Re: Pointing Model vs. OAG with PHD2


Roland Christen
 

I have used a pointing model at my observatory which has a 175 F8 refractor. I find that for imaging up to 20 minutes the model does just fine but my mount has encoders and essentially zero periodic error. For a non-encoder mount you might be more limited in time to perhaps 10 minutes unless you have excellent periodic error corrections.

The main problem with SCT scopes is mirror flop which cannot be modeled since it is a random error. Using a separate guide scope again limits the exposure time because of mirror flop which causes differential pointing errors between the two scopes (differential flex). The only way to assure proper imaging with SCTs at their native focal length is off-axis guiding.

Rolando



-----Original Message-----
From: Bruce Donzanti <donza2735@...>
To: main <main@ap-gto.groups.io>
Sent: Thu, Apr 23, 2020 9:37 am
Subject: [ap-gto] Pointing Model vs. OAG with PHD2

I would feedback from those doing pointing models if would be advantageous for me to try it based on my specific setup on stick with OAG.

I am in a permanent observatory above my garage roof which is about 12 feet above street level. In the observatory, my C11" EdgeHD (with an 80mm piggybacked onto it) is on an AP1100 which is on top of a Pier Tech adjustable steel pier (can be raised 18 inches).  The steel pier lies on top of a cement pier that runs down below street level.  So, the scope itself is about 8 ft above the observatory floor, requiring me to use safety ladder to get to it but just about everything is automated now with few exceptions. My setup is well built, has excellent polar alignment and cable management.  I would have to recheck my PE on my 1.5 year old AP1100.  For guiding, I have been using PHD2 with an OAG for about 2 years but I find it to be finicky and I often have to get up on the ladder to re-focus the guide camera (one of the few non-automated steps).  Seeing and transparency in my area is generally not very good and I do not get many clear nights.  Typically, my seeing is around below average/average with many nights of poor and few above average/excellent. I am thinking of using the 80mm as the guide scope for the C11 since it has an autofocuser but I am still dealing with PHD2 and my so-so skies. 

With this as an introduction, would taking the time to learn to build a pointing model save me a lot of aggravation (i.e.,up and own on the ladder for the OAG adjustments or using the less effective 80mm) in the long run or am I just exchanging one set of problems for potential another set?  Would a pointing model be more or less advantageous for guiding in my sky conditions or just the same as using an OAG approach?

Bruce

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