Callibration Steps

SFn8CieR

Hi, Can someone please explain the calibration Step and MAX RA and DEC Duration figures so that I will know how to use them to guide better with my scope?

The current AA+ defaults are 2000, 2000, 2000 and I have read that if you are using a ZWO mini Guide scope with FL=120mm these figures should be in the vicinity of 8000 to 10000? Is that right?

I know when you do a calibration, these higher values move the mount more than if you use the default 2000 values and requires less Steps to calibrate. So is that better with less steps to calibrate?

I read somewhere that if you are using an OAG with the FL of the Guider being the same as the FL of the scope, then these figures should be in the range of 250 to 500ms. Is that right? I am just trying to understand all of this.
thanks..

Kevin_A

No…. 8000ms (8s) is grossly inaccurate and just plain wrong.

Kevin_A

A guide scope of 120mm using a 120mm mini camera that has 3.75um pixels is around 2000ms, a guide scope of 162mm using the same camera is 1350ms, 240mm guide scope is around 900ms, 448mm scope (OAG) is about 500ms and a 644mm scope using an OAG is about 300ms. These numbers are based on Phd2 moving the guide star 25pixels in 12 steps which is the nominal amount for accurate results. Always calibrate near the meridian in the south and then reuse the calibration for the whole session.

SFn8CieR

Thank you for explaining that.
Is there a formula for working this out (Guide Scope FL, Cam Pixel size, and calibration step) or is it based on PH2 moving the star 12 pixels?
I dont use PH2 for guiding, just the AA+ which is based on PH2 I believe.
So when you say to calibrate in the south near the meridian then I can use that for imaging at any location?
thanks again..

Kevin_A

SFn8CieR yes, you can use that calibration in the south for any location afterwards in the sky. Just click the button in the guiding camera page for re-using existing calibration in asiair after you calibrate. Make sure when you start calibrating you clear calibration in the guide graph page.
I do not use phd2 much anymore but have it on my laptop so I can still open it and go into the guiding tab that will allow you to punch in the numbers and output the correct step size. If you reduce the step size slightly it will slightly increase the number of steps to maybe 14… and that is not a bad thing as it is slightly even more accurate, but not necessary. If you are imaging in the east, calibrate in the south just east of the meridian… same applies to west… just west of the meridian in the south.

SFn8CieR

Kevin_A
Thank you so much for that. My guiding has been much better lately around 0.2 to 0.5 rms thanks to your advice!
When you said above for a 448mm scope (OAG) its about 500ms, I take it you mean using an OAG on a scope with FL of 448? So do I change the all 3 calibration steps to 448 (from 2000,2000,2000)? or have I got that wrong? What about MAX and DEC duration figures, do they change as well or is that the same thing?
cheers..

Kevin_A

SFn8CieR the focal length when using an Oag is the focal length of the scope.
So phd2 calculates that any 448mm scope using a asi120mm mini camera with a 3.75um pixel at 500ms for the step size. It does not care if it is a guide scope or Oag. A Oag will require more gain however as it has less light thru the main scope. The maximum Ra duration and Dec duration should be based on your mounts worst slope. So your mount guide setup could be set at 500 step size, 400ms Ra max duration, 400ms Dec max duration… using 1 second exposures with Ra aggression at 45% and Dec aggression at 40%. Those would be typical settings.
Calibration step size is based on guide camera and scope only and max durations and aggressions are based on mount slope error correction requirement.

Kevin_A

SFn8CieR I have created a page under mounts section for typical AM5 setup numbers.
The only one you need to keep as is, is Calibration Step Size as we have calculated that as 500ms.
Here is a link…. Cheers!

https://bbs.zwoastro.com/d/17446-what-you-need-to-know-about-guiding-swg-mounts/27

SFn8CieR

Hi Kevin, thank you again,
I am using an Ioptron CEM70EC2 mount with a Sky Rover 80mm ED APO scope (native FL=480) with 0.8 reducer (FL=384mm) with a 30mm guidescope FL=125 and ASI120 mini guide camera, so from you previously said, my Step size should be 2000 with MAX RA & DEC Duration being the same, is that right? I think I understand this and where you got it from. I need to learn PHD2 to figure this stuff out too.

I am also going to image with my C11 at F10 and F6.3 one day with OAG, so native FL=2800 and with reducer FL=1764 with ASI 174 Guide Camera with 5.86um pixels, what figures would you recommend here?

If I buy a Hyperstar V4 the C11's FL will be 532mm at F1.9, so I would also need to buy a larger guidescope at FL 500mm or so and use the ASI 174 on that. So extrapolating from your initial figures above the step size would be around 530? Would that be about right?

Also do you know if there some info on how to guide a mount like mine with encoders. I have RA & DEC Aggression set to 20% and calibration data as per your figures and it works ok. It would be nice to know how guide exposure time affects these encoder mount as I would assume the less corrections (ie longer exposure time) would be better, but am limited to max 5 secs on ASIAIR and I really could not tell if it made a difference, sometimes better, sometimes worse.

thanks mate!
cheers..

Kevin_A

SFn8CieR no…. The step size and durations are not the same.

Kevin_A

SFn8CieR on the cem 70…. 2000 step size, 2500 Ra duration, 2500 Dec duration.
For a worm gear mount the exposure time would be 2-3 seconds. Harmonic mounts 0.5-1.0 seconds.
I thought you were using an harmonic mount so since you are not then 2-3s guide exposures.
Max Durations just need to be less than exposure lengths so 2500ms for 3s and 1500-2000ms for 2s exposures. As far as encoders go… I am not an expert on these.

SFn8CieR

Kevin_A
Hey Kevin,
I just downloaded PHD2.
You recently said " I do not use phd2 much anymore but have it on my laptop so I can still open it and go into the guiding tab that will allow you to punch in the numbers and output the correct step size" .
How do you use it if its not connected to your equipment. It wont let me do anything. No matter if I go to Calibration or Guiding Assistant it want to connect to the equipment. I run everything from the ASIAIR + and so not sure what to do.

SFn8CieR

Thanks Kevin, you're a champ!!

Kevin_A

SFn8CieR you just create a profile with a guide cam type, focal length with no mount then open it, then hit the brain icon at the bottom and go to the guiding tab. You do not need to run it, just open it. Punch the numbers in the advanced tab under guiding tab.

Kevin_A

SFn8CieR you hook up nothing, you do not even run it… just open phd2, create profile, go to brain at bottom of page and enter pixel size and focal length of guide scope. It will give you a suggested cal step size.

Kevin_A

SFn8CieR here are the steps… open phd2 and hit brain.

You can fill in the guide scope focal length and pixel size in the very last page if you wish to try changes for different setups.
Kev

SFn8CieR

Hi Kevin,
Got it working, thanks again, you are very helpful !!!
Wow this is a game changer for me and I have learned a lot.
Cannot thank you enough..
cheers..