Hi John:

I have run into a situation where CCDAP is selecting an inappropriate guide star. Is there any way to force selection of a particular guide star?

Don

Don,

CCDSoft, Maxim and CCDAP all select the brightest star in the guider FOV for guiding. If there is a nearby star of similar brightness, you need to adjust the target framing with an eye to the guider FOVI so that the nearby star is well away from the guider FOVI.

There is an option in the meridian flip set up box for automatic selection of the guide star. If this box is unchecked the program will pause and allow selection of a guide star. Maybe this option could be added to the initial guide star selection process as well?

Hi John:

I think that Joe's suggestion is an excellant one. In some instances reframing is not an option to accomidate a guide star.

Don

Hi Don,

Are you using the standard version of CCDAP3?

Hi Frank:

No. I am using the mult-target version.

Don

Hi Don,

In reading this thread, I'm curious about about the definition of an "inappropriate" guidestar and if manual selection is needed, what criteria would cause a guidestar, other than the one with the highest SNR to be used? My limited understanding of guiding is that the higher the SNR the better the centroiding routines work, usually resulting in better guiding. Higher SNR, I believe, translates into higher ADU of the guidestar, which brings it back to selecting the brightest star in the FOV should give the highest SNR.

I guess what I am trying to understand is what happens that would cause the normally brightest star being selected for guiding to be wrong. TIA.

Frank:

The instance that I have run into most recently are stars that are too close togeather that confuse the autoguide algorithm.

Don

Hi Don,

One possible workaround way to handle that would be when the guidestar selection is made by CCDAP3 and you have that scenario, before the exposure starts, go to CCDSoft and abort the guider, take a guider image, select the guidestar you want to use and restart guiding. As long as you don't need to focus, I think the dithering routines will work on the currently selected guidestar, but not 100% sure. If I get some clear skies tonight, I'll try and confirm the procedure.

Frank's comments are exactly correct. However, there appears to be some confusion about how CCDAP works in selecting a guide star. Let me explain a bit.

1. A guide star selection is best performed in TheSky. If you are concerned about a possible double-star being selected, then use TheSky's object information box to determine if it is a double. Even if it is, it would be problematic only if both components have very similar magnitude. If that is the case, then you need to reframe the target by changing the FOVI, either in translation, rotation or both. I have successfully guided across the meridian with dissimilar magnitude doubles in the past.

2. During dithering, the guder is stopped and restarted. CCDSoft keeps the reduced frame guide box during this sequence; Maxim does not, i.e., Maxim takes a full size guide frame and reselects the guide star. Again, this is not an issue if the guide star is chosen properly in step 1.

3. During an off-target focusing and return slew, a full frame guide exposure is taken to reselect the guide star, regardless of camera control program used. This is the only change in this regard from 3.30.x to 3.31.x. Again, this is not an issue if the guide star is chosen properly in step 1.

4. After a meridian flip, a full frame guide exposure is taken to reselect the guide star, regardless of camera control program used, as described in 3, above. This has been the method of operation since CCDAutoPilot v2.x!

The pause for slection of a guide star, which occurs when Auto guide star select is not checked, allows users running semi-attended to adjust pointing to find a guide star. This would happen for example when someone has a narrow field guider without a rotator and needs to rotate manually to find another guide star. Clearly this is not an unattended operation and AFAIK, very few people use this option. In fact, in looking at the code, I noticed a programming error that would have prevented it from being successful! (I'll fix this in the next maintenance release.)

So, bottom line, when running unattended, whatever guide star you select for your target framing will be maintained during off-target focus slews and meridian flips (if you are using a fork mount or a rotator). Even without a rotator, if you plan your target framing such that a suitable guide star is 180° away from your chosen one, the automatic guide star acquisition capability will keep you going.

BTW, in some early testing, it appears Paramount users having ProTrack enabled may not get as precise a post-meridian positional placement as those having it disabled. All of my testing has been with ProTrack disabled.