F4 Newt lunar imaging and trying a near IR filter

[timh]

More quixotic attempts at trying to optimise my F4 Newtonian for lunar/ planetary imaging - a single instrument suitable for all uses.

A fundamental limitation of course is that - when uncorrected - the ~ coma-free field at such a low F number is limited to about 1.4 mm on axis or less than an arcmin.

One solution is to use a coma corrector - no barlow - and a camera -ASI 715 MC - with tiny (1.45 um) pixels. This seemed to work pretty well viewtopic.php?t=9542 but limits resolution - i.e. sampling at 0.28 arcsec/ pixel which is insufficient to sample the theoretical Dawes limit for a 300 mm telescope of ~ 0.4 arcsec.

So here using an ED 2X Barlow to make the system F8. Based on the coma-free field diameter in mm = f^3/45 mm being now about 11.4 mm and a good fit to the size of the ASI 715 MC chip - the image should be ~ coma free across its maximum field of 3864 x 2192 pixels or ~ 8.8 x 5.5 arcmin at ~ 0.137 arcsec/ pixel

VX 12 300 mm F4 reflector, CEM70G mount, 2X Barlow, Baader Clicklock, ASI 715 MC camera. Captured using 1 minute Sharpcap .ser files. Processed using BiggSky and Affinity photo. Filter was an Astronomik ProPlanet 642 (R and near IR) or a Baader L2 equivalent UV/IR cut.

Gain was 100 (less than unity) and exposure 6 ms with Biggsky selecting ~ 30% of 2640 frames over 1 min.

The seeing on the night of the 27/28 October was horrible and I couldn't clearly even get focus. I just thought that this comparison of the RGB image and the IR image of part of the Mare Crisium and crater Picard might be of interest? I am amazed at just how good the ProPlanet filter was at minimizing the seeing effects -- there are tiny craterlets visible probably less than a mile across and not far off the (IR) limiting resolution of ~ 0.5 arsec. The UV/IR cut filtered image on the other hand is very poor and at least twice as blurred.

Thanks to Dave for highlighting the filter.

[admin]

Hi Tim,

it is quite a drastic difference in quality - very interesting. Looking at the image on the right, it almost has a look to it that the stacking has struggled with alignment - that there is a hint of a misaligned copy in there? Maybe it's just the poor seeing, or maybe the individual frames were bad enough to upset the alignment at times?

I should get an IR pass filter

cheers,

Robin

[turfpit]

Tim

Poor seeing and/or low elevation tend to have adverse effects on the data captured.

For lunar, I have 3 filters that I use:

• Altair IR685 pass, works well for high elevation or when imaging before dark

• Baader 610nm red passthrough, helps with low elevation or when the seeing is unsteady

• Baader UV/IR-cut, used with the 183C usually for capturing high % illumination mosaics

I have also had decent results with a Wratten #56 & #58 (green) filters.

It has taken me many hours of experimentation to try to make sense of all this.

Dave

[timh]

Thanks for the comments. I think that you are right Robin because I am also seeing something that looks like misalignment in images of Saturn. I am doing something wrong in stacking the OSC ser files but not sure what it is ?

Tim

PS Got it! I hadn't ticked the channel alignment box in BiggSky