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No Stars Using ZWO OAG

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#1 Squeezer04

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Posted 16 June 2021 - 12:59 AM

Just set up my new ASI2600mm Pro kit which includes the M68 OAG and 7 position 36mm filter wheel. My telescope is the Williams Optics GT81. My previous setup was a DSLR, ASIAir Pro, GT81, ZWO 120mm mini guide camera mounted to the 30mm mini scope. Everything has gone OK on the new setup with the exception of the OAG. When I loop 1-2 second exposures through the OAG/Guide camera I get no stars. The main camera focuses perfectly. The OAG has an integrated focuser and I have slid the guide camera in and out of the tube in an attempt to see/focus stars. No luck.

 

I don’t believe it’s possible to mount the OAG backwards but I looked down the OAG tube with the camera removed during the day and I could see that I was looking out the GT81.

 

I guess I can go back to the mini scope but I bought the OAG and would like to make it work.

 

I’m out of ideas. Any help greatly appreciated.

 

Mike



#2 bobzeq25

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Posted 16 June 2021 - 01:17 AM

Just set up my new ASI2600mm Pro kit which includes the M68 OAG and 7 position 36mm filter wheel. My telescope is the Williams Optics GT81. My previous setup was a DSLR, ASIAir Pro, GT81, ZWO 120mm mini guide camera mounted to the 30mm mini scope. Everything has gone OK on the new setup with the exception of the OAG. When I loop 1-2 second exposures through the OAG/Guide camera I get no stars. The main camera focuses perfectly. The OAG has an integrated focuser and I have slid the guide camera in and out of the tube in an attempt to see/focus stars. No luck.

 

I don’t believe it’s possible to mount the OAG backwards but I looked down the OAG tube with the camera removed during the day and I could see that I was looking out the GT81.

 

I guess I can go back to the mini scope but I bought the OAG and would like to make it work.

 

I’m out of ideas. Any help greatly appreciated.

 

Mike

Setting up in daytime can help.  It becomes clearer what you need to do to get to focus.  Go NOWHERE near the Sun, you want a distant tree.

 

Most of us find the excellent 120MM unsatisfactory with an OAG.  A  290MM is useful, there are other choices.


Edited by bobzeq25, 16 June 2021 - 01:17 AM.


#3 rgsalinger

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Posted 16 June 2021 - 01:44 AM

You will be surprised at how close to perfect focus the guide camera in an OAG needs to be. I suspect that's your problem.

 

Make sure that the stalk of the OAG is parallel to the long side of you main imaging chip and just above it. That will put it as far as possible in the light cone but should not cast a shadow. Do a cursory measurement of the back focus of the main camera chip and the OAG camera chip fully inserted. You want to have the main camera a bit farther out than the guide camera when you start.  

 

What I do is to use 10 second bin 2 exposures to find the focal point. I start with the guide camera all the way in the OAG. I move it out in small increments checking after each move to see if I've got star. I try to do this pointing at the Milky Way or some other area of the sky with a lot of stars. The stars will initially look weird and washed  out - use a high contrast stretch so that you can see them. 

 

If I move the camera all the way out and cannot get focus then I move it back all the way in. I now do one of two things. Either I add some spacing between the OAG and the filter wheel (or camera) or I move the focuser in until I can see some stars. 

 

Once you get focus use a par focal ring or just a piece of tape to mark the focal point so that you just have to fine tune it if you need to set it up again. I keep my imaging trains all together to avoid having to set up my OAG's over and over again. 

 

Rgrds-Ross



#4 Squeezer04

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Posted 16 June 2021 - 09:41 AM

Thanks for the input.

 

Bob, I’lll try again today.

 

Ross, I’m confused regarding your suggestion to possibly add spacing between the OAG and filter wheel. My assembly is per ZWO’s recommendation to get 55mm back focus to my imaging sensor. Won’t adding spacing mess that up? Besides my filter wheel is behind the OAG in the imaging train. If I add spacing I’ll just be moving the filter wheel/camera back. the distance between the OAG and telescope will be unchanged. I’ll try again tonight using your 10 second bin 2 exposures. Thanks.



#5 davidparks

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Posted 16 June 2021 - 12:44 PM

Here you go:

 

ZWO's manual for the OAG, page 5:  Manual

 

There is a small 5mm extension on the OAG that should be removed before you put the helical focuser on.  This will allow 1.25" barrel style cameras (like the 120 mini) to be spaced closer to the prism.

 

Your thinking is correct in every way.  Maintain your 55mm back focus,  OAG/EFW/2600, as this allows the Guide and Main cams to remain parfocal.


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#6 Squeezer04

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Posted 16 June 2021 - 12:57 PM

Thanks David. I think the problem is with my guide camera. I think it's broken. While looping when I cover the scope with my hand nothing happens. I may have burned out the sensor during the day. new one on order.



#7 bobzeq25

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Posted 16 June 2021 - 01:12 PM

Thanks for the input.

 

Bob, I’lll try again today.

 

Ross, I’m confused regarding your suggestion to possibly add spacing between the OAG and filter wheel. My assembly is per ZWO’s recommendation to get 55mm back focus to my imaging sensor. Won’t adding spacing mess that up? Besides my filter wheel is behind the OAG in the imaging train. If I add spacing I’ll just be moving the filter wheel/camera back. the distance between the OAG and telescope will be unchanged. I’ll try again tonight using your 10 second bin 2 exposures. Thanks.

Do you have a flattener or reducer flattener?  If not there is _no_ 55mm spacing recommendation from ZWO.  Many flatteners have a 55mm spacing requirement.  As a courtesy ZWO shows you how to achieve it.  It has nothing to do with the scope (assuming no flattener) or the camera.

 

If you've been trying to maintain 55mm, that may be part of your problem.


Edited by bobzeq25, 16 June 2021 - 01:13 PM.


#8 rgsalinger

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Posted 16 June 2021 - 04:09 PM

If you move a spacer from in front of an OAG to the back of an OAG you do not change the spacing between the main camera and the back of the scope. So, if you had 55mm before you still have 55mm.

 

Note that this is for the case where you cannot get the OAG close enough to focus. After this adjustment, you should be able to move the guide camera out just a bit and get focus. Your problem in this case is that you cannot get in enough to achieve that 55mm of spacing that you think that you need. 

 

FWIW, if you have the reverse problem then either you need an extension to the OAG or you can move spacers from in back of the OAG to being in front of the OAG.

 

I tend to keep my wheel directly attached to the camera and add any necessary spacers between the wheel and the OAG. You really don't want to move the filter away from the camera more than a tiny amount. 



#9 Squeezer04

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Posted 17 June 2021 - 01:05 AM

I used ZWO’s recommended imaging train. My telescope is attached directly to my OAG (16.5mm). The OAG is attached directly to the filter wheel (20mm). The filter is attached to a 2mm spacer. The spacer is attached directly to the 2600mm Pro. The camera back spacing is 17.5mm. With this recommended setup the total back spacing is 56mm. If I remove the 2mm spacer, it’s 54mm. I don’t think I have anything to play with here. Still waiting for the new guide camera to arrive.

 

Thanks again for all the suggestions.



#10 Squeezer04

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Posted 21 June 2021 - 01:42 AM

Looks like I’m gonna have to throw in the towel on this OAG. I ordered the ASI290MM Mini thinking my 120 mini was bad. Neither are bad. During the day I can get the guide camera to focus through my William Optics GT81 on objects out to about 30 meters. Focus on the main imaging camera is spot on. The focal length with my .8 reducer/flattener is 384mm. Beyond about 30 meters there is nothing I can do to get objects in focus. I’ve tried every available position of the guide camera within the OAG housing. Tried again tonight with no luck seeing a guide star. Unless somebody can tell me what I’m doing wrong I guess I’ll go back to the mini guide scope.



#11 AhBok

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Posted 21 June 2021 - 02:00 AM

Are you sure the stalk is not in backwards?
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#12 Blackhawk163

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Posted 21 June 2021 - 05:50 AM

Are you sure the stalk is not in backwards?


I second this! Make sure it's not facing the wrong direction. I struggled for three days before I realized my error. 🤦🏽‍♂️

#13 Squeezer04

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Posted 21 June 2021 - 10:51 AM

Would I be able to focus on anything if it was in backwards?



#14 t-ara-fan

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Posted 21 June 2021 - 10:55 AM

You will be surprised at how close to perfect focus the guide camera in an OAG needs to be. I suspect that's your problem.

 

Make sure that the stalk of the OAG is parallel to the long side of you main imaging chip and just above it. That will put it as far as possible in the light cone but should not cast a shadow. Do a cursory measurement of the back focus of the main camera chip and the OAG camera chip fully inserted. You want to have the main camera a bit farther out than the guide camera when you start.  

 

What I do is to use 10 second bin 2 exposures to find the focal point. I start with the guide camera all the way in the OAG. I move it out in small increments checking after each move to see if I've got star. I try to do this pointing at the Milky Way or some other area of the sky with a lot of stars. The stars will initially look weird and washed  out - use a high contrast stretch so that you can see them. 

 

If I move the camera all the way out and cannot get focus then I move it back all the way in. I now do one of two things. Either I add some spacing between the OAG and the filter wheel (or camera) or I move the focuser in until I can see some stars. 

 

Once you get focus use a par focal ring or just a piece of tape to mark the focal point so that you just have to fine tune it if you need to set it up again. I keep my imaging trains all together to avoid having to set up my OAG's over and over again. 

 

Rgrds-Ross

All sounds good but I recommend setting up in day time using SharpCap to show 30fps video from the guide camera.  I write on my ASI174MM mini guide camera with a Sharpie so I know how far out the guide cam should protrude.

 

ZWO has some detailed instructions on their website about spacing for various combinations of their equipment.


Edited by t-ara-fan, 21 June 2021 - 11:00 AM.


#15 rgsalinger

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Posted 21 June 2021 - 11:17 AM

If it's installed backwards, then you will not see any stars. The flat side of the OAG prism has to be pointed toward the telescope. You can also check by looking from each side of the OAG. Only one side will show what's directly above the OAG. I use my hand mostly. 

 

Rgrds-Ross



#16 RogerM

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Posted 21 June 2021 - 03:22 PM

A couple of illustrations albeit there is a difference between your setup and mine (I have an EFW between the main imaging camera and the OAG).  But, this simply means that you would need a 20mm spacer in lieu of the EFW.  This sets up the main imaging camera's sensor and the guide camera sensor equidistant from the OAG Prims center.  Close enough to where focusing the OAG camera will be fairly straightforward.  If you do not have the additional spacing for the main imaging camera then you will not be able to move the guide camera inwards enough to focus it.

 

sml_gallery_339350_14633_57311.jpgsml_gallery_339350_14633_14754.jpg

 

Edit:  I forgot, I also have the helical focuser so if you are not using one then you do not need an additional 20mm spacer...the main imaging camera then connects directly to the OAG using the included M48 adapter.


Edited by RogerM, 21 June 2021 - 03:25 PM.


#17 Squeezer04

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Posted 21 June 2021 - 05:09 PM

Problem solved!

 

First of all I want to thank all of you for hanging in there with me on this problem.

 

The issue was I could not get the guide camera sensor CLOSE enough to the prism. I kept messing with spacing in the image train with spacers and that wasn’t doing it. I finally took the OAG off and slid the prism MUCH closer to the guide camera. I now have room on both sides of the OAG focuser to fine tune my adjustments.

 

Thanks again.


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