Setting up off axis guider

[Craig a]

Received my zwo OAG and in the process of setting it up, my question is when measuring the distance from prism to guide cam sensor to get it the same as the main camera sensor, do I measure from the bottom of the prism or centre of the prism?  

Edited December 10, 2020 by Craig a

[vlaiv]

49 minutes ago, Craig a said:

Received my zwo OAG and in the process of setting it up, my question is when measuring the distance from prism to guide cam sensor to get it the same as the main camera sensor, do I measure from the bottom of the prism or centre of the prism?  

Center of the prism, but don't worry about it much, you won't be able to dial in exact distance due to field curvature - just get it reasonably close and then you'll focus guider properly under the stars.

First you focus imaging camera and then adjust guider to match it. By the way, you don't need to be perfect in your guide camera focus - you can even guide a bit better when slightly unfocused. Algorithm will still calculate proper centroid but seeing effects will be smaller.

[Craig a]

Thanks I’ve done the measurements and think I’ve got it reasonably close, I’ve got to bring the rig inside this weekend to fit the rail kit to the neq6 but from my window I can see the Welsh hills about 15 miles away so I can finish the focusing in daytime on them 

[ollypenrice]

8 hours ago, vlaiv said:

 

 By the way, you don't need to be perfect in your guide camera focus - you can even guide a bit better when slightly unfocused. Algorithm will still calculate proper centroid but seeing effects will be smaller.

I'm told that this is no longer what's recommended for PHD2. It was certainly what Craig Stark said for earlier versions but I gather that sharp focus is now preferred. (For all that, I haven't touched my guide scope focus literally in years and it seems fine!)

Olly

[Craig a]

Thanks for that olly, I shall try to get focus as sharp as possible then, I’m really hoping I will be able to pick stars up in what’s going to be a really small fov on the OAG 

[michael8554]

The PHD2 developers expect best guidescope focus, and a PHD2 Guide Assistant run will suggest improving focus if the HFD is high.

Remember once you have the imaging and guiding cameras both in focus at the same time, in future you only need to adjust the imaging camera focus - the guide camera will then be in focus too.

Michael

[wimvb]

All the above  about focus is for a guide scope, basically. For an oag you can get elongated/misshaped stars when defocused, and guide programs may not even be able to find a star to guide on, if focus is off. Besides, even guiders (oag or scope) are subject to star light spreading due to atmospheric conditions, and will have a star width at best focus. Just make sure that the guiding camera isn’t too much undersampled.

[prusling]

Another factor you may need to take into account is the variation in main camera focus point when using filters, either where they are non-parfocal or you are using a refractor without good colour correction.

I don't have a problem with this when using a triplet with near-parfocal Baader filters but have found that there is a significant variation in focal distance between Ha and Oiii when using a doublet, and this is enough to put my OAG focus well out.

I could focus the OAG mid-way between the extremes and have it a little out of focus all of the time, but have presently reverted to using a finderguider.

Edited December 11, 2020 by prusling
typo

[Craig a]

Ok thanks all, I managed to focus the OAG on a distant roof I could see, I focused the main cam first then focused the OAG took abit of fiddling but may need a final tweak under the stars but so far it working just need these relentless clouds that have been here for over a month to disappear so I can try it out, but the weather for the foreseeable future is looking rather bleak