A couple of moon pics and revisiting the basics

[timh]

The attempt to do planetary and lunar imaging led to much deeper scrutiny of the optical set up. Lucky imaging may afford the possibility of resolving down to near the Dawes limit -- but the optics, alignment, optical train camera etc. then need to be correspondingly excellent. So a fair bit of time examining things that - for deepsky stuff -- I really hadn't considered that much.

To cut a long story short -- Careful examination of the F4 VX12 Newtonian monitoring intra and extra focal patterns revealed low but significant levels of spherical aberration (maybe 0.2 wave higher order as guestimated by simulation with aberrator) -- a major problem with poorly controlled and variable registration of the optical train into the Baader focuser leading to varying levels of tilt - and a focuser mislaligned with respect to the optical axis leading to uneven image illumination since part of the secondary mirror was occluded.

So a few things on the way to getting improved -- firstly priority a clicklock to get consistent registration of the collimating laser and the optical train.

One fundamental limitation of of an F4 Newt is that - when uncorrected - the coma-free field is limited to about 3 x 3 arcminutes on axis. But I quite like wider field lunar images because of the context and orientation of major features that they provide. So here an F4 aplanatic coma corrector and no Barlow. Because of the tiny pixels in the ASI 715MC camera it still (in theory) leads to a reasonable resolution of ca 0.28 arcsec/ pixel.

VX 12 300 mm F4 reflector, SW aplanatic coma corrector, CEM70 mount, ASI 715 MC camera at about 8 ms, gain 150. About 3000 frames over 80s - 3864 x 2192 pixels. Processed using BiggSky and Affinity photo.

The first picture shows the region spanning the 3 well known craters Gassendi, Tycho and Copernicus. The other image shows the region including Plato and the lunar seas down to Tranquillity. The pictures were taken on the evening of 27 May with moon just past a half and rather low (about 20 degrees or so) in the Southern sky so not perfect atmospherics.

[admin]

Hi Tim,

it's always fun to try something new A couple of arc-seconds of seeing blur certainly does mean that the collimation requirements for deep sky are not as refined as those for lucky seeing imaging.

Have you tried the 'barlowed laser' approach to collimation? By putting the laser collimator into a barlow lens, which turns the beam into a divergent light cone, the requirement for precise alignment of the laser collimator with the draw tube is far less stringent.

cheers,

Robin

[timh]

Hi Robin,

Thanks for your comment

Indeed this high precision lunar and planetary stuff has been a new world of learning -- there is just no end to it in this hobby (which is why it is so absorbing) and suddenly you see why things like Strehl factors might matter. Anyway an interesting challenge - or possibly a fool's errand - to see how far an F4 Newt can be tuned so that it at least performs acceptably for lunar and planetary while at the same time having the light grasp and transportability (to dark skies) necessary for deep sky stuff.

Yes on the collimation-- the Tublug / Howie Glatter laser system is a sort of barlowed laser with an attachment that diverges the beam and - as far as I can see - it is dead accurate, reproducible and fits so tightly into the 2 inch focuser that there is zero lateral movement - neither movement of the spot or of the reflection of the spot on the Tublug screen on rotation etc. There is something very reassuring about the precision engineering of the Howie Glatter although it was expensive. So I think that collimation is good. The real trouble has been that I just didn't realise how floppy every other fit into the focuser has been. So all set up nicely using the laser/ tublug accurately centred in an albeit slightly tilted focuser-- but then -significant tilt introduced every time the optical train was put into the 2 inch compression ring. The observed tilt in the out of focus rings around the Airy disc varies according to which of the three compression screws I start to tighten first.

I think that the solution to the tilt problem will be to do away with the compression ring fitting altogether and replace with a Baader 2 inch clicklock fitted to the focuser screw thread. The uneven illumination requires that the focuser base be adjusted -- but improving the Strehl factor of the mirror will cost and mean repolishing it to a higher level of accuracy.