Guiding Failure

Hardware and software setups, autoguiding and plate solving parameters, etc..
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Posts: 20
Joined: Mon Nov 12, 2018 12:53 pm

Mon May 18, 2020 2:49 pm

I am using an AP 1100 with APCC. and a Lodestar X2 as a guider. Recently I have not been able to auto guide successfully. I do slit spectroscopy using the fixed position option. Measuring the X and Y position on a image of the guide camera agrees with the X and Y positions used by PHD2
The slit is not in the center of the full size image. I set the star close the the designated X Y position and hit start and the Guide image appears in the 40 pixel image but not centered see the attached "Guide Star" shot. Note that the Cross hair is not on the slit. At some point the star is lost and tracking inspector reports excessive errors.

Should the green cross hair be on the slit?
I assume after a successful calibration Prism knows the location and direction of RA and DEC.
The AP mount knows which side the tube is on so I assume I do not need to check either of those boxes

tracking inspector.png
Guide_Star.png (34.93 KiB) Viewed 630 times
Posts: 53
Joined: Thu Dec 14, 2017 4:49 am

Wed Jun 03, 2020 1:18 pm

Hi there
I also do slit spectroscopy and use 'Advanced guiding' in Prism with a specific set point.
The green cross hairs show your chosen set point, which should be estimated by holding the cursor over the star when its on the slit (as observed in your guide cam via a looped preview). The coordinate readout is given in the lower left corner of the image. If this point is within reasnable bounds of the stars image, the latter will get 'captured' by the guiding and you should be away. Be careful about using alternative readouts as the origin of the image coordinates may be different.

Some points to note:
You will benefit from having small values of RA 'agression', especially with faint stars that dont 'spill over' across the centre of the slit. Otherwise teh guiding oscillates back and forth across teh slit.
Try to use as long an exposutre as possibel on teh guide cam. Thsi gives a more solid star shape to work on (and also avoids chaing the seeing). I ay typically use 6-9 seconds for mag 8+ stars
You can dynamically alter the set point after guiding has commenced (there is a tab for it in the guiding panel). This is really useful as its ofen not clear at first where the precise centre of the slit is. Use the scrolling controls available there rather than putting a number directly in the box. This feature is also useful for 'foooling' the guiding if it is struggling to accomodate a nearby star as well, or with a spurious stellar image caused by internal refection across teh slit-mirror.

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