going to be a maximum of 2 pixels resolvable separation using the native pixel resolution.
So possible but right at the bleeding edge and quite possibly a complete waste of decent imaging time.  🥴

Other main limiting factors…
•    Seeing – I’m in the Northern England so it usually sucks (1.5” if I’m lucky, 0.9” on the odd occasion I win the lottery).
•    Focus – autofocus in NINA using a Moonlite CF2 with a stepper pretty much nails that.
•    Mount. I still use a 2015 vintage AZ-EQ5 GT on spine-friendly grounds on one of the original and much-maligned tri-piers. The pier has a DIY ‘shrinker’ that does an excellent job of stiffening it. The mount head was recently rebuilt by DarkFrame Optics to their StellarDrive standard. As a result it has negligible stiction so balance is very important. My imaging rig has quite a long moment arm so DEC is particularly tricky – an ADM dovetail clamp as a small moveable counterweight is very useful for fine tuning. 
•    Light pollution - the objects are magnitude +16.7 so not particularly challenging in terms of sky brightness.

 

Data acquisition and processing. 10 May 2021.
•    1x1 binned luminance data with a IDAS LPS P2 filter.
•    Stable temperature and a carbon fibre tube so very little focus drift.
•    Over a period of 3hrs guiding total RMS (excluding dither) was 0.63” so less than 50% of the imaging pixel scale – a decent figure for a EQ6-R but this is an AZ-EQ5 carrying 22lbs so not too shabby.  😍
•    I stacked 29 five-minute dithered subs for the final result in AstroPixel Processor.
•    To reduce the ‘Minecraft’-like stars in the crop it was 2x drizzled and sharpened slightly in the Topaz Labs Detail plugin.
 

Conclusions…
1.    I definitely managed to split them! Galactic Hunter used a Stellarvue SVX130, AstroPhysics Mach1 and QHY600C so my setup is rather more on the budget end 😂
2.    Scope and camera are well matched – no shock there as they were chosen that way.
3.    The Stellardrive AZ-EQ5 drops out of the equation with my current pixel scale - I don't need a larger mount for the time being which is good news for my back. It’s tracking far below 1 arcsecond RMS, so subs are usually excluded due to ‘Musk-related events’.
4.    The drizzling feature in APP works really well and doesn’t always need as much in the way of drizzling statistics as I expected. This was a potential concern with the fewer, longer subs typical using the CCD versus a typical CMOS
5.    I now know I can get very close to the resolution limit of my setup – going further would require lots of pennies 😳 (EdgeHD and QHY268 together with their price-tags come to mind). 
 

Now I can say 'Been there - Done that - No t-shirt available'.  Come the return of darker nights in late August/September I can go back to my nice picturesque large clouds of glowing gas……..  😎

Clear skies
Pam
 

Edited June 8, 2021 by whitfieldp
typo in focuser model