Celestron OAG Guide camera Vignetting

[maxchess]

Hi,

I am setting up the Celestron OAG (93648)  but get severe vignetting on the guide camera. The scope is a standard Celestron 8" SCT with a 6.3 focal reducer. The main camera is a Canon 450D (APS). I have two camera I have tried as guide cameras. The Orion SSAG and the ZWO ASI 178MC which I already have. I am trying a daytime setup using a distant tree. I can get both the canon 450d and either guide camera both in focus, but the guide image shows severe vignetting, such that I get a moon shape half image, but I can still see the tree. Is this normal or should I expect a full image? I have not tried adjusting the prism depth as the manual cautions against it and I am worried about intruding into the main image space. Any ideas., or will it work like this.  I would rather not buy yet another guide camera.

[michael8554]

Is the image train C8 -  Reducer - OAG - DSLR/Guidecam?

The Reducer makes the path of the light rays steeper, which might be the reason the OAG prism is being vignetted.

So the chances are the prism isn't in the DSLR frame, so you should be able to lower it to reduce the vignetting.

Of course this may or may not make getting both cameras in focus a problem   :-<

Also are you able to get the DSLR the correct distance from the Reducer, which is usually 105mm ?

Michael

[maxchess]

My image train is:  C8 - F Reducer - OAG - Spacer -DSLR.    and OAG - spacer  - guide camera ZWO ASI 178MC

Does anyone have this combination working without vignetting, if so what is the spacing? I used the 11mm spacer to the DLSR and 6mm to guide camera as recommended in the OAG manual. I can get both the  DSLR and guide camera in focus, but after trying last night I found that both cameras have vignetting.

On closer reading I noticed that the Celestron OAG instruction book does not include any mention of use with a FR. Odd really since I would guess that most imagers use one with an SCT.  Checking round the forums I find that this is a common problem especially with larger Edge SCTs. However I have the basic 8" SCT.  It is also clear that the OAG is quite fat and places the camera further away from the telescope back than my standard 2" nosepiece.  So I have just done a daylight test with no spacers or nosepieces on either camera and they both focus and it looks like vignetting has disappeared on the DSLR and is much reduced on the guide camera with increased image brightness,

So I will test it on the next cloudiness night.  However I would still like to hear from anyone who has the C8/FR/OAG/DSLR combo working

Max

[alacant]

Hi. Take a flat frame with the prism at the centre of the long edge of the dslr. If it's clear, push in the prism repeating the flat until you see the prism shadow. Now back off the minimum amount. That should minimise the vignette. The Stars via the guide camera don't have to look perfect and if you guide using phd2, it will choose a sensible star anyway. HTH.

[maxchess]

OK tried to test the new set up in between the cloud with no spacers in the OAG having achieved daylight parfocal as described before. . After focusing tried to image M27 Dumbell Neb. The DLSR bit worked with only mild vignetting , but my guide cameras could not find any stars. Only had a short time to try swapping guide cameras and playing with settings. Wondering if Orion SSAG and the ZWO ASI 178MC just not sensitive enough. 

[michael8554]

You could try adding your dslr fov frame in a planetarium on M27 and see what stars should have been available.

Worth routinely  trying before any imaging, as you can then find best orientation of the oag for brightest guide stars.

Michael 

[wimvb]

I don't use an oag, but have you looked into the oag/dslr assembly from the scope side? The oag prism should extend into the light path, but not past the sensor edge. Here's a schematic.

https://www.sciencecenter.net/hutech/mitsub/oag6.htm

To get as much light as possible, I would try to push the prism as far as the schematic shows. Possibly a mm less.

M27 is within the Milky way, so there should always be a star to guide on. You can increase the guide camera gain and exposure time.

[Shibby]

Just checking, when you say vignetting, do you mean circular? That would be odd, suggesting that it's caused by something between the prism and guide camera.

If the vignetting is from the edge of the main imaging circle then, as others have said, you need to move the prism as close as you can to the main camera's sensor without overlapping. Moving the prism in/out has no effect on focus - remove the whole reducer/OAG/camera assembly and look back towards the main sensor while moving it into position.

[maxchess]

I used a combination of all the advice above and made progress. By adjusting the prism I got the guide scope fully illuminated and increasing the gain gave brighter images. However I am finding that with my two cameras I only get a few indistinct  stars that my software won't guide on. I am using the INDY EKOS internal guider which worked well when I was guiding a 90mm scope with the Orion SSAG.  I am coming to the conclusion, after my own experience and reading various forums that I need a better guide camera to guide using an OAG, something like a lodestar.  (maybe I should try PHD2, but the stars are so indistinct, even using darks that I am not optimistic)  My next step is to try guiding through the 90mm scope (f=500) mounted on top of the C8. I have read that I might get "Flexure"  but I am using a Focal reducer with only a C8 so I hope it will not be a problem. The mounting uses what looks like very substantial rings from ADM.  Now waiting for a clear night.

[alacant]

1 hour ago, maxchess said:

I need a better guide camera

Hi. The focus is critical for the oag and unless you have a helical focuser to hold the guide camera, very (read VERY) difficult to aceive by push-pull and hope for the best trial and error even on a distant object in daylight. Get close in daylight and then make the most miniscule of movements under the stars. Take something fragile to throw hard against the ground whilst you're doing this or it could end up being your telescope;) Fortunately, you only need to do it once.

One other thought...If it won't focus on a ssag, it won't focus on a lodestar. Have you got the oag in the correct place? Have a look here.

HTH

[maxchess]

The Celestron OAG I use has a helical focuser on the guide camera fitting, so achieving focus was not the issue. I focused during the day as you suggest and I could get focus when pointed a a bright star. I used Altair, but when the scope was pointing at my intended real target the potential guide stars were too feint. I could see them, but they were very dim and lost amongst noise of high gain. I tried lots of settings for gain and various transformations available in EKOS and I used darks.  This lark is difficult enough already and using a poor camera just makes it harder. It all worked fine through a 50mm mini scope, just not the tiny prisim of an OAG at extended focal length.

[maxchess]

Alacant, I notice from another thread that you use a monochrome Datyson T7 with your OAG. Would that be any better than  my Orion SSAG or ASI 178MC? its a lot cheaper than a Loadstar.

[alacant]

11 hours ago, maxchess said:

Would that be any better

Hi. JTOL...

Unfortunately we're not comparing like with like. Our Newtonian reflectors have secondary mirrors which illuminate the prism adequately. Your reducer makes the c8 light cone drop rapidly toward the edge; maybe the oag was meant for f10. Try a field flattener with the 6.3 perhaps?

The t7m is identical to the asi120mm and fwiw, we've never failed to find guide stars in the fov, even in barren zones. We've never had a ssag to be able to do a comparison. Maybe the best way to solve this is to join a club; find a guy with a lodestar and a ff and tell him his telescope is the best... You just got yourself a demo...

**Is there any way you could get the oag on the telescope side of the reducer? Otherwise, we're out of ideas...

Good luck anyway.

  

[ollypenrice]

Alacant's original suggestion was the best. I've no idea why Celestron advise against altering the prism depth but this is daft advice because it can't be the same with or without reducer.

So make sure the prism is going to be at right angles to the middle of the long side of the chip and then, taking rough flats, insert the prism deeper till its shadow shows on the flats and back it off a tad. then focus.

From the guide software's point of view perfect focus is not to be desired (according to Craig Stark who ought to know) but if it's too far out of focus the stellar image may not be bright enough to register.

Olly

[maxchess]

I followed that advice and now get the prisim fully iluminated, by day I can see distant trees clearly full frame in the oag guide camera and the main camera image seems unaffected. The problem is that at night the very few available stars are too dim to guide on. The Orion SSAG and the ZWO ASI 178MC do not seem sensative enough despite adjusting gain, applying darks and various transformations. I can guide ok with a 50mm guidescope but not with the oag.

Everything I have read elsewhere confirms this view and while it might occasionally possible to use a cheaper camera , to get reliable OAG guiding at long focal length you need a very sensitive guide camera, like the lodestarx2. So I am going to plan B and mounting a 90mm fl500mm frac on my C8 and try to guide with that. Should be a lot more photons.

[michael8554]

I guide an 8" SCT at F6.3 with a 200mm guidescope,  a 90mm F500mm guidescope seems to me to be too big, heavy, and prone to flexture.

I'd ideally like to OAG but I'm put off by the many problems like yours that I read.

Michael 

[maxchess]

Ok, as i write i am finally  guiding using the 90mm fl500mm frac on my C8 with the Orion SSAG camera. I am using the internal guider with Indi Ekos on a Rasperry Pi3+. This allows me to sit inside and track progress via VNC on a tablet or laptop,  All seems fine and the camera can find stars with the gain turned up to 70 out of 100 on the SSAG. Seems to be keeping within the 2 arcseconds limit most of the time, but my Polar Alignment probably needs some work. I also think I am guiding at the limit of my EQ5-pro weight. Currently the total weight of the two scopes and the cameras etc is 10.2 kg.  I think the max for imaging is 11 kg.

I am only doing 200 sec exposures because its M2 and quite bright, but the surrounding stars look sharp.

[ollypenrice]

26 minutes ago, maxchess said:

Ok, as i write i am finally  guiding using the 90mm fl500mm frac on my C8 with the Orion SSAG camera. I am using the internal guider with Indi Ekos on a Rasperry Pi3+. This allows me to sit inside and track progress via VNC on a tablet or laptop,  All seems fine and the camera can find stars with the gain turned up to 70 out of 100 on the SSAG. Seems to be keeping within the 2 arcseconds limit most of the time, but my Polar Alignment probably needs some work. I also think I am guiding at the limit of my EQ5-pro weight. Currently the total weight of the two scopes and the cameras etc is 10.2 kg.  I think the max for imaging is 11 kg.

I am only doing 200 sec exposures because its M2 and quite bright, but the surrounding stars look sharp.

You need to know your imaging scale in arcseconds per pixel... 

To calculate pixel scale we divide 206.265 by our focal length in mm and multiply this by our pixel size in microns.

In order for your guided mount to support a given pixel scale your guiding RMS needs to be about half the pixel scale. This takes away much of the subjectivity. Round stars are often taken as evidence of satisfactory guiding but this is wrong because equivalent errors in both RA and Dec will produce round stars even when those errors are enormous.

Olly