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Off Axis focus is challenging me!

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#1 EAF

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Posted 14 September 2019 - 01:10 PM

 I have a ZWO ASI294, a ZWO ASI174, and a ZWO off-axis guider unit.  I have made numerous attempts to get the two camera's to focus at the same time with no luck.  I can get the OTA camera, ASI294 to focus quite easily but have had absolutely no success in getting the ASI174 in the off-axis guide port to come to focus.  Several attemp[ts with spacers in front of each camera has not worked.  I am hoping someone can suggest how to set up the mechanical separation between the units to make this work.   I really suspect it is an Inward Focus problem, maybe.

 

Ed

 

 



#2 dswtan

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Posted 14 September 2019 - 01:41 PM

It's challenging for most of us! Checking you have some basics:

- You're doing this in daylight on a distant object, right?
- The ASI174 is producing an image, right?
- The pick-off mirror is sufficiently low enough into the light train, right?
- The pick-off mirror is facing the right way, right?
- You're following the diagrams and photos in the ZWO documentation, right?


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#3 Akwilliams

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Posted 14 September 2019 - 02:02 PM

Have you tried moving the stalk of the OAG up/down?    By moving the stalk physically down in the OAG can give you the necessary movement of the camera to get into focus.  


Edited by Akwilliams, 14 September 2019 - 02:03 PM.


#4 EAF

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Posted 14 September 2019 - 04:38 PM

dswtan- The answer to Q1 and Q2 is yes, I next checked to make sure I had Q3 and Q4 set right.  On Q# the prism is set to where it is just above the main camera sensor.  Sort of centered between the top of the main sensor and the OD of the train at that point.  The reason for rechecking is that when I take an image with the guide camera I get a large circular field of view about 30 to 40 percent of the screen width on a black rectangle of the view window.  Of course, if I zoom in the circle will fill the view field I am using ZWO software to do this initial set up.  Somehow that seems off to me.  

 

By moving the stalk I can come close for a faraway tree but not perfect, There are about 3000 steps, at a position of 25,0000, on my Moonlite focuser difference in where one camera is focused versus the other..  Hopefully, with a little more tweaking I will get there.

Thanks for comments, Ed



#5 Der_Pit

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Posted 14 September 2019 - 06:02 PM

Is the 174 the mini?

In general, the ZWO setup for OAG is designed in a way that you have the filterwheel between OAG and main camera. If you don't have one you will need additional spacers between (main) camera and the OAG.  If that distance is too short, the focus in the guide branch will lie within the prism stalk, and you have no way of reaching it there.

Check the diagram on the ZWO site, here

So you need the 11mm on the camera, and the 21mm (placeholder for the filterwheel), only then the OAG.



#6 TinySpeck

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Posted 14 September 2019 - 08:01 PM

In general, if your imaging cam and guide cam don't have different lens elements in their paths (like a reducer in one and not the other), the distance from the prism to the two camera sensors should be the same.  You can get a ballpark idea of that with just a ruler.  If you have a separate focuser in the guide cam path it helps in getting them parfocal, too.



#7 EAF

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Posted 14 September 2019 - 10:27 PM

I was only using the 11 mm ring, I have the 21 and will get it added.

 

Thanks all - Ed



#8 choward94002

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Posted 14 September 2019 - 10:29 PM

Focusing an OAG can be a challenge ... here are some tips that I've used when I spin mine up ...

 

First you need to get the primary mirror focused, mainly because when you change the focus on the primary path you're going to change the focus on the OAG ... so do whatever you do to get the primary imager dialed in.  I use a realtime collimation viewing program (there are a lot of them out there) to watch Polaris until the focus is as good as possible ...

 

Next you need to get the OAG camera stalk focused, which I do by:

 

- Do the math to figure out the proper focus distance, then measure everything to get a rough idea of the distance of the stalk ... remember to measure from the center of a pick prism!

- Next, get a rough focus using a Baader vari-lock focuser and using a realtime camera viewer and a bright star like Polaris.  I'm not a fan of doing this during the daytime because of the cone error is just too much for the OAG.  This is also where using an OAG with a "real" stalk and not a "soda straw" will come in handy.  Get a measurement of the total stalk length using the Baader

- Finally, get a helical focuser (you can get them on eBay, or the OAG will come with one), use spacers to adjust to roughly (+/- 3mm) the same stalk length and do a fine focusing again with Polaris.  At this point you've gotten the OAG stalk properly focused ... it doesn't need to be airy limit focused, BTW, the guide software actually does better somewhat out of focus 

 

At this point you've got the primary imager focused, the OAG imager focused and you're ready to begin!  As you adjust focus on the primary imager you'll throw off the focus on the OAG camera but again, PhD won't really care so it (shouldn't) make a difference ...

 

Clear skies!


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#9 EAF

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Posted 17 September 2019 - 09:15 PM

I appreciate all the input as it collectively got me over the issues.  When I put enough spacers between the OAG and the main camera all the other suggestions worked out perfectly.  I am not surprised how easy it is to solve these problems if you can get good advice!

 

Ed


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