RA guiding: one sided corrections

jaminite
21 Sep, 2016 18:05
Greetings. I'm hoping someone can tell me if I have polar alignment problem or something else. My RA corrections are always in one direction. I know for DEC this can be a good thing.  But in RA, close to 99 of 100 corrections are all in the same direction. PHD2 guiding assistant  tells me my polar alignment is off. I've been polar aligning using the QHY Polemaster camera. I get the alignment dead on, then run it twice more to confirm. Yet, still one sided RA corrections and PHD2 guiding asst saying PA is off. Trees prevent me from doing an ideal drift align. I have tried drift aligning the best I can, but RMS increases, subs are worse, and the QHY Polemaster confirms bad PA. With the one sided corrections, I can usually guide with 1-1.5 arc second RMS. With my setup I can get 10 minute subs with just a hint of star elongation. I'd love to be able consistently guide with less than 1 arc second rms.

I'm using an Orion Atlas, Orion Starshoot autoguder, Orion short tube 80 guidescope, PHD2, ascom pulse guiding, no st4.
Edited 21 Sep, 2016 18:09
jaminite
21 Sep, 2016 18:15
Apologies, I should have posted this in the Get Help discussion group. I'll re-post it there.
siovene
21 Sep, 2016 20:42
This is the correct forum. The "Get Help" forum is for AstroBin related questions. I will delete the other post and the discussion can continue here.
jaminite
22 Sep, 2016 01:04
Ok, thanks Salvatore. smile
Slawomir
22 Sep, 2016 04:14
Hi Jammie, while I do not know what causes your RA corrections to be in one direction only, it would not hurt to check the following: 1. Is the mount being set up on a hard surface or on grass? 2. Any flex in the main OTA's optical train and/or the guidescope? 3. Polar alignment / guiding can be affected by atmospheric refraction. When doing drift alignment, try not to point the telescope below 40-45 degrees. That's all I can think of at the moment, assuming there are no mechanical issues with the mount. Hopefully you will sort it out quickly smile
Edited 22 Sep, 2016 04:15
jaminite
22 Sep, 2016 04:58
Thanks very much for the tips. I do set up on a heavy duty tarp on grass. I have checked the mount bubble periodically to see if one of the legs are sinking. As far as I can tell the mount has remained level throughout each set up.  I've been mulling a pier for some time, maybe it's time. There's no obvious differential flex that I can detect by hand or eye. I have an oag with the needed spacers, just haven't taken the time to sort it. Trees give me no choice but to drift align above 40°.

The fact that all the RA corrections are all in one direction tells me it  has to be something correctable. I'm bothered that phd says my PA is off but the qhy Polemaster says it's dead on.  One of them has to be wrong. Turning off guiding leads to drift, so I'm inclined to believe PHD over Polemaster, but how could Postmaster be so inaccurate? Could the optical tubes not be perpendicular with the mount somehow? One of the rings on the 8 inch newt is a little wonky. If my skies are clear this weekend I'll setup just the mount and guide scope and see if the problem persists. Then finally get the oag working.

Thanks very much for your thoughts. Also, I checked out your images. I really like your work. smile

Jammie
Slawomir
22 Sep, 2016 05:16
Maybe try using at least 300mm by 300mm (more would be better) pavers under each leg? I believe that the optical tube does not need to be perfectly orthogonal with the mount, but a wobbly ring can cause a slow sag in the OTA under gravity so that would be one of the first places to attack smile Glad you like my astro-photos smile
jaminite
22 Sep, 2016 05:58
Thanks again. I love the pavers thought. I should have doing that all along anyway. I'll pick some up. The ring isn't wobbly at all. One ring closes nicely. The other one needs a little pressure to get closed enough around the tube for the bolt to reach the threads on the other side. I'll play with it, reverse its position on the bar and swap places with the other one. Thanks for taking time to help. Much appreciated.

Jammie
Miguel_Morales
24 Sep, 2016 04:16
If you can, it would be even better if you removed the grass that would be under the pavers so that they are in contact with compacted earth. A very tiny bit of movement (sinking) will affect your PA quite a bit. An arc second is very very small.
jaminite
24 Sep, 2016 15:32
Thanks. I'm going out tonight. I'll try it.
KuriousGeorge
26 Sep, 2016 01:57
Likely polar align off. Aim at a star  near where you like to shoot, turn off PHD guiding corrections and adjust AZ & EL so no drift. You should see RA drift before you start. Pier helped me the most by far.
Edited 26 Sep, 2016 01:59
keving
21 Aug, 2017 13:26
There was a discussion on this in relation to my mount - the MESU 200…
I had an imbalance in RA corrections and  it  was due to the drive electronics (from Scitech) being slightly miscalibrated so it was trying to drive the RA too fast/slow. I tweaked this (after doing a measurement of drift with  unguided star) and I could correct the imbalance.
Are  you using EQMOD? I think there are some settings buried deep in there that allow you to fine tune the RA drive rate…?
 
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