Guidescope : 125mm focal length.
Guidecamera: ZWO ASI120MM mini
To answer your question: I just found it more convenient and less straining on my DSLR to use the guide camera. As the FOV of both are more or less comparable, I figured it was good idea idea.
Do you see any problems with this approach ?
I see one point that could lead to some questions : if the guide is not exactly aligned with the primary scope or DSLR, the alignement might not be ok ? I am not sure at all, but i would say yes. Maybe someone else could better answer this question.
SW 150/750P (Newtownien) - SW ED72/420 on HEQ5 pro using EqMod
ASI071 pro - CC Baader MPCC III (for newtownien)
Guiding with 60mm refractor and QHY5 II mono
Kstars/Ekos on LENOVO Thinkcenter M71 - Linux Mint 21 & Stellarrmate Pi4 8GB
In the absence of differential flexure (and it would need to be variable), there would seem to be no reason why you should get variable results just from using the guide scope during PA. A couple more questions for you:
a. Can you describe which flavor of PA you are using (direct polaris solve, or rotations at off-polaris sky positions)?
b. Have you tried to do a drift align with Phd2 for comparison?
I had a similar problem with the Ekos PA software on my Celestron CGX-L mount (rotations away from polaris approach). I also experienced variable results, and lost faith / gave up on that software. A PHD2 drift align solved this variability issue for me, so I suspect the implementation in Ekos (vs the mount or other possibilities). You might consider trying a PHD2 drift align, followed by running the Ekos PA routine (multiple times without driving either knob on the RA or DEC axis) to do the same variability analysis as you've already done. That will be a good data point for both quantifying / verifying that Ekos returns a similar numerical value as is reported by PHD2, AND verifying that Ekos returns the same value over multiple runs (since the knobs aren't changing)..... CS Doug
In response to your questions:
a. I am using the EKOS polar alignment. I do the following: I try to get more or less polar aligned and then I use the procedure to take pictures at -60 degrees, 0 degrees and +60 degrees RA. I make the camera point straight ahead so declination is 90 degrees then I guess.
b. No, I have not but I plan to.
It's interesting you point out you had a similar experience with the Ekos PA. Can other people confirm this as well ? I will look at the PHD2 align when time and weather permit.
Thanks for your feedback.
Taking pictures spanning 120 degrees could be one reason to this variability. I never attempted this before, often just limited myself to 15 or 30 degrees per rotation. While I have seen variability before, it's often within tolerance. Today, I will try to repeat this for a few times and record the results. For reference, I have EQ8 mounted on a pier.
I actually rotate by 60 degrees twice because I thought it would be _more_ accurate.
Note that I am doing a manual slew so by no means I am accurately getting 60 degrees every time. I am assuming this does not matter that much ? Or is this a wrong assumption ?
It could be that a manual slew would *not* work (although in principle the software *should* overcome this by not using the user rotation value for anything other than a mount command and then figuring out the "real" rotation by plate solve). If the SW isn't already doing this, your manual slew could be part of the problem. Again, although slower, I'd recommend a careful baseline PHD2 drift align (once), followed by another set of Ekos PA runs (back-to-back) to compare both precision and variability. In case it's not obvious, *don't* change the Alt or Az mount knobs after the PHD2 align for any of the Ekos repetitions! Then report your findings. Good luck and CS, Doug
Edit: Also, to be fair to Ekos, please be careful with your PHD2 drift align! You need a good/flat (stable) drift over at least a few minutes! Also, if you don't have PEC sorted out for your mount, that could pose problems too. It's worth spending some time on PEC too to beat that component of variability down....
I usually do 2-3 runs of the new PA routine with Ekos (no polaris view) and find it very consistent between runs, maybe some differences of max 1’ between iterations. I’ve run the routine close to the pole and in other regions using my guide scope with 120mm FL and the guide camera QHY5LIIM. I have to say though, that I need to set the FL to 128mm in order to match the FOV reported by Ekos, seems strange to me to have this 7% difference, however plate solving is working fine with this FL so I have been using that since the beginning.
However I then run PHD2 guiding assistant (setting FL as 128mm as well) and the error reported is much higher. In my last try after getting under 1’ error with Ekos i got 45’ of error in PHD2, which I think was correct as the drift was evident. I then run drift alignment in PHD2 and got under 3’ error so I was able to continue with my imaging session. Unfortunately I wasn’t able to run Ekos PA right after to get the error reported as it refused to plate solve any image (didn’t happen to me before)
Any ideas of what could be the issue? Maybe related to the FL not being set correctly?
In my next session I will definitely run PHD2 drift alignment first and then try Ekos PA, without touching the knobs but just trying different FLs to see if can get close results between both methods.
I'd recommend setting the focal length to the actual system focal length and not changing it (neither for Ekos nor for PHD2). Changing the FL changes the image scale, so of course the error will change too. Just set the proper focal length values in both software, and let the chips fall where they may. Otherwise, you're just minimizing/normalizing the error to an adjusted/fake focal length.