Help With Focus Please


#1

I have been using SGP for focussing for some time now and have had no issue with it. I recall that there used to be sliders for star count and nebulosity (or something like that). There is a new system that involves setting a minimum star size at 1x1 pixels. I have never had to adjust this from its default of 6 - it has just worked.

Last night, however, I was imaging M33 in Ha. I noticed that the focus routine was picking up some of the Ha in the galaxy - NGC 604, in fact. So I had to look at the manual. I tried various settings for min star size - up to 8 and down to 2. I was never able to exclude NGC 604. As it turned out, even though the reported HFR was around 1.6, I think I got good focus anyway. I slewed off to B-Andromeda and checked with a Bahtinov. But I would prefer to be focussing just on stars if I can.

Any tips?

I will link to a screenshot showing what was happening - https://dl.dropboxusercontent.com/u/54557410/Focus.jpg

Thanks in anticipation


#2

You can tune this value with only a single camera image. Just take one (using Frame and Focus) that mimics AF filter, binning and exposure time and then click the “HFR” (star) button.

Detected stars will be circled in green. As you adjust the minimum star size, the detected stars will adjust with it in order to show you what effect you are having on the detection routine.

Did you get to read through this guide:
http://www.mainsequencesoftware.com/Content/SGPHelp/UnderstandingAutoFocus.html

Specifically, the section titled “So… How do you set it up for your rig?”


#3

Thanks Ken. I will try fine tuning using Frame and Focus next time. The other night NGC 604 ended up being circled - and it had something like 11 x 11 next to it. I assume that I should be reducing the minimum star size to try to exclude it?

I have read the manual about setting up the rig and I think I have the step size and things like that well sorted out. I have had no issues with other objects.


#4

In all likelihood, you will not be able to exclude it. That said, unless the region is extremely star-poor, it should not matter.


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