Re: Nudge Guiding last night


Roland Christen
 

PHD2 can be set up the same way. Very low Min Move. Very low aggression. I like to use 10 sec guide exposure in Maxim in order to minimize random sky motions. Set the delay between guide exposures to 5000 msec (5 sec) or even more if the seeing is very steady. MaximDL has good algorithm for finding the centroid even if the guide star is saturated, as it was in my case. I'm not sure if PHD2 can do that, so it would have to pick a fainter star.

One thing to be careful about is good cable management. You cannot have cables dragging or you will get sudden jumps that take longer to settle because of the slow guide cadence. I would do this type of guiding only for longish exposures, such as 20 min and longer for narrowband deep sky. If you dither, it will take longer to settle also. Make sure that the main camera exposure length doesn't saturate the object that you are recording. In my case the 1hr exposure did not saturate the tiny squiggles inside the Bubble nebula. It would very much saturate the core of a typical galaxy with a wideband LRGB filter set.

Rolando



-----Original Message-----
From: Tony Benjamin <tonybenjamin@...>
To: main@ap-gto.groups.io
Sent: Tue, Sep 22, 2020 1:55 pm
Subject: Re: [ap-gto] Nudge Guiding last night

You are using Maxim…how would you set this up in PHD2? Just very low aggression and greater than 5 sec exposure time?
 
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: September 22, 2020 10:34 AM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Nudge Guiding last night
 
Hi Astronuts,
 
I posted my guiding results from a 5 hour imaging session last night on Astrobin:
 
I wanted to show how an Absolute Encoder Mount can enhance the guide accuracy and thereby improve the image resolution when using medium to long focal length telescopes.
 
There are always discussions on the various newsgroups that question whether an encoder mount is really necessary if you're going to guide anyway. The idea doesn't make sense until you realize that the encoders provide the mount axes with a very precise position according to the input commands of the servo controller. It means that if you can detect sub-arc sec errors in the position of the guide star, you can correct for them at a very precise level. You can also take 90% of the guide burden away for really long exposures by modeling, and thus driving both axes at a custom rate according to the model.
 
The encoder loop is not only accurate but very linear down to the sub-arc sec level. For example, a non-encoder mount can have non-linear response and discontinuity in Dec during direction reversals, even if the gears are belt driven. That can cause delays and even oscillations around the zero point. All that goes away with encoder control.
 
Last night was not especially great seeing, Clear Sky Clock rated it as a 3 out of 5, but it was good enough for the nudge guiding plus model tracking to track down to the 0.2 arc sec Pk level. Without the model and just normal full guiding (2 - 3 sec guide exposure) the system would be guiding at around 0.5 arc sec Pk (which isn't shabby).
 
Rolando

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