PHD2, Alt/Az mounts and Rotator options

[eow...@gmail.com]

I have a planewave L350/CDK14 mounted Alt/Az with a PlaneWave rotator. When I did photometry I never bothered to guide. But I have switched to spectroscopy and for that I do need to guide.

I notice that when I connect to PDH2 I have an option to connect PHD2 to the rotator. I did not do that and while guiding was good, the spectroscopy was poor, ADU count on the spectrum was very low given the exposure time.

Is it necessary to connect to the Rotator  in PHD2 as well as the rotator on the CDK14 when I image?

I can imagine that this is the case.

Thanks for the help,

Ed

[Dale Ghent]

First, Alt-Az is not the axis orientation intended for use with guiding. Guiding is oriented on the basis of one physical axis representing declination and the other representing RA; it speaks only on N/S/E/W terms. I do not know how PWI4 handles pulseguide commands in those directions while in an alt-az configuration but it would have to work some magic internally to translate N/S/E/W pulseguide commands into corrective movements for not 1 axis, but *3* (alt, az, and rotation). Generally speaking, the decision to run in an alt-az configuration implies a conscious decision to dispense with guiding.

But, yes, if you have a rotator in any case, PHD2 needs to be made aware of it so that it may adjust its calibration model to match the position angle of the sky as seen by the guide camera. Connecting PHD2 to the rotator driver is mandatory in that case.

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[eow...@gmail.com]

Many thanks for the quick response. The information is very helpful. I am not sure how the magic works with the Planewave system, but it does guide and I will try connecting PHD2 to the rotator on my next run. You are quite right about alt-az and dispensing with guiding, I have had this rig for several years and never guided when I was doing photometry. Spectroscopy is a bit different.

Ed

[Brian Valente]

Ed

>>>Is it necessary to connect to the Rotator  in PHD2 as well as the rotator on the CDK14 when I image?

Yes, if you are rotating the image train after calibrating, that invalidates the calibration. Attaching the rotator allows PHD to compensate for changes to rotation. 

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Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[Bryan]

A bit of a side topic.  Interesting article about guiding an alt-az mount.  https://telescopemount.org/guiding-the-panther-alt-az-mount-with-phd2/

Bryan

[eow...@gmail.com]

Actually the point about calibrating is very important. At least with a Planewave it needs to know how to move in different parts of the sky, so recalibration is more-or-less manditory unless your next target is pretty close to the one you are sitting on now. PHD2 complains, especially when the target is high in declination, but I disregard it and recalibrate when I more more than 5 degrees.

I tried the rotator connection last night. Yes, it is needed and many thanks for that tip.

Ed

[Bruce Waddington]

Maybe you could expand on your current solution.  Does the PW rotator move continuously while tracking in altitude and azimuth in order to counteract field rotation?  PHD2 only reads the rotator position when guiding is started so it is unaware what the rotator is doing while guiding.  How long can you run on-target with acceptable guiding results?  If you don't connect PHD2 to the rotator but re-calibrate for every target, what difference does it make?

Thanks,

Bruce

[Brian Valente]

>>>> Maybe you could expand on your current solution.

yes some additional info would help. Do you have a wedge for the mount?

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[eow...@gmail.com]

There is no wedge, as said, the mount is Alt-Az, not equitorial.

Yes, the mount derotates continuiously. For regular imaging (not spectroscopy) I never guided at all. Errors were always less the seeing even when the seeing was good for the site.

Yes, I have found that recalibrating for every target that is at some (yet to be determined) distance from the last target is needed. If not, guiding fails. Since "some distance" is undefined, I do it for every target more than five degrees away from the last target.

It is too early in my experiments to say definitively how well PHD2 is doing in guiding with the rotator option checked, keeping a star's center exactly on a 23um slit if not a trivial proposition, but I would say at least 300 seconds without significant manual correction and it seems (no quantative data) that is does better than if the option in PHD2 is not checked.

Ed