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Cannot focus OAG...


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Am trying to use an OAG with my old Vixen VC200L fitted with a x 0.71 focal reducer.  The OAG is attached to the reducer, followed by the filter wheel, then camera.  I absolutely cannot find a focus point for the guidescope in this configuration.  Is the reducer putting me inside the focus point of the OAG?  When I take the guide camera off and look down at the prism, I can only see the shadow of the secondary mirror, no matter how far in or out I put the prism.

Anyone out there with experience of OAGs and SCTs with focal reducers?

 

 

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Firstly I'd get your imaging camera focal distance right ( normally 55mm) 

Then measure from the prism to the camera sensor, that measurement should be the same from the prism to the guidecam sensor( remember the backfocus is to be incorperated into this figure, and it's usually different from the imaging camera)

Another tips friend of mine uses is to point it at a globular cluster for fine tuning focus..

Hope this helps

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To expand on newbie's advice:

With the bare OAG off the scope, look down the guidecam port, you should see a reflection.

 So is the prism pointing towards the scope (correct), or to the main imaging camera (wrong) ?

Mount everything and get the imaging camera in focus.

Measure the distance from the imaging camera sensor, to the centre of the prism.

Now measure from the guide camera sensor, to the centre of the prism.

That distance must be the same.

Once you have set that distance correctly, mount everything, and focus the imaging camera.

Now, for test purposes, lower the prism into the middle of the light path, to be sure of getting stars.

Without defocusing the imaging camera, finetune the guidecam focus and lock down.

Looking at Flats, position the prism just outside the long side of the imaging camera's frame.

Then in future only focus the imaging camera, the guidecam should then be in focus too.

You can do all this during the day on a distant landmark, using very fast exposures on both cameras.

Then on stars, focus the imaging camera, then the guide camera.

And as before don't alter the guidecam focus, only the imaging camera focus.

Michael

 

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Thanks for the replies.  The main camera comes to focus just fine and the prism of the OAG is most definitely in the light path (and facing the right way!).  I have a set of street lights and a brightly-lit bridge a couple of miles away and the area serves as a good target when I am trying to sort out focus on new kit.  Moving the guide camera in and out across the full range of its focus gives nothing at all, even though the main camera gives an in-focus image.  The guide camera is working - if I shine a torch down the scope the guide cam display goes from black to a fully saturated white.  Measurements from prism to guide cam tally up with prism to main cam distance - this is an SXV set-up and everything is supposedly engineered to be parfocal anyway.

It's not the first time I've set an OAG up (though the first for this equipment configuration) and it's normally straightforward - focus main cam, check prism isn't shadowing its chip, then focus guide cam, no problems.  I'm scratching my head on this one.  I'm assuming people must use OAGs with SCTs or newts, but when I slide the guide cam out of my set-up and look down its draw tube, the secondary mirror and a support vane fills most of the prism view - there's not a lot of sky to be seen. 

Will have another play when it's dark tonight (can't try the Lodestar in the day - it's too sensitive even at fastest exposure speed).  I do have a guide scope I can use and I might give up and use that instead! 

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Try getting in focus without having the maincam in focus.   You’ll probably find that the amount of play in the guidecam side of the oag is too short or too long. 
 

Or...and that got me once, the prism is indeed facing the wrong side.   It will show light using a torch in either direction.

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1 hour ago, Robindonne said:

Try getting in focus without having the maincam in focus.   You’ll probably find that the amount of play in the guidecam side of the oag is too short or too long. 

That's a good idea, thanks, I'll try that one.

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3 hours ago, Hallingskies said:

That's a good idea, thanks, I'll try that one.

And I did!  Sure enough, the guide cam comes into focus at a point well past where the main camera is in focus.  The focal reducer is obviously pulling the "light cone" in to a point outside of the range of the OAG focus adjustment.  Oh well, it's Plan B then (guidescope)... 

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There is always a reason that the guidecam can't focus.  Always always it is because the distance between objective and imaging sensor that you get into focus on the main camera has to equal the same distance to the guidecam sensor as well.  Bullet camera sensors can get deep inside the OAG drawtube - so a lesser distance, whereas the "hockey-puck" ones sit outside the draw-tube and are further away.  Almost 100% certainly you have not got these distances correct and matching.  Play about in the day time with a remote roof or chimney and get them into focus that way.  Then, at night, a slight tweak and you are a-ok.

Seriously, once you crack it you'll wonder why you ever bothered with guidescopes.

Good luck.

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39 minutes ago, kirkster501 said:

There is always a reason that the guidecam can't focus.  Always always it is because the distance between objective and imaging sensor that you get into focus on the main camera has to equal the same distance to the guidecam sensor as well.  Bullet camera sensors can get deep inside the OAG drawtube - so a lesser distance, whereas the "hockey-puck" ones sit outside the draw-tube and are further away.  Almost 100% certainly you have not got these distances correct and matching.  Play about in the day time with a remote roof or chimney and get them into focus that way.  Then, at night, a slight tweak and you are a-ok.

Seriously, once you crack it you'll wonder why you ever bothered with guidescopes.

Good luck.

I have always used an OAG for my refractors and indeed, prefer not to bother with guidescopes.  However, I decided to blow the dust of of my old VC200L.  I used an OAG with this many years ago, and had no problems as I recall.  However, I’ve thrown a focal reducer into the mix and this seems to be causing a problem, though I can’t work out why.  I can focus the guide cam or the main one, but not both. I have the rig set up pointing at some distant streetlights and that works quite well for testing purposes.  I have the SX OAG and its focussing is a bit primitive (and actually quite sloppy - not impressed really) and there is not a lot of it. The lodestar fits on a C mount rather than being pushed in to a socket.  The focal reducer ties me to a specific back focus for the main camera and it seems there just isn’t enough adjustment in the SX OAG to accommodate the guide cam.  

I’m not too bothered to be honest - I have a really nifty old Vixen guidescope of about 450mm fl with rock solid sweep adjustments.  It fits on the VC200L a treat and took about 30 seconds to set up! As I plan to be galaxy imaging, quite often in narrow and sparse star fields, the ability to easily shift the guide cam field might come in handy.  With the focal reducer bringing me down to 1278mm fl, I think any problems with guidescope flexure will be minimal.  There’s no mirror flop with the VC200L of course.

Guess I’ll see how it works out...

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12 hours ago, Hallingskies said:

Sure enough, the guide cam comes into focus at a point well past where the main camera is in focus.

Sorry, that makes no sense.

To be in focus, the guidecam sensor MUST be the same distance from the prism as the imaging camera.

And both are after the FR, so equally Reduced.

You must be measuring wrong.

Perhaps an image of your setup would clarify ?

Michael

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Posted (edited)
4 hours ago, michael8554 said:

Sorry, that makes no sense.

To be in focus, the guidecam sensor MUST be the same distance from the prism as the imaging camera.

And both are after the FR, so equally Reduced.

You must be measuring wrong.

Perhaps an image of your setup would clarify ?

Michael

No need to apologise - you are right, it makes no sense at all.  And it was really bugging me.  So this morning I decided to try it all out in the daylight.  As you asked, this is the set-up.  From the left, VC200L draw tube, focal reducer, OAG housing and guidecam, filter wheel and main camera (you can see the Vixen guide scope - Plan B - on the top of the scope...) 

VC200L.jpg.db1246542b598ea3b1a6812969cda507.jpg

Here's what the guide camera is seeing down the prism draw tube, so this shows that at least the prism is seeing some light...

prism.jpg.727d7b93a96e23f50b1908b59da075a0.jpg

The Lodestar is way too sensitive on an 8 inch aperture to be ran in the daylight, so I made up a crude aperture mask and stuck a couple of layers of neutral density film over the hole in the mask...

mask.jpg.c09fb20a0f5e75455c4b5388f76f1e0d.jpg

That done, I set the prism and the guidecam at about "half way" positions, fitted the guidecam, aimed the scope at a distant bridge, and set the guidecam running...

990196069_guide2jpg.jpg.da20e2ee8edbf2159c4617ac6985d513.jpg

...and it works!!!  Yes, a grotty image, but taken through a window while it was raining.  Rotating the whole imaging train improved the vignetting which I think was being caused by one of the thick secondary supports...

1895926240_guide3jpg.jpg.07dd9d86aff7dcc1d53e3aeab9b041ad.jpg

The main camera was showing roughly the same view...

main.jpg.49bb329c404e9b92ed0faa88a6142880.jpg

I think what had been happening was that in the dark, the guide cam was simply not seeing the street lights I could see in the field of the main camera.  The odd glimmer I did see in the guide cam was probably a reflection in the window.

So the moral of the story is .....   TEST IN THE DAYLIGHT!

Thanks to everyone who replied.  This may count as another Duuurrr moment...

 

Edited by Hallingskies
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9 hours ago, Starflyer said:

I've found the moon useful when setting up an OAG too, big and lots of detail for focussing on.

...there's never a moon around when you want one...

  • Haha 2
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  • 1 month later...

I tried an OAG setup last evening and I was facing the western horizon from the first floor balcony. I chose an electricity pylon as the main camera target. Bad idea. The pylon did not extend as far as the prism and I was getting all kinds of random foliage in focus in the guide camera. I need to find a brick wall at a good distance. 

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Much better going tonight! Used a tiled roof and managed to get the guide camera (ASI 120MM) in (rough) focus at the same time as the (slow, tiny) Atik16ic with the ZWO OAG. I hope it is good enough for guide stars. ZWO manual filter wheel in place, too, and the Baader MPCC coma corrector. My first RGB galaxy beckons!

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23 hours ago, themos said:

chose an electricity pylon as the main camera target. Bad idea. The pylon did not extend as far as the prism

Once you'd focused the main camera you were free to make a tiny slew to put the pylon on the prism ?

But you got there with the roof.

So accurately focus a star on the main camera, then focus the guidecam without touching the main camera focus, looking for lowest HFD on the PHD2 Star Profile window.

Lock everything down, and I expect you know that once you have the main camera focussed, the guidecam will always be focused too.

Michael

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