My OAG needs 18mm in-focus - I have 9.5mm max - how can I solve this problem?

[Giles_B]

I am using a RVO deluxe OAG - a large prism is needed as my focal length is 975mm.

I have the guide camera sitting on top of the OAG stalk. I can reach focus with my OAG if it is placed an additional 18mm away from the filter wheel and the camera, i.e. 18mm toward the objective. This makes the total image train length 64.5mm - comprising 20mm (OAG) + 18mm (Spacer) + 20mm (EFW) + 6.5 (distance to ASI1600mm sensor).

This is 9.5mm more than the 55mm needed if I use a reducer (0.8x takes the FL to 780mm). I have experimented with a 1.5x barlow on the guide cam and can't get the guide cam to focus, so I assume this gives too much back focus.

Am I missing anything that could help me reduce the spacing distance by half? Do different OAGs have differences in light path sufficient to make it worth experimenting with a different OAG?

Edited May 29 by Giles_B
Clarity

[Oddsocks]

Not sure that you need to specify such a big prism requirement for a 975mm FL OTA, that is more often suggested for long focal lengths typical of SCT's of 1.8m plus where the OAG is seeing such a small FOV that a large prism and large guide cam sensor is needed to have a chance of finding a bright guide star.

I think that at 975mm native FL, and a reduced 780mm FL, for most areas of the sky you could get by with a slim OAG of just 9mm or 10mm thickness and smaller prism.

With a smaller prism you will sometimes have to jog the OTA away from being perfectly centred on the target so that a bright guide star falls on the guide cam sensor but with a modern CMOS guide camera this is not required nearly as much as was once the case with less sensitive CCD guide cameras.

The 55mm overall backspace requirement for your flattener/reducer puts a hard limit on what you can fit in the gap between the main camera, filter wheel and flattener/reducer.

[david_taurus83]

Why the 18mm spacer? Can you not change it to a 10mm or 7.5mm?

Edit: Sorry, I read that again and realised you need to push focus out for the guide cam. Is it a nosepiece type guide camera? You got a picture of how everything sits together?

Edited May 29 by david_taurus83

[Giles_B]

Thanks - yes @david_taurus83, the guide cam currently needs to be much closer to the objective than the camera.

@Oddsocks Would a slimmer prism need less in-focus? Rather than  more? and if so, would it gift me around 10mm less than my current prism? I realise it could be a case of just trying it out, but given the cost of an OAG is not inconsiderable, I'm loath to spend money with no idea of the chances of success. I've tried in vain to find some information about the light path of OAG prisms/mirrors...

Sorry I should have thought to add a picture (ignore the 2" extension, it is in place because I could not get a good grip on the imaging train with the focuser's twistlock mechanism):

[Oddsocks]

I had a diagram somewhere of a typical OAG light path but can't find it ATM, I'll edit the reply later and add the diagram if I can find it again.

Basically, the distance from the centre of the prism/mirror in the OAG to the main camera sensor must be  identical to the distance from the centre of the prism/mirror of the guide camera sensor so that both reach focus.

If you changed the OAG for a slimmer version that would move the prism/mirror closer to the main camera sensor and you would need to move the guide camera further in towards the OAG prism/mirror so that both sensors are still at focus.

So to answer the question whether you could still reach focus on the guide camera with a slimmer OAG would depend on how long the OAG stalk is and how the guide camera is attached to the stalk.

The limiting issue with your current setup is how far inwards you can move the guide camera in the Baader focus mechanism that is attached to the OAG stalk

If you removed 9.5mm from your current reducer-main camera spacing are you able to move the guide camera the same distance inwards towards the OAG prism/mirror so that both sensors are again the same distance apart, I'm guessing not, which is why you are stuck.

Maybe a cheaper solution that replacing the entire OAG would be to find a simpler T2 attachment for the guide camera coupling instead of the Baader mechanism that is currently pushing the guide camera too far out so that you can't reach focus on the guide cam when you have the correct 55mm distance for the flattener.

Edited May 29 by Oddsocks

[Clarkey]

47 minutes ago, Oddsocks said:

Maybe a cheaper solution that replacing the entire OAG would be to find a simpler T2 attachment for the guide camera coupling instead of the Baader mechanism that is currently pushing the guide camera too far out so that you can't reach focus

Agreed. The camera is a long way from the prism. If you get rid of the Click-lock and add a micro-focuser it is much simpler.

4 hours ago, Oddsocks said:

Not sure that you need to specify such a big prism requirement for a 975mm FL OTA

Also, I would agree with this. I am using a 'standard' prism and 120mm mini at 1000mm and I have used a similar set up at 1200mm FL without issue.

[Giles_B]

I think appearances may be deceptive - the camera UV/IR window is sitting directly on top of the prism stalk, so there is no inward movement - all the spacers in the camera have been removed, and the baader focusing eyepiece holder is not restricting inward movement at all - it is a bullet style guide camera, a 220mm, so goes down past the visible focuser mechanism.

Interesting what you say about the 120mm mini - I was specifically advised against this at one point, which is why I opted for the 220mm mini.

It sounds like I'm unlikely to reduce the in-focus requirement by much if I switch the OAG, although given your experience with standard prisms it might be worth keeping an eye out for a second hand one if it comes up.

I assume I'm on a hiding to nowhere trying to use a barlow to add back focus to the guide camera? I've had no success so far, but potentially could be something I continue to experiment with if it's got any chance of working.

 

[Clarkey]

Looking at the picture the bottom of the camera definitely looks quite a long way out compared with mine.

I'm not sure who said not to use the 120 mini. Yes, they are a little old now and the 220 will undoubtedly have a better QE, but they still work. I upgraded to a 290 which is marginally bigger - but here is not much in it.

[Clarkey]

1 hour ago, Giles_B said:

I think appearances may be deceptive - the camera UV/IR window is sitting directly on top of the prism stalk

Must be a long prism stalk.....

[Oddsocks]

Is the T2 plate welded to the prism stalk or secured with grub-screws?

If grub-screws you could shorten the stalk by the required 9.5mm and remove 9.5mm from the flattener-main camera BF distance.

That would bring the T2 plate down to just touch the body of the OAG, judging from the photo, an exact distance measurement for the gap between the base of the T2 plate and the body of the OAG would confirm if that is doable.

Edited May 29 by Oddsocks

[Giles_B]

14 minutes ago, Oddsocks said:

Is the T2 plate welded to the prism stalk or secured with grub-screws?

If grub-screws you could shorten the stalk by the required 9.5mm and remove 9.5mm from the flattener-main camera BF distance.

That would bring the T2 plate down to just touch the body of the OAG, judging from the photo, an exact distance measurement for the gap between the base of the T2 plate and the body of the OAG would confirm if that is doable.

It's got a grub screw! This is a fantastic idea if (I can make sure I don't scratch the prism). I'll take everything apart now and get it measured, but it sounds like it should work

Indeed, now I've got it apart I can see that the stalk has been adjusted and probably is protruding an unusual amount beyond the baseplate, which would be why you'd initially queried the guide camera position!

[Giles_B]

Thanks again for talking this through and offering advice. I, successfully shortened the stalk, applied some blacking and it's all good as new. Total distance of the image train is now 54.5mm