Focusing - Difference in binning?


#1

I am new to auto focus and have found the following - I wonder if anyone can shed any light on it.

I am using the beta version as I believe this had a better focusing routine. When I auto focus I find I get the following difference in my focusing poinn when using different binning methods.

1x1 bin -3921
2x2 bin -3905
3x3 bin - 3906
4x4 bin - 3899

These have been run consectuviely, each using the luminance filter, 8s exposure and 40 steps and at the same temperature. I have found that when the moon is full I am needing to up the binning in order to focus, but the difference here concerns me.

Can anyone explain the difference and why it’s there? I can say that according to the B mask and B grabber the figure of 3921 is accurate. So how can i get accurate focus during a moon?

I look forward to hearing some ideas.


#2

There are many factors that can affect focus. You don’t mention what equipment you are using so it’s hard to know exactly, but I suspect that the different focus points are still within the critical focus zone. A difference of only 22 steps may not really affect the focus, if it’s within the range of the CFZ.


#3

I’m using an 8"RC, While the focus may be in the CFZ, it’s not acceptably good (for example I can get it much less than +30 in grabber and this is roughly what SGP gives me) - An image is all about focus and there IS a big difference between a focus just within the CFZ and really nailed. SGP never seems to nail it for me - always about 20microns out.


#4

Since binning can make the stars look brighter, you might want to reduce exposure times when you change binning (from 1x1 to 2x2 to 3x3 to 4x4). Over saturating stars can result fewer unsaturated stars for SGP to select. I don’t believe SGP uses saturated stars to measure the star sizes during auto focus run.

Are you focusing on a single bright star or a star field?

Peter


#5

I haven’t reduced the exposure times with the binning change as with the moon about I found that I just wasn’t able to focus even. I assume that I am focusing on a star field - this the default for SGP focusing I assume?


#6

If you focused at four different binnings on a single night, the sky brightness is the same but next bin level will result up to four times brighter stars and better chance of saturation. So I suggest you to try at different exposure times for different binnings so that different binned images will result approximately the same number of unsaturated stars SGP will select.

The manual suggests to bin the next level after imaging. In other words, if you plan to image an object at 1x1, then focus at binned 2x2. Or image at 2x2, then focus at binned 3x3. But if the download speed is fast for imaging, then use same bin levels for both imaging and focusing.

Peter


#7

2x2 or 3x3 are probably your best bets fro AF. The reason being is that stars are round and CCDs try to display them on square pixels. When you are at 1x1, the distribution of a given star across pixels is minimized and, because of this, so is our ability to effective calculate HFR. When you spread the same star across 4 or 9 times as many pixels you are giving SGPro much more data to work with for HFR calculations.

The added bonuses here are also:

  • Faster download time
  • Less exposure time

#8

OK - Thanks for the comments guys - I will use 2x2 bin and up the exposures if necessary and hope that I can get the focus to somewhere near what the Bahtinov mask gives me.


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