Eta Carinae with SGP/MetaGuide OAG autofocus and dither


I have been working with SGP autofocus and find it is doing pretty well with my Edge11 as long as there are enough bright stars in the scene. It was working well enough that I did my first unattended and automated autofocus session with OAG. This requires some coordination between SGP and MetaGuide to turn off, then on the guider - but it worked very well.

This is 5x10m exposures with autofocus after the 4th exposure. There is very little focus drift at all, and nice small and round stars with sub 1.5" fwhm.

Details here:



Wonderful detail in this image Frank. I am really impressed. Good news that the SGP autofocus is working with the Edge11. I am running an RC12 with .8 reducer for effectvie 1950mm and .56"/pixel with my SBIG8300. I can’t use autofocus during an unattended image session but I use it to start and use temperature compensation during the session, which works sort of ok, but not great.
How do you like your Edge 11 and how long have you had it? I might be interested in moving to an Edge 11 or Edge 14. Any advice you might give me?
Thanks, Jerry


Hi Jerry-

Thanks for the comments. Yes I like the Edge and have used this Edge11 for about 1.5 years. Before that I used Edge8 at f/10 in Dublin, Ireland - and before that I used C11 from just north of NYC. I have used all of them with robofocus on the primary - but previously I had used my own focus code combined with Maxim - but lately have been using SGP for acquisition including focusing and dithering with MetaGuide - and video OAG.

I am very happy with the optics and performance of the Edge11 with 0.7x reducer. I have used the sgp autofocus for a long time - but this is the first time I didn’t help it along manually by turning off autoguiding in MG, doing autofocus, then re-guiding. In this case SGP did it all -which was nice to see.

My main limitation for full automation is that with my cge-pro I can’t exactly center the field well enough so that the guidestar will land properly on the oag sensor. But that stuff should work well with a high end mount and sgp.

You don’t say if you’re using OAG - but if you are and your image scale is the same as mine, then acquiring a specific guidestar will be a challenge for automation. But if you just use long guide exposures you may be able to find guidestars anywhere - but fainter.

I don’t know the rc12 very well - but many people complain about focus shift in Edge and prefer a separate focuser. But if you use autofocus with the primary focus knob, the actual focus shift is smaller than people think - for me around 20" - and with autofocus it just does its thing and 3 minutes later you have excellent focus. And the 20" shift only happens when racking back focus; when you rack it back in the star returns to where it was and you can start guiding again - and dither.



Hi Frank,
All sounds good to me. I guide using a Short Tube 80mm with a ZWO ASI120MM mono guide camera, so I have lots of guide stars no matter where the scope is pointed, and no rotator required. And I have been using PHD2 for guiding. I have Metaguide installed but was not able to get it working successfully. I gave it my correct fl, pixels, etc. but it insisted they were wrong and would not guide for me. I did use it to get a really good focus on my guide camera. Next time I return to my obs (mid April) I will have another go at it.

So are you saying you get excellent focus V curves? And that the donut issue is not a problem for you? With my rc12 I only rarely get a decent V-curve, and that is staying close to best focus and well away from donut range. So in practice I just use temperature compensation to make a small focus adjustment over the night. My ota is carbon fiber and focus changes very little over a 20 degree temp change, which actually is working ok for me. It may be I have some mechanical issue that I have overlooked.

Is the fact you are using OAG a major factor in getting your sharp images? Would I be able to get comparable images using my separate guide scope? Also would there be a difference using Metaguide vs PHD2?

Thanks for all your input.



For any cassegrain I would definitely use OAG and you should see a big difference in sharpness as long as your focus is good. If your seeing is really bad then the mount and guiding won’t help much. But if your seeing is good, MetaGuide and low latency video guiding should help - but it won’t help as much with a high end mount as with a mid-range mount.

Yes - the sgp autofocus routine does work well for me and I don’t have an issue with donuts messing things up - but I only take the focus curve near focus - so donuts don’t really appear. My problem with it is the occasional jagged curves due to small stars that aren’t real - and when the nebulosity rejection isn’t tuned properly. But having a number of decently bright stars near the center, coupled with 2s exposures, gives me fairly reliable curves. It’s ok for some jaggedness to appear as long as the very bottom part of the curve is sampled well by three consecutive real and low points - based on my understanding of how the routine currently works.