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Where does the OAG go?

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#1 wesnikon

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Posted 24 April 2019 - 01:23 AM

I recently purchased a ZWO OAG and am now wondering, where in the image train it has to go. As I have a Newton there's also the inevitable coma corrector and the only way I have been able to fit the OAG in was right after the 16.5mm spacer that sits on the main camera, thus removing the 20mm spacer that followed the 16.5mm spacer. I didn't actually expect that to work and it didn't. There's no manual in the box and the one I found online only shows a refractor (w/o CC), so that didn't really help either. Is there someone here who can shed some light on this?



#2 ChrisWhite

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Posted 24 April 2019 - 05:29 AM

My image train is camera + Filter wheel + spacer + oag + corrector.

My guess is you were unable to reach focus with the oag? The sensors of both the oag and the imaging camera must be the same distance from your corrector to reach focus at the same time. So fool with spacing until you get it right. I so mine in the daytime with my scope pointing out the window at the neighbors fence.
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#3 the Elf

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Posted 24 April 2019 - 09:34 AM

There is no hard rule where to put it but a few things to consider:

- when imaging narrowband you want the NB filter behind the OAG. Otherwise the image is too dark for guiding

- you need to find a position that allows the guide camera to be in focus. Withyout any extra optics in the path the distance OAG to main camera and OAG to guide camera are the same. The geometry of your equippment determines the position

- things become a lot easier if you put a 0.5x reducer in front of the guide camera. The stars become brighter and the tiny field of view is also lager. This moves the guide camera closer to the OAG than the main camera.

- the further towards the scope you go the higher is a chance of bizar vignetting with mirror optics (RC, SCT). A large black spot in the middle of the guide camera occurs. That does not harm the guiding as long as you put the stars in the corners

- the further towards the scope you move the OAG the lager the vignetting on the main camera will be.

- don't wory about the position of the coma corrector. Most guide softwars accept egg shaped stars for guiding

Check my YouTube channel, there are two videos about OAG. Send me a PM if you perfer a reply in your native language.


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#4 Scott1244

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Posted 24 April 2019 - 01:37 PM

I could be wrong, but my understanding is that OAG's work best with a refractor and not so good with a Newt. For a reflector, a guide scope is a better choice.



#5 Alex McConahay

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Posted 24 April 2019 - 04:19 PM

>>>>> I could be wrong, but my understanding is that OAG's work best with a refractor and not so good with a Newt. For a reflector, a guide scope is a better choice

 

I use an OAG with a reflector quite a bit. And it is preferred on a reflector. It helps control differential flexure that can come when you have multiple mirrors and other optics that can flex around.

 

As far as where it goes, you need a diagram. Closest to the front side of the filter wheel is the best place. But you have to diagram out where the flattener/corrector is, and how much space it is from there to the imaging plane. If the filter wheel and OAG fits in that area, adapters and all, that is where it should go. If it does not, then you have to put the OAG in front of it. The primary concern is that the corrector be properly spaced to the imaging chip. 

 

Alex


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#6 ChrisWhite

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Posted 24 April 2019 - 07:17 PM

I could be wrong, but my understanding is that OAG's work best with a refractor and not so good with a Newt. For a reflector, a guide scope is a better choice.

 

I use an OAG with my Newt.  Started out with a guidescope and experienced differential flexure. OAG fixed this, and performed even better with guiding than before. 


Edited by ChrisWhite, 24 April 2019 - 07:18 PM.

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#7 rockstarbill

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Posted 24 April 2019 - 09:29 PM

I use an OAG with my Newt.  Started out with a guidescope and experienced differential flexure. OAG fixed this, and performed even better with guiding than before. 

+1 to what Chris said here. I use an OAG on my Newt as well and it works wonderfully. The Skywatcher Coma Corrector should allow for 55mm of backspacing, IIRC. The ZWO cameras come with spacers in the box that will allow you to configure their OAG for the proper spacing for your corrector. 


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#8 wesnikon

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Posted 25 April 2019 - 04:17 AM

I could be wrong, but my understanding is that OAG's work best with a refractor and not so good with a Newt. For a reflector, a guide scope is a better choice.

I don't see why an OAG would be a bad choice for a Newt, for me it's about bringing the weight down and a guide scope adds quite some weight.



#9 wesnikon

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Posted 25 April 2019 - 04:18 AM

There is no hard rule where to put it but a few things to consider:

- when imaging narrowband you want the NB filter behind the OAG. Otherwise the image is too dark for guiding

- you need to find a position that allows the guide camera to be in focus. Withyout any extra optics in the path the distance OAG to main camera and OAG to guide camera are the same. The geometry of your equippment determines the position

- things become a lot easier if you put a 0.5x reducer in front of the guide camera. The stars become brighter and the tiny field of view is also lager. This moves the guide camera closer to the OAG than the main camera.

- the further towards the scope you go the higher is a chance of bizar vignetting with mirror optics (RC, SCT). A large black spot in the middle of the guide camera occurs. That does not harm the guiding as long as you put the stars in the corners

- the further towards the scope you move the OAG the lager the vignetting on the main camera will be.

- don't wory about the position of the coma corrector. Most guide softwars accept egg shaped stars for guiding

Check my YouTube channel, there are two videos about OAG. Send me a PM if you perfer a reply in your native language.

Thanks Elf, I'll check those videos out - I think I watched your PixInsight videos the other day, the name sounded familiar :-)



#10 wesnikon

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Posted 25 April 2019 - 04:22 AM

>>>>> I could be wrong, but my understanding is that OAG's work best with a refractor and not so good with a Newt. For a reflector, a guide scope is a better choice

 

I use an OAG with a reflector quite a bit. And it is preferred on a reflector. It helps control differential flexure that can come when you have multiple mirrors and other optics that can flex around.

 

As far as where it goes, you need a diagram. Closest to the front side of the filter wheel is the best place. But you have to diagram out where the flattener/corrector is, and how much space it is from there to the imaging plane. If the filter wheel and OAG fits in that area, adapters and all, that is where it should go. If it does not, then you have to put the OAG in front of it. The primary concern is that the corrector be properly spaced to the imaging chip. 

 

Alex

I shoot with an OSC - for the time being (a mono camera is on my list), so I put it right after the main camera. Unfortunately with the adapters I have I'm 6mm short of 55mm back focus, I'm in touch with a shop right now to see if there is any such thing as an M48 male to female adapter.



#11 wesnikon

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Posted 25 April 2019 - 04:25 AM

+1 to what Chris said here. I use an OAG on my Newt as well and it works wonderfully. The Skywatcher Coma Corrector should allow for 55mm of backspacing, IIRC. The ZWO cameras come with spacers in the box that will allow you to configure their OAG for the proper spacing for your corrector. 

The spacers from ZWO will not suffice, unfortunately. The 21mm spacer is M42(?) female on the scope side, the CC is M48 male though. The 16.5mm spacer that came with ASI294 is not enough to get the main camera in focus again, so I'm now looking for a 6mm M48 male to female.



#12 wesnikon

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Posted 25 April 2019 - 04:26 AM

I use an OAG with my Newt.  Started out with a guidescope and experienced differential flexure. OAG fixed this, and performed even better with guiding than before. 

That is one of the reasons why I'm switching, weight reduction being another one.



#13 wesnikon

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Posted 25 April 2019 - 04:30 AM

My image train is camera + Filter wheel + spacer + oag + corrector.

My guess is you were unable to reach focus with the oag? The sensors of both the oag and the imaging camera must be the same distance from your corrector to reach focus at the same time. So fool with spacing until you get it right. I so mine in the daytime with my scope pointing out the window at the neighbors fence.

I'm going for Main camera -> OAG -> Spacer (-> Spacer) -> CC to make sure the prism won't be blocking the beam and cause vignetting. I can't get focus on the main camera at the moment, funnily, the OAG camera can and the image is much better than I expected; I wouldn't be able to tell whether or not the image is from an OAG if I didn't know.



#14 ChrisWhite

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Posted 25 April 2019 - 05:35 AM

Well, if your oag can reach focus, but your camera can't (and needs more outfocus) you just need more spacing between the oag and the imaging camera. You are close. Why do you need an m48 x m42 female again? Post a pic of all the adapters and accessories you are using lined up with each other but not threaded together. There is often more than one way to do this and you might be making it more difficult by using some stock adapters. For example, if it is a zwo oag you could thread directly to the CC with m48 on one side and use the m42 plate on the other side of the oag to accept your imaging camera and spacers.

Don't worry about vignetting from the oag prism. You will have some no matter what you do, but it will calibrate out with flat frames.
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#15 wesnikon

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Posted 26 April 2019 - 01:34 AM

Well, if your oag can reach focus, but your camera can't (and needs more outfocus) you just need more spacing between the oag and the imaging camera. You are close. Why do you need an m48 x m42 female again? Post a pic of all the adapters and accessories you are using lined up with each other but not threaded together. There is often more than one way to do this and you might be making it more difficult by using some stock adapters. For example, if it is a zwo oag you could thread directly to the CC with m48 on one side and use the m42 plate on the other side of the oag to accept your imaging camera and spacers.

Don't worry about vignetting from the oag prism. You will have some no matter what you do, but it will calibrate out with flat frames.

If I add a 6mm spacer I'll get the exact same back focus as without the OAG - that was the idea.

 

Here's a picture of all the parts I have available at the moment, top row is the imaging train that gets OAG in focus, bottom row is left over parts.

 

_D853293.jpg


Edited by wesnikon, 26 April 2019 - 01:35 AM.


#16 the Elf

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Posted 26 April 2019 - 01:57 AM

Did you try? You can test it at daytime provided you have a life view of main camera and guide camera. Use a target at least a kilometer away, neighbour's house might be too close to reach focus at all. A distant tree at the horizon is perfect. Make sure the distance between coma corretor and main camera sensor is as specified, the correctors are sensitve to spacing.

You will need a way to rotate the setup in order to find a guide star. If you slip in the camera into an eyepiece clamp that is how you rotate. If you want a threaded connection you need a rotator somewhere. The other option is an extremely sensitive guide camera that works even on dim stars.

It is also helpful to put a sticker or felt pen mark on the guide camera to have it upright. The guiding will work in any orientation, but if you are missing a star and have to slew the mount it helps a lot when main and guide came image are in the same orientation. The guide cam image is mirrored of course, that can probably be undone in the life view. Given the Einstein quote in your signature I'm sure you are not part of second infinite thing and get the task accomplished without problems. It's not as difficult as it sounds.

 

... one more remark: you can and you have to adjust the length of the prism stalk. I recommend you point the scope to the cloudy sky that we have right now and check the vignetting while moving the prism in and out. You want it as far out as possible to keep the vignetting low. If you go too far there is vignetting in the guide camera. This changes focus of course. I put a cheap parfocal ring on my guide camera to keep the position. See image:

 

V-Power_2_low_parfocalring.JPG

 

The upper edge of the prism will add a single diffraction spike to bright stars at one side of the image. The yellow star at the upper left in this image is an example:

http://www.elf-of-lo...merica2018.html


Edited by the Elf, 26 April 2019 - 02:08 AM.


#17 wesnikon

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Posted 26 April 2019 - 02:12 AM

Did you try? You can test it at daytime provided you have a life view of main camera and guide camera. Use a target at least a kilometer away, neighbour's house might be too close to reach focus at all. A distant tree at the horizon is perfect. Make sure the distance between coma corretor and main camera sensor is as specified, the correctors are sensitve to spacing.

You will need a way to rotate the setup in order to find a guide star. If you slip in the camera into an eyepiece clamp that is how you rotate. If you want a threaded connection you need a rotator somewhere. The other option is an extremely sensitive guide camera that works even on dim stars.

It is also helpful to put a sticker or felt pen mark on the guide camera to have it upright. The guiding will work in any orientation, but if you are missing a star and have to slew the mount it helps a lot when main and guide came image are in the same orientation. The guide cam image is mirrored of course, that can probably be undone in the life view. Given the Einstein quote in your signature I'm sure you are not part of second infinite thing and get the task accomplished without problems. It's not as difficult as it sounds.

Didn't get a chance to try yesterday, what I have tried so far is the imaging train as of the above picture and it gets the OAG in perfect focus, but the main camera doesn't quite get there, because - as you say yourself - I need to get the back focus for the coma corrector right. That's why I'm thinking about getting a 6mm spacer unless I follow ChrisWhite's advice and move the OAG further down the imaging train (in which case I could use the 21mm spacer and be done with it), which, according to your previous post is a bad idea, because the vignetting will increase the closer the OAG gets towards the scope. So obviously there's more than one way to do this.



#18 the Elf

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Posted 26 April 2019 - 02:36 AM

When you move the guide camera's sensor too far from the tiny little hole you'll have horrible vignetting indeed. If you move the OAG more towards the scope put this on your guide camera:

https://www.teleskop...tergewinde.html

Any reducer moves the camera towards the scope. You can use spacers between guide camera and the small reducer to adjust the reduction factor and the focus position.

Watch this at 7:40  https://www.youtube....h?v=S3lZcLt_zDU




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