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Need help on OAG

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#1 Fukinagashi

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Posted 21 May 2019 - 04:14 AM

To cope for possible mirror flips and tube flexures on my cheap GSO Newton 10" I've recently bought an OAG.

Having little backfocus (my corrector is 55 mm) to put my AP setup (wheel + camera) I opted for a TSOAG which adds a mere 9.5 mm to the whole.

I need to point out following issues:

 

1) First of all the star pattern is very very far from the Gaussian profile I would expect. This is true in particular for brightest stars. I haven't figured it out yet how can a prism deform the pattern of a star in this way.

2) It is very difficult to me to find the optimal distance of guide camera sensor plane to reach a proper focus. Given the unusual star pattern generated by the prism indeed it is even impossible to use a Bathinov mask. I calculated that I'm approximately 0.15 mm off proper focus.

3) I noticed that the intensity of the star image on the sensor changes as I move the scope. Some stars even disappear. Moreover this generates "star lost" problems during calibration in PHD2.

4) I make use of both NB and BB filters that are not parfocal each other. This means that if I get a proper focus with NB filters I'm off for BB filters and viceversa. How can I cope with this given that the difference can be so tiny that I cannot use even 0.5 mm extenders?

 

I rely on your experience to find solutions to my problems...

Thank you in advance!


Edited by Fukinagashi, 21 May 2019 - 04:16 AM.


#2 AhBok

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Posted 21 May 2019 - 06:53 AM

I have an OAG on my 8” F4 and am quite familiar with your issues. Mine were similar at first. Take a look at just the OAG connected to the camera. Make sure the prism is placed as close to the chip as possible without intruding into the light path. One reason for ugly stars is the OAG uses off axis stars, that is stars on the extreme edge of the field that your main imaging cam cannot see. Getting your prism as far down as possible into the field helps quite a bit. Also, place the prism tangent to the longer side of the chip if it is rectangular. Again, this will help you get the prism farther down.

I focused mine during the day on a distant object. First, I focused my main camera and then the guide camera. I actually used SharpCap to focus the guide camera because video allows better control over exposure than PHD2. I find that once focused this way, that my PHD2 presents round stars across the entire field. Also, when I switch to narrowband, PHD2 still works fine.

Now, the focus point does change between filters somewhat. I’ve played with the idea of using my autofocuser to measure the step counts for each filter and then place the focuser at the average of the step counts. Then adjusting the guide cam focus to this point. But I’ve not needed to do this yet.

You can use a Bahtinov mask on your guide cam at night using SharpCap to focus rather than PHD2. The video and gain controls in SharpCap make it easy to see. Afterwards, focus will be excellent for PHD2.

Hope this rather long answer helps!

#3 Fukinagashi

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Posted 21 May 2019 - 07:01 AM

Thank you very much for exhaustive answer!

 

Make sure the prism is placed as close to the chip as possible without intruding into the light path.

 

Cannot explain better this statement? Isn't the prism-to-sensor distance fixed (according to focus point)?


Edited by Fukinagashi, 21 May 2019 - 07:17 AM.


#4 WadeH237

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Posted 21 May 2019 - 07:15 AM

Thank you very much for exhaustive answer!

 

 

Don't the two statements contradict in any way?

No, they don't.

 

Your telescope has an imaging circle, where the best image is at the center of the field.  The farther away from the center, the more aberrations will affect the shape of the stars.

 

You want the best stars to be on your main imaging sensor so that your picture looks good.  You also want the prism for the OAG to be somewhat close to the center of the field, but not so close that it gets in the way of the main sensor.

 

It's common that keeping the prism out of the way of the main sensor results in it seeing some fairly aberrated stars.  That's generally fine, though, because the guide software is pretty good at finding the centroid even of non-round stars.  With one of my setups, the guide sensor see stars that are so comatic that they look like tiny, capital 'T' letters - but it still guides fine.


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#5 Fukinagashi

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Posted 21 May 2019 - 07:44 AM

No, they don't.

 

Your telescope has an imaging circle, where the best image is at the center of the field.  The farther away from the center, the more aberrations will affect the shape of the stars.

 

You want the best stars to be on your main imaging sensor so that your picture looks good.  You also want the prism for the OAG to be somewhat close to the center of the field, but not so close that it gets in the way of the main sensor.

 

It's common that keeping the prism out of the way of the main sensor results in it seeing some fairly aberrated stars.  That's generally fine, though, because the guide software is pretty good at finding the centroid even of non-round stars.  With one of my setups, the guide sensor see stars that are so comatic that they look like tiny, capital 'T' letters - but it still guides fine.

 

So I can put the prism at the center of the image circle when focusing and then I move it a bit away when imaging. Right?



#6 WadeH237

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Posted 21 May 2019 - 12:01 PM

So I can put the prism at the center of the image circle when focusing and then I move it a bit away when imaging. Right?

No.  Moving the prism will change the position of the guide sensor.

 

You need to do a one-time exercise of getting the guider to be par-focal with the main camera.  To do this, get the best focus on the main camera that you can.  Then focus the guide camera using whatever means you have (sometimes, you have a helical focuser on the OAG, and sometimes you just need to slide the guide camera in or out of the OAG port).  Once you do that, you only ever need to focus the main camera, and the guide camera will also be in focus.

 

In my case, I have a guide camera that fits into a 1.25" eyepiece barrel.  I set the spacing so that when it's pushed fully into the barrel, it's just a little too close to reach focus.  I then focus the guide camera by pulling out of the barrel just a bit.  I have my imaging software set to take repeated exposures from the guide camera, and I monitor both a 400% view of the star I'm using and follow the FWHM reported by the imaging software.  I also have a par-focal ring for the guide camera.  This is basically just a collar with a 1.25" hole in it and a thumbscrew.  I put it on the guide camera before inserting it into the OAG barrel.  That way, once I have the guide camera in focus, I can slide the par-focal ring all the way down and secure it with the thumbscrew.  That way, if I need to remove the guide camera, or rotate it, or whatever, I can just insert it back into the barrel until it hits the ring, and I know that I'm par-focal with the main camera again.

 

I hope that this helps,

-Wade



#7 premk19

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Posted 21 May 2019 - 12:07 PM

So I can put the prism at the center of the image circle when focusing and then I move it a bit away when imaging. Right?


Yes you can. While focusing lower the prism stalk as low as it'd go, adjust guide camera position to get best focus, and then raise the prism stalk away from the main imaging sensor. As long as you haven't moved the guide camera relative to the prism you should still be in focus.

#8 AhBok

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Posted 21 May 2019 - 01:36 PM

Premk19–Are you sure about this? The distance to the imaging sensor and the guide sensor must be equal. The prism has no appreciable effect on this. Correct?

#9 premk19

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Posted 21 May 2019 - 01:56 PM

Premk19–Are you sure about this? The distance to the imaging sensor and the guide sensor must be equal. The prism has no appreciable effect on this. Correct?


I was suggesting to lower/raise the guide camera + prism together as one unit to find better guide stars to focus on. Many OAGs allow this kind of adjustment.


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