AM3 Excessive RMS Error

Peetconrad

Hi there.

I have recently purchased an AM3 mount along with an Asiair Mini. I am relatively new to astro imaging. Over the few clear nights I have had, my RMS error has been far in excess of what I was expecting from the mount, and I am hoping someone with greater technical expertise will be able to help to make it lower

My total RMS error has been averaging around 1.5". I have seen other AM3/5 users regularly achieving 0.5@ RMS, I would like to achieve similar, or near enough. I image in Bortle 4 skies.

The equipment I use is as follows: -

MAIN SCOPE: WO Zenithstar 73ii APO
MOUNT: ZWO AM3
GUIDESCOPE: SVbony SV165 120mm F4
GUIDE CAMERA: ASI120MM Mini
ASIAIR MINI

My current settings in the ASIAIR app are as follows: -

GAIN: 29
CALIBRATION STEP: 2000ms
MAX DEC DURATION: 2000ms
MAX RA DURATION: 2000ms
EXP: 2 secs

Guide Camera Bin2: Off
Stability: 2' - Time 5s

If anybody needs more setting information to address my issue, by all means, please do let me know. I spent a long time ensuring an accurate polar align (within 6" arc seconds according to the app). The tripod is mounted on a solid, level, concrete platform. There was no wind to speak of and my setup is relatively light (certainly below the limit of the mount). I have checked my cables aren't pulling and snagging. I power the mount directly using a 12V power supply via mains electricity.

As far as I am aware, these settings are the default standard. I also have a guide log from my imaging session the previous evening where I was trying to change things (without much success). I would consider 1.5@ RMS in clear skies to be a little excessive. If anyone can help, I'd be highly grateful.

Brgds

Peet

Kevin_A

Your guide setting are the first problem and you can improve that further in the future by getting a bigger guide scope and better guide camera. Harmonic mounts need fast 0.5-1.0s guiding. Your calibration step size is ok but the max durations need to drop to about 500ms on both Ra and Dec. Aggressions need to be about 40-65% on both and start off usng 1s guide exposures. Then try 0.5s guide exposures. The problem with shorter exposures required by the AM3 and AM5 is that your guide scope is small so not good signal and the 120 guide camera is gain limited. Bump up your gain to about 70 at 1s exposures. These mounts are too fast to use slow 2s guide exposures. All harmonic mounts react differently so these settings are just a good base setting and you will need to adjust accordingly to your own mounts characteristics. Cheers!

Peetconrad

Hi Kevin_A

Thank you for your response :-) I'll be implementing the changes you suggested once I get clear skies... It may be a while.

From other advice I've read elsewhere I most definitely have my guiding exposures too slow, and my max durations much too high. I was wondering, seeing as you seem to be able to offer advice on the subject, what these parameters actually mean (and do)... Particularly the durations of the DEC/RA and calibration step? Apologies, am not looking to be 'spoon fed', but as I said in my post I am new to astrophotography, so what may seem like 'obvious' terms to seasoned guiders is actually brand new to me. I really want to learn more, hence my question. If you could point me to a beginners guide, or any other beginner friendly source, I'd be most grateful.

Once again thank you so much for your input. Those simple suggested settings were exactly what I was looking for, as I don't have the experience to understand them myself... yet :-)

peet

MSki

Kevin_A Hey Kevin, thanks for the suggestions with regards to the settings for guide camera. I currently have the ZWO ASI120mm and the ZWO 30mm guide scope. With your suggestions for ASIAIR settings, I was able to better my RMS from 0.9 to 0.5. I am considering purchasing the Askar 52mm guide scope and upgrading my camera. Do you feel I will get much performance increase by doing this? And would you have a suggestion for a guide camera?
Thanks
Mark

Kevin_A

Peetconrad max durations are basically the maximum correction pulse duration time.
The best way is to go online and look up phd2 best practices, that will show you how to calculate calibration step size… but phd2 was mainly designed around worm gear mounts so everything applies except for the 2-3s guide exposure lengths. Harmonic drives need 1s or shorter.

Kevin_A

MSki I have used a few different guide scopes and bigger apertures do better, but quality matters. Some mounts need all the help they can get while others need no help and will guide with any old setup. I did try using a premium 50mm apo scope as a guide scope and it worked well. The only factor I found annoying was the focus did shift over the night due to all the glass elements climatizing when it got colder in the night. Otherwise it gave excellent stars. I am about to test out the premium Askar SD 52mm guidescope once my skies clear. Asker offers a regular and SD version of the same aperture. The SD version is a doublet too with 1 SD glass element for improved clarity. With only 2 glass elements I think that focus shift will be far less than my other apo guidescope that had a flattener too. I still prefer my asi290mm mini as the optimal guide camera but alas they stoped making it and replaced it with a bigger pixel 220mm mini. The 290 pixels were smaller which is better in my opinion. The asi120mm mini can work fine but it is gain limited. I also have a asi678mm with 2um pixels and it works great but you need to use it in a usb3 port or it will not run well.
Cheers and I am glad your numbers were reduced so much by my suggestions! Glad to help out!
Adding a better guide scope will help but you are already guiding near the maximum I would expect from this grade of mount. A bigger guide scope will probably just make your numbers more consistant near 0.5rms…. which is a very respectful result already!