Settings to Use with SCT and OAG?

[Mike I.]

For use with my 9.25" EdgeHD SCT at f/10, I recently purchased an off-axis guider and a QHY5L-II camera. I'm wondering what initial settings to use with this setup.  Mount is a Tak EM-200 Temma2. I use a KWIQ guider (similar to Orion mini guider with QHY5 guide cam) when guiding at f/7 (1645mm). With that setup I use the default settings in PHD.  The focal length of the SCT/OAG at f/10 is 2350mm. Image scale with the QHY5L-II is 0.33 arcsec/px.  I've got the calibration steps changed based on PHD2's auto calc step calculator to 100ms, and it calibrates fine at 10 steps. Exposure setting seems good at 2s. Should I increase the minimum move settings for both RA and Dec from the default of .15px, and by how much? How about aggressiveness/hysterisis, and the max. duration for both RA and Dec? I'd be very interested in the settings used by those who have SCTs and use an OAG at f/10. I know the settings are dependent on individual mount characteristics and other factors, such as local seeing, but I"m just looking for initial settings which I can tweak after seeing the log and initial graphs.  Need some clear skies for that, which we haven't had for a while here. Thanks.
Mike

[bw_msgboard]

You should start by using the “new profile wizard” to set up a profile for this new configuration.  That will set up default values for the guiding algorithms as well as the calibration step size.  The wizard uses the best information we have available and you are likely to make a pretty good start with those.  I use them myself with an guider image scale of 4 a-s/px and a focal length of 2540 mm and I get good results.

 

Bruce W.

 

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[Mike I.]

Thanks Bruce. That's what I've done, but looking at the values calculated by the wizard, shouldn't the minimum move setting be greater than the .20px the wizard calculated for a FL of 2350mm and a guider image scale of 0.33 arsec/px or am I wrong about that? Thanks.
Mike

[bw_msgboard]

I just ran the new profile wizard, specified your camera, a focal length of 2350 mm, and got a min move of 0.50 pixels.  Are you sure you specified the correct 2350mm focal length in the wizard?  The wizard wants values for the pixel size and focal length on the first tab, not image scale numbers.

 

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[Mike I.]

Thanks Bruce for the help. I was using v.2.3.0 not 2.3.1, got it right now. I took it out for a test drive last night. I was getting oscillation in RA, with corrections frequently alternating between east and west. I tried lowering the aggressiveness from the wizard's calc of 70 to 30 and increasing RA min. move from .5 to 1.02, but neither had much effect. Any recs to tame the oscillation or is it expected with such a long fl and mediocre seeing?
Mike

[bw_msgboard]

I’d guess this might just be “welcome to the world of long focal length guiding.” L  I went through exactly the same thing just last week.  The adjustments you tried certainly made sense, they are exactly what I did, and I saw no improvement either.  What saved my session was that the seeing simply got better after midnight - I had already reset all the guiding parameters to their starting values, and I was back in business.  There will always be some oscillation in RA of course – I just watch the “RA Oscillation” number and try to keep it in the range of 0.2 to 0.5 over long stretches of guiding.  In your case, what you probably need is just a night of good seeing so you can get a feel for how things should work when conditions are right.  It also helps to develop some independent sense of the seeing conditions, such as the FWHM of stars after a focus run or as shown in an exposure of 10 seconds, or how other imagers are doing at your site that night, stuff like that.  Even the seeing forecast in the clear sky clock can be useful – if the forecast is poor, you can perhaps avoid tearing your hair out when the guiding performance looks bad.  Lastly, you might want to disable corrections for a short time and just watch how the guide star is bouncing around.  On a bad night, you might quickly realize there’s simply no way to guide that stuff out and quit worrying about the guiding parameters.  This all sounds easy, but there’s nothing easy about it when you’re trying to get some imaging done. J 

 

Good luck.

Bruce W.

[Mike I.]

Thanks Bruce for the help and suggestions. Here's one of the test images from first OAG light last night with the 9.25". Stars don't look too bad, I think. Single 6-min. sub taken with a Canon T2i.
Mike

[bw_msgboard]

It looks like you’re on the right track – I’ve certainly seen (and personally captured) much worse… J

 

Bruce