Bhatinov looks good but HFR=2.4


#1

Greetings - I am trying to get autofocus down and have used a bhatinov mask to get what it shows is good focus but my HFR is still above 2. Is this normal? I have recently replaced my stock focuser with a new Moonlite and the autofocus routine runs properly (decent V curve).

I am wondering if I have a collimation issue or something.

I am using an orion ED80/

Thanks!

Tom

capella%20bhatinov|661x456


#2

The unit of HFR is pixels, so its meaning completely depends on your pixel size and focal length. If you know your pixel scale is e.g. 2 arcseconds per pixel, a HFR of 2 pixels would mean 2*2=4 arcseconds.


#3

Thanks - I am running at a pixel scale of 1.9"/pixel and bin 2x2 with the DSLR when focusing. Would I ever expect to be getting thse 0.8 HFRs that I see people discuss here in this forum?

Thanks again

Tom


#4

I see that my bhatinov image did not upload correctly in my earlier postcapella%20bhatinov


#5

That looks close but not quite perfect. The centre line is closer to the lower than the upper.

The HFR at focus will vary depending on the pixel size and at least the scope focal length. If you want smaller numbers try getting a camera with bigger pixels or binning.


#6

I guess you are right now that I look at it - I will work on that. Either way, one would think that the routine would get me to the best spot irrespective of my ability to interpret bhatinov patterns.

If I were to bin 4x4 or something would that improve the end focus even though I will not be binning my images when I am gathering subs? In other words, if I bin to focus and not to image does that degrade anything?

Thanks

Tom


#7

People do bin to get higher quality data but this won’t change the focus position and it will only reduce the HFR because the binned pixels are bigger.

The best focus for your system is at minimum HFR, the actual minimum value doesn’t matter. Your system could be perfect and give a HFR of 2.4, we can’t tell.


#8

Get a copy of BahtinovGrabber.exe - works very nicely to get best focus. I’ve been using it for years.

John


www.mainsequencesoftware.com