Help Please - OAG camera looks “cut off”

[Vinnyvent84]

Hoping everyone can take a look and give some advice on this as I’m a newbie and I’ve tried so hard to get this sorted out but to no avail. 
 

I just put my 8 SCT on my new AM5. I know from my reading that at that focal length it’s advised to use an OAG. In addition from what I’ve read it’s best to use the Celestron OAG as it has a large format prism of 12.5mm and matches mixed with the larger sensor of the ASI174mm mini. So I purchased both. Added them to my imaging train on the 0.63 reduced SCT 8. 
 

I know I have proper back focus of 106mm (added 1mm to account for filter). I read that it’s best to do flats as a test to ensure that the prism is not intruding on the main cameras chip; now honestly that is not needed because I’m only using a 678mc planetary camera as the main imaging camera as I await delivery of my 533mc. As expected no intrusion but this is when things get weird. 
 

I ran a test flat on the guide camera. And it displayed what seems like the sensor getting cutoff to the left. Seems like only 80% of the sensor is getting light. I then dialed up the gain like crazy and you can see that as well in one of the images below. In addition I took a screenshot of the focus tool in the ASIair and you can tell in that image it’s very dark to the left. 
 

I literally adjusted that prism all the way from one extreme to the other along with different movements of the camera in the helical focuser of the OAG. I put the prism of the OAG so deep inside the OAG that it almost fell out. I can’t understand how I wouldn’t be getting use of the whole sensor. But then when I go outside and Focus on a power line for example it looks like the whole sensor is being used. It’s baffling.  Any advice that someone can give would be massively appreciated. Thank you! 

[Elp]

Note when using the focus mode it's a zoomed in view due to the camera and screen needing to update quickly so that might explain why you see the power line shot in full view rather than cut off.

You have tried rotating the camera around the central axis of the OAG haven't you? The adjustment should be there so you can align the OAG prism square with one of the sides of your imaging sensor. The way to do it initially is do it physically with the OAG+spacers+imaging camera off the scope and make sure it's square and just off the edge of your main imaging sensor, then mount to your scope and refine. Either you can rotate the prism stalk part or you could rotate the imaging camera.

Edited May 27 by Elp

[Vinnyvent84]

11 hours ago, Elp said:

Note when using the focus mode it's a zoomed in view due to the camera and screen needing to update quickly so that might explain why you see the power line shot in full view rather than cut off.

You have tried rotating the camera around the central axis of the OAG haven't you? The adjustment should be there so you can align the OAG prism square with one of the sides of your imaging sensor. The way to do it initially is do it physically with the OAG+spacers+imaging camera off the scope and make sure it's square and just off the edge of your main imaging sensor, then mount to your scope and refine. Either you can rotate the prism stalk part or you could rotate the imaging camera.

Thank you for the reply Elp. Yes I have tried rotating the camera around the central axis and prior to mounting the imaging train to the scope I ensure the prism was squared with the long side of the sensor on the main imaging camera. But the thing is the main imaging camera chip is tiny because it’s a 678mc planetary camera so the prism, even when it’s as far into the OAG as possible doesn’t even come close to getting near the sensor edge. 
 

Thanks for the observation about focus mode view I didn’t know that and it is good to be aware now. However, if you see the guiding view with the green cross hairs notice the gradient of black background. It goes from a lighter black on the right to a pitch black on the left; same location I think the sensor gets “cut off” at. If a star enters that error the guiding will stop as it says guide star lost. 

I even took the guide camera out and tried pointing my cell phone camera in the hole to see the view and this image is the best I could get - not sure what to make of it. 

[knobby]

Are you using a diagonal with the OAG ?

[knobby]

Scrub that, just realised that's the view down the OAG itself.

[knobby]

Have you tried the OAG while not connected to the scope to see what it looks like ? I seem to remember my 8 inche HD reducer made the image circle smaller, maybe worth checking OAG in different ways of connecting ?

[Elp]

Looking down the OAG side, what's that dark semi circular shape on top of the illuminated circle (bottom half)?

[Vinnyvent84]

5 hours ago, knobby said:

Are you using a diagonal with the OAG ?

Nope no diagonal being used. Here is a picture of the imaging train. 

[Vinnyvent84]

4 hours ago, Elp said:

Looking down the OAG side, what's that dark semi circular shape on top of the illuminated circle (bottom half)?

Honestly I have no use lol there’s nothing in the OAG but the prism itself attached to its stalk so I’m unsure what’s causing that. There’s no debris or loose pieces of anything and I wore surgical gloves when I made the adjustments. 

[Vinnyvent84]

5 hours ago, knobby said:

Have you tried the OAG while not connected to the scope to see what it looks like ? I seem to remember my 8 inche HD reducer made the image circle smaller, maybe worth checking OAG in different ways of connecting ?

Sorry if I sound silly but can you please elaborate? How can I check it while not connected to the scope? 
 

I know the image circle on an 8 SCT does shrink when using a reducer but to what amount I honestly do not know. 

[Vinnyvent84]

4 hours ago, Elp said:

Looking down the OAG side, what's that dark semi circular shape on top of the illuminated circle (bottom half)?

Actually come to think of it it may just be the prism stalk and it looks that way due to the perspective of the angle I was holding the cell phone. If you look closely I’m coming in at a higher angle I think and pointing it downward so that may be the reason it looks that way. 

[Elp]

Are you using an Edge HD8 as it doesn't look like the reducer for that scope. It also looks like it's screwed on the wrong way around, reducers normally scope side are fatter then taper in camera side.

[bosun21]

The Celestron 0.63 reducer is not suitable for Edge SCT's apparently? This is what some suppliers are indicating.

 

[Oddsocks]

22 hours ago, Vinnyvent84 said:

Hoping everyone can take a look and give some advice on this as I’m a newbie and I’ve tried so hard to get this sorted out but to no avail. 

I just put my 8 SCT on my new AM5. I know from my reading that at that focal length it’s advised to use an OAG. In addition from what I’ve read it’s best to use the Celestron OAG as it has a large format prism of 12.5mm and matches mixed with the larger sensor of the ASI174mm mini. So I purchased both. Added them to my imaging train on the 0.63 reduced SCT 8. 

I ran a test flat on the guide camera. And it displayed what seems like the sensor getting cutoff to the left. Seems like only 80% of the sensor is getting light. I then dialed up the gain like crazy and you can see that as well in one of the images below. In addition I took a screenshot of the focus tool in the ASIair and you can tell in that image it’s very dark to the left. 
 

The partially illuminated guide sensor is to be expected with your system components but it should not prevent you using the OAG effectively.

The reason that the guide sensor is not fully illuminated is because you can’t physically move the prism/mirror stalk far enough into the primary beam, which is the part of the focal path that is exposed to the full aperture of the SCT.

If you look at the diameter of the focal reducer compared to the diameter of the OAG, and also remember that the reducer is effectively reducing the diameter of the image circle at the focal plane (main camera sensor) then you will realise how far you would need to move the OAG prism/mirror inwards to fully intercept the beam path as it converges towards the main camera sensor.

You can see that directly by eyesight alone from the other end of the OTA.

With the OAG and cameras mounted, If you peer into the mouth of the OTA and shine a flash light down the tube, then looking through one eye at close to the centre of the aperture, you should be able to see the main camera sensor in the centre of the field and off to one side you should be able to see the major part of the prism/mirror of the OAG.

The part of the prism/sensor that you can see exposed when your eye is positioned central to the OTA aperture will be the part that is seeing on-axis light.

Now move your head/eye over to the side of the OTA mouth opposite the side that the OAG prism/mirror is currently and you’ll see that more of the prism/mirror is visible, but in your case probably still not all of it.

The part of the prism/mirror that you can see with your eye to the side of the aperture is exposed to off-axis light entering the OTA.

Lastly, move your head/eye across to the other side of the OTA aperture so that it is on the same side as the OAG prism/mirror and you will only be able to see a very small part of the prism/mirror is still visible.

With the above demonstration you can now visualise why you see a bright semicircular image that does not fully cover the entire sensor when exposing and stretching a flat field image on the guide camera but when you expose a general scene, or star field, you will see stars across all parts of the sensor that has some exposure to the on-axis and off-axis light rays entering the OTA.

Any part of the OAG prism/mirror (and therefore the guide sensor) that you could not see when peering into the mouth of the OTA with a flashlight, wherever you placed your eye/head around the circumference of the mouth of the OTA, will always be dark and no image will be formed. As long as you can see the full prism/sensor from one side of the mouth of the OTA then you’ll always be able to find a guide star to guide on no matter how poor the flat field looks.

Because the diameter of your flattener/reducer is much smaller than the diameter of the OAG all you can do is move the OAG prism/mirror stalk inwards as far as it will go while not obscuring the main camera sensor, and while still being able to reach focus on the guide camera, and then accept that the fully illuminated field will not cover the entire guide sensor, even so, you should be able to find and guide on stars over pretty much the entire field but there will be a big drop in star brightness depending where the star is positioned in the field.

HTH.

[Vinnyvent84]

14 hours ago, Elp said:

Are you using an Edge HD8 as it doesn't look like the reducer for that scope. It also looks like it's screwed on the wrong way around, reducers normally scope side are fatter then taper in camera side.

Hey Elp, nope it’s a none edge - standard C8. It’s a Celestron 0.63 reducer. It’s screwed on correctly as it only has only one side that will correctly attach to the rear of the OTA. The image posted by bosun is the one that I have. 

[Vinnyvent84]

13 hours ago, bosun21 said:

The Celestron 0.63 reducer is not suitable for Edge SCT's apparently? This is what some suppliers are indicating.

 

That’s correct. I’m using the non edge sct. It’s just a regular C8 that came on the evolution mount. 

[Elp]

I think you've confirmed this already but the prism stalk is as far into the OAG it can go? Looking at your image train I've got a feeling it's partially looking at that tapered piece which is connected between the OAG and the reducer, which doesn't really make sense because if you were using a larger sensor the issue would be worse. I don't have that OAG at hand so can't look at it, both the ZWO one I had and the Celestron Radial Guider I have were/are much simpler in design and not as bulky in shape.

[Elp]

If you could take a few exploded images of the individual bits split apart with the prism and main imaging sensor in view it may help us diagnose.

 

[Elp]

1 hour ago, Elp said:

tapered piece

I think this is your issue, all the images I've seen of that part show it assembled this way below which makes sense, most tapered flatteners also orient like this, fat end on scope, thin end camera side, you might have the OAG on the wrong way though I don't know how due to prism orientation unless it's removable and can go back in both ways looking front and back:

[Mr_Cat]

2 hours ago, Elp said:

I think this is your issue, all the images I've seen of that part show it assembled this way below which makes sense, most tapered flatteners also orient like this, fat end on scope, thin end camera side, you might have the OAG on the wrong way though I don't know how due to prism orientation unless it's removable and can go back in both ways looking front and back:

Looking on Flo that photo is of an adaptor for the larger SCT's with a 3.25" SCT thread and using with a full frame cameras
 

Here is my 925 with .63 reducer and Celestron OAG. To get the OAG to work with the reducer I've had to move the prism housing as far as I could into the light path - but my prism housing seemed to have an end stop so when I loosened the allen bolt it only moved "so far" - no risk of it falling out despite what the instructions say..
 

22 hours ago, Vinnyvent84 said:

Nope no diagonal being used. Here is a picture of the imaging train. 

 

Looks remarkably similar to mine!
 

[david_taurus83]

The Celestron OAG is a great unit but the stalk is a bit short. See below pic of mine when used with the correct 0.7 Edge reducer for my C8 Edge HD. The prism looks like it would barely pick up any light through the reducer. That said, I have no problem getting guide stars across the chip of my ASI174 mini. The sensor is massive compared to other guide cameras. How does it look if you look at it like below with your reducer?

[Elp]

32 minutes ago, Mr_Cat said:

Looks remarkably similar to mine!

So it's likely looking at that flanged piece. Does that OAG have prism tilt adjustment as that may help, my radial guider does.

[Vinnyvent84]

3 hours ago, david_taurus83 said:

The Celestron OAG is a great unit but the stalk is a bit short. See below pic of mine when used with the correct 0.7 Edge reducer for my C8 Edge HD. The prism looks like it would barely pick up any light through the reducer. That said, I have no problem getting guide stars across the chip of my ASI174 mini. The sensor is massive compared to other guide cameras. How does it look if you look at it like below with your reducer?

I only have this picture currently but I’ll be home from work soon to take the one your requested. I hope it helps. If memory serves I have the stalk down as far as it will go. If it’s not there is only a minuscule amount more I can lower it of that I’m sure. I appreciate the help on this!

 

[Vinnyvent84]

3 hours ago, Elp said:

So it's likely looking at that flanged piece. Does that OAG have prism tilt adjustment as that may help, my radial guider does.

I’m not expert but I do not believe it does. I just see the grub screw that can be loosened to move the stalk vertically in either direction. 

[Vinnyvent84]

4 hours ago, Mr_Cat said:

Looking on Flo that photo is of an adaptor for the larger SCT's with a 3.25" SCT thread and using with a full frame cameras
 

Here is my 925 with .63 reducer and Celestron OAG. To get the OAG to work with the reducer I've had to move the prism housing as far as I could into the light path - but my prism housing seemed to have an end stop so when I loosened the allen bolt it only moved "so far" - no risk of it falling out despite what the instructions say..
 

Looks remarkably similar to mine!
 

It does! I notice a key difference though. Your guide camera is much further into the OAG than mine. Perhaps that’s my issue. I originally thought having the spacer in front of the main imaging camera was impacting the stance requirements that need to be match in relation to the guide camera, the main and the prism. But if you’re having no issues and you have the spacer located in the same spot then I guess I can rule that out.