Eta carina with OAG guiding

[bmitchell82 (Brendan)]

im not 100% sure but there are 2 types of binning, Software and Hardware, almost like Graphics acceleration. the HW version is always alot better.!

[gregbradley]

I find binning the guide camera 1x1 gave better guiding than binning 2x2 or 3x3. If you are using SBIG self guide then yes 2x2 or 3x3 may be needed for those little 237 chips.

If you are getting blocky stars at 3x3 binning or more then a solution is a higher pixel guide camera. The Starfish is 1.3mp and is cheap with subsecond guide exposures available.

I think its way worthwhile to invest a bit extra in the guide camera as it is a vital part of the autoguiding setup.

From your image though it looks like your autoguiding was superb.

Greg.

[Bassnut (Fred)]

Greg. Guiding binning depends on image scale. bin 1 is best on a wide field guide scope, and bin 2 or 3 with internal or OAG at long FLs. bin 1 is no advantage at long FLs, wasted res, so you bin to get brighter/less noise stars. It can make a diff to the point where bin1 is so dim, its unusable and binning is at a given exposure time..Actually, S/N ratio is what counts more whilst guiding, not resolution so much, which is why the cooled SBIG guide cam is so usefull.Blocky stars arnt too much of a problem if you defocus.

Having said that, for standard external guide with a wide field ST80 for instance, I found the QHY5 small pixels and large FOV perfect for the usual 2 sec exposures (as the starfish would be), there was always a bright enough star in view.

Its with internal/OAG that things got ugly.

[AlexN]

Greg - Do you know what size pixels the starfish has?

My thoughts have always been to try and guide at a similar arcsec:pixel resolution for best results... Or even guiding at higher resolution if possible. I have done tests with 200mm FL guiding 1756mm FL etc and the results can be ok given you really tweak the setup to high hell, however I find best results come from having equal or slightly better guider resolution... My setup is an example of this.. I guide with a QHY5 with 5.2um pixels.. I image with the KAF8300 with 5.4um pixels. I guide using an OAG which means that the guider is getting ever so slightly more resolution than the imager. If I can achieve guiding around 0.3 pixel corrections, my guiding is going to be perfect in the final image. Where as if I were guiding with 0.3 px corrections with the QHY5 in the TMB 80/480, and the KAF8300 in a 12" SCT at 3050mm, the the 0.3px movements in the guider would equate to multiple pixels on the imaging sensor...

The same affect can be assumed when binning a guider in an OAG, or binning the guider in a short FL scope... if your arcsec:pixel ratio starts to undersample horribly, and stars appear as a singular pixel, the guider will not recognize star movement until its moved an entire pixel. Hardly what we want..

[Bassnut (Fred)]

Originally Posted by AlexN

Greg - Do you know what size pixels the starfish has?


... if your arcsec:pixel ratio starts to undersample horribly, and stars appear as a singular pixel, the guider will not recognize star movement until its moved an entire pixel. Hardly what we want..

mmm, well when I was external guiding at 3m with a QHY5 on a ST80, the stars were never 1 pixel and many pixel guide stars allowed sub pixel corrections, worked OK, just.

OAG guiding at 3m with say a 9um imaging cam (at 0.62 A/P) and a QHY5 guiding cam (5.2um, 0.36 A/P), single pixel guide stars means a pixel guide shift is still less than an imaging pixel. And anyway, at that FL, 2 arc sec seeing would see very little image shift even if guiding danced around 2 or 3 pixels (or more), so binning is viable then even if the guide star was a single pixel.

[AlexN]

Agreed Fred, Its when you start using a 50mm finderscope as a guider getting 6 arcsec per pixel that long focal lengths become really difficult to guide. With the ST80 you'd have been looking at 3.3~ arcsec per pixel I would think (something around that figure) and stars would still be spread across a few pixels..