Owl PN at F20, SW200 +x5 powermate

[Tommohawk]

So - the main idea here was to see how much coma there is with a x5 powermate. The manufacturer's explain that the powermate can be used with their coma corrector, so they obviously feel there is something of an issue.

Reason being, when using a powermate for planetary work any coma wouldn't really matter, but if using it for lunar full frame work it could. Obviously there wouldn't be any stars to show the coma in that case but presumably there would be loss of resolution in the periphery. 

In practice I would probably only do  this with  a smaller high frame rate sensor, possible the ASI290 which I usually use for planetary, but I figured it would be useful to test with the ASI1600 just to see what one might get away with.

I thought I would also combine this with an effort at longer FL DSO imaging just to see what the result was. I've often wondered where the practical limit lies, especially given this is all driven by an HEQ5, not a top end mount. In other words which is better for a smaller target - modest focal length and then crop, or longer FL but probably issues with tracking/guiding/image quality  especially given the higher gain and longer exposures required?

BTW I've minimised the powermate to sensor spacing so although its nominal 5x, it's actually only 4.5x give or take. 

Trouble is I underestimated how the reduced field would reduce the number of stars, so its a bit difficult to find well placed stars near the edge to evaluate coma. In fact my first effort had so few stars neither DSS or APP would stack it. I really need to repeat this with a test specific for coma, with stars placed exactly where I need them near the edges and forget combining with a DSO.

Guiding was a bit variable - generally it was 0.67" RMS, but occasional big spikes reduced this overall to 1.41". Irritating, because the previous day with  identical set up I got 0.7" overall. Hey ho. 

My impression is that even with the ASI1600 coma isnt too bad, and probably wouldn't be an issue with a sensor like the IMX385  which is less than half that size.

So what do we think?

1. Significant coma? Looks to me like slight collimation issue or camera tilt. (quite possible given weight of camera + 8pos EFW with extensions required to get focus)

2. Imaging at 4500mm FL is a good idea? Would really appreciate any input on either issue!

SW200PDS + 5x powermate, ASI1600MM -20deg 99x 90s LUM gain 300, offset 64. No calibration files, minimal processing.

[MarkAR]

Nice report.

1. No idea

2. Go for it

As a note, I ran your set up through a tool calculator and you are sampling at 0.16 "/px. Apparently that is well oversampled and supposed to be a no no.

[Tommohawk]

7 minutes ago, MarkAR said:

Nice report.

1. No idea

2. Go for it

As a note, I ran your set up through a tool calculator and you are sampling at 0.16 "/px. Apparently that is well oversampled and supposed to be a no no.

Thanks for that Mark. Yeah as you say way oversampled on paper. Always pushing my luck!

[MarkAR]

Can always bin 3x3 or 4x4.

[Tommohawk]

9 hours ago, MarkAR said:

Can always bin 3x3 or 4x4.

That's a good call - and a reminder that if I do get another camera the ability to bin would be helpful.

Do you know if the binning option is inherent in the camera itself or the software? 

[Merlin66]

AFAIK, all CMOS cameras cannot be binned at the sensor level. Any binning must be applied by software later.

All the CCD cameras can bin at sensor level....

 

[MarkAR]

Thanks @Merlin66, you've just saved me from making a faux pas.

Thought the 1600 being mono was a CCD.