How hard is this hobby?


#1

I’m new to SGP, and trying out my kit with it. Last night was a run to try auto focus. Having tried and failed miserably with Maxim in the past, so was pleasantly surprised to find sgp gave nice repeatable v curves.
So tonight, after a bank of clouds, I set up to see if my filters were parfocal. Tried for three hours, and could not get a single good v curve, on lumminence, let alone the other filters. Thinking of taking up knitting, at least I could do it in the warm

Huw


#2

Huw - can you share your settings - aperture, microns per step, exposure, filter … - perhaps some screen grabs. I had issues when I first started. Coming from Maxim/Focusmax, I was used to short exposures. Initially with SGP I did not give it sufficient exposure - the faint donuts were being misread as low HFD. Once I gave sensible exposures (typically about 10s for luminance, 15 for RGB and 30 for narrow band, depending on the stars - I have had, fingers crossed, very very few dropouts.


#3

The main reason why autofocus doesn’t work for me is when the exposure time
is too short. Or that I’m nowhere near the focal point.

 MarkS

#4

We’ve all been there Huw. Those who persevere are the ones who eventually get good images. Like buzz says we need much more information regarding equipment and settings before we can be of much help.

If you say you had perfect V curves at one time, what changed between that and the failure?


#5

Hmmm, now that I’m in the warm, I can apply a bit of reasoning. It had been raining earlier this evening, and then went very cold. There were clouds initially passing over, which cleared.
I’m imaging at f4, with a focuser that gives 10 microns per step. CFZ for f4 should be about 50 microns, I set autofocus every 10 steps, and 7 positions. The resulting V curves were mostly zig zag lines, not Vs. Possibly very bad seeing would cause this, will wait for another clear night and try again, with present weather, will update you next year.

Huw


#6

Few more details, imaging at 300mm fl, with 9 micron pixels. Both nights I was using 1 second exposures on L filter only, both 2x2 and un-binned.
H


#7

Is it possible that the drop in temperature changed the focus point so much that SGP couldn’t get a decent start? Do you get roughly in focus before running autofocus?

When AF runs and you get your first data point, how different is the HFR measurement from where rough focus is? A good rule of thumb is to aim for a HFR value that is 3-4x where your in focus HFR value is. So if good focus is around 1, your HFR V curve should go out to about 4, back down to 1 and then back up to 4 (in theory!). If you are not getting good V curves try increasing or decreasing your step size and/or # of positions. Your step size of 10 and 7 positions seems rather conservative to me. Try increasing both and see what happens.

What scope? AF issues can be vastly different for refractors vs scopes with moving mirrors for example.


#8

I would definitely increase your exposure time.


#9

Trying to test kit under patchy cloud is incredibly frustrating because you never know if changes are because of wisps of cloud or with the equipment.

What telescope are you using? Does it have a central obstruction?

There does seem to be some concern about how the auto focus algorithm in SGP works if you have a mount with an obstruction because once it sees a doughnut it tends to lock onto one side of the doughnut instead of the whole thing and give smaller HFR values.


#10

V curves started about 3- 4 ish, down to 1. Scope is a refractor. Transparency was pants, so quite probably thin high cloud was playing its part. Will try again when we get a clear night, whenever that might be, hopefully before the trial copy of SGP runs out.


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