HFR question

I am presently using V 2.x as I only use it for focusing my Moonlite focuser. I display image statistics in the left hand margin. I noticed today that the HFR value in the statistics list changes now after every image. In the past I seem to remember that value only as a result of a new focusing cycle. Could someone verify that this is the normal operation or did I hit a button or option somewhere that is now updating that value after each image.

Let me try one more time…if I do auto focusing once an hour while imaging and it results in a HFR value in the Image Statistics module, will that value stay the same until auto focus cycles again an hour later, or will that hfr value vary with each subframe hfr result?

The statistics module reports numbers from the current image, so you shouldn’t expect it to stay constant between autofocus runs. There is also the image history (I think that’s the right name) module that will show star count and hfr values for the current run so you can see any drift or the impact of clouds, etc.

Thanks Terry, though I assume the focus point is not changing, only the
real-time HFR snapshot.

Unless you are set up to use SGP’s ‘Temperature Compensation’ feature (which seems unlikely given your opening question), SGP will not change the focuser position between AF runs unless of course you manually make a change.

You can readily monitor this for yourself using:

a) SGP’s Focus Control window which displays the current focuser position

b) There is a parameter ‘FocPos’ contained in the FITS header of each captured frame which should also show there has been no SGP invoked change in the interval between your AF runs.

Good point Mike, I do use the focuser module on the screen and I will
have to check that next time.

Odd thing is that the final HFR at focus completion is typically
1.5-2.5. This week I notice the final focus is 1.5-2 range on the module
but then immediately kicks up to 3-4. Pretty sure this is new and I was
wondering whether I need an SGP reinstall…


As Terry said above HFR is a calculated on a per image basis, and for a given imaging train, is highly influenced by focus accuracy. However focus accuracy is not the sole determinant of a measured HFR. For example:

a) Temperature can change during a long exposure affecting focus accuracy and hence HFR. Over a longish sequence of exposures without a refocus, this could become significant.
b) Gusting wind or other issues may affect tracking accuracy between exposures causing star flux to become elongated or smeared over a different number of pixels thereby affecting an HFR measurement
c) Star flux reaching your sensor over a period of time can be affected by varying atmospheric transparency and height of target over horizon. As a result of this fainter stars may fail to reach the required threshold to be included in a given HFR calculation and thereby affecting the result.

While in stable conditions and barring tracking mishaps I would expect there to be minimal variation in measured HFRs across a modest sequence of exposures, over a longer sequence there is clearly greater opportunity for other HFR influencers to become more apparent.

For an unexpectedly large change in HFR between frames in the first instance I would be inclined to suspect a major seeing fluctuation or some incident affecting tracking rather then anything in SGP.

My thoughts for what they are worth.

Appreciate the analysis. It all makes sense. The night I noticed it
there may have been big fluctuations of HFR due to the jet stream being
directly over me. I will give it another check on my next run.