TS-OAG Setup help needed.

[Merlin66]

Wasn't me either - honest!!

[ribuck]

Well i've just stripped down my gear and reassembled as follows

1) Prism Holder on same Axis as QHY5 ccd Chip.............Check

2) place Prism back in to OAG housing. Is Prism holder square with housing..... Check

3) Is prism axis aligned with Main ccd Axis..............Check

4) Looking at the Prism, can i see the ccd chip of guider reflected.............Check

5) Does reflection of guider chip axis look right................Check

6) Attach Coma corrector and ensure it's not catching........Check

Now wait for daylight tomorrow to see if i can take short exposures with guide cam on distant object and achieve focus.

and relax........lol

[ribuck]

One last question for now......

Say for arguments sake that the Main ccd chip is 25mm away from front of OAG prism, does the Guide ccd chip in theory have to be the same distance away or is there no set rule on this ?

Rich.

[Merlin66]

Rich,

The distance to the imaging focus behind the the centreline of the OAG is equal to the distance from the optical axis to the focus of the guide ccd. Call this 50mm.

As Bern, I think has said; from the optical axis to the top of the OAG body is 26mm, so the guide focus would be (50-26) = 24mm above the top of the OAGbody. I'll prepare a sketch which shows the relationship - much easier to visulise.

Ken

[ribuck]

Thanks Ken,

A diagram would be fantatic to help me better understand the relationship.

Rich.

[Merlin66]

Richard, etal,

The attached diagram shows the typical OAG layout.

The light from the telescope objective is entering from the left.

The focal plane of the telescope is at Ic and lies (obviously!) on the imaging CCD.

Beam A is the maximum ( sometimes slightly vignetted) edge of the FOV; depends on focal length

Beam B is the illumination of the imaging CCD. The CCD is usually smaller than the maximum FOV.

Imagine a star diagonal or a flip mirror or beamsplitter in the optical path. If the centre of the mirror/ beamsplitter is at point O then the beam will be reflected at 90 degrees to point Ig.

This is where a guide camera would be located. The distance from O to Ig is the same as the distance from O to Ic.

Ok, now replace the beamsplitter with a small pick-off prism (D).

It still picks up the same converging beam as the imaginary beamsplitter and focus Ig remains unchanged. ( Note that the beam from the prism is not aligned with the axis of the OAG - in real life, the prism is usually tilted slightly to correct this)

The problems of focusing a guide camera on an OAG are due to the offset (O-P). If the construction is correct the distance (O-P) is approx the same distance as the prism is from the optical axis, which can be measured. So, this distance from the optical axis for the guide CCD becomes: (P-Ig) = (P-Ic) + (O-P)

Hope this helps.

Ken

[ribuck]

Thanks Ken,

I appreciate the time you have taken to draw the diagram. Just need to figure out how i measure points "O" & "IG".

Rich.

[Merlin66]

Richard,

A close approximation would be to measure the "throat diameter" of the OAG and the intrusion of the prism. This would let you calculate the distance of prism to the optical axis. This is equivalent to (O-P).

The distance (P-Ic) can be measured from the centre of the OAG body to the camera CCD chip.

Ken

[Catanonia]

About to try exactly the same with a QHY9 / QHY5 and Lumicron OAG

My plan is as follows.

1. My QHY9 is bang on focus with my ED120 and locked.

2. Put in the OAG and manually adjust to compensate by eye on the length added.

3. Use a Bahnitov mask with the QHY9 to get back to focus and LOCK.

4. Slot in the QHY5 guide and move it around to find focus. I do have a slip collar to help with this for the barrel.

5. Cross fingers.

Lets see what happens eh

[Merlin66]

I assume you have the Newtonian Lumicon OAG???

You may need a spacer behind the OAG body and the CCD camera.

The original design was for a film slr camera with a 55mm backfocus.

If your imaging camera is too close to the rear of the OAG body you'll find you can't get close enough to focus the guide camera.

I used the Lumicon for many years with an 18" f4.5 and 10" Lx200.

IMHO It's one of the best OAG available.

Ken

[ribuck]

Well good lucky,

I've got storm clouds here so it's a no go tonight but they might clear.

It's quite spooky as my Bahnitov mask just arrived this morning, so similar apar from scope and OAG.

Rich.

[Catanonia]

I assume you have the Newtonian Lumicon OAG???

You may need a spacer behind the OAG body and the CCD camera.

The original design was for a film slr camera with a 55mm backfocus.

If your imaging camera is too close to the rear of the OAG body you'll find you can't get close enough to focus the guide camera.

I used the Lumicon for many years with an 18" f4.5 and 10" Lx200.

IMHO It's one of the best OAG available.

Ken

Based my purchase of the lumicron OAG on the reviews. I got the shortest version, so not really holding much hope out, but will give it a go.

Really want to get it working as a Ha 8nm filter is on its way and I want to get back to 20min exposures for it.

If it doesn't work, then Bern gets more business sorting out extensions

[ribuck]

Well after many mnay hours fiddling over 2 nights i finally got the TS-OAG to focus, but i've now hit another problem. I took a 60sec guided exposure and the stars were horrible and elongated even at the centre of the image even though i was guiding.

I now need to wait for another clear night to do further testing as i ran out of time before dawn beat me.

I'm going test the TS-OAG without the Coma corrector in place and see what i get as it's possible the problem could be related to that if it's position isn't right, but now need to order the 2" nose piece adapter that fits the TS-AOG......more cost.....it's never ending.......

Rich.

[Psychobilly]

Please post the warts and all image...

If your using a focal reducer or MPCC infront of the OAG then for it to work properly you should maintain the correct spacing between it and the imaging CCD and then adjust the guidecam to suit...

[ribuck]

I'll drag the image off my other laptop, and post up. The diastance of the Coma corrector is approx 2mm out and i also noticed a strange halo in the image which i had never seen before. Hopefully tonight will be clear so i can do some further testing.

Rich.

[Psychobilly]

trouble is you get so little time to do any testing this time of year that these things seem to drag on forever

[ribuck]

sods law, i forgot to save the image so i'll have to try and do more testing tonight and post the results from that instead.

I know exactly what you mean about running out of time and stuff dragging on. This time of year i can't star imaging until 11pm with having work the next day, it only really leaves the weekend.

But i'll try and sneak in an hour tonight. I'm going to try with the old guide scope tonight and see if i get similar results as i do with the OAG, this should at least tell me if it's anything to do with coma corrector.

Rich.

[Merlin66]

Rich,

I assume PHD showed the guiding was OK?

and still there was streaking on the image.....

All things being equal you have to therefore assume somrthing was moving between the guide camera and the imaging camera????

[ribuck]

Hi Ken,

I need to do some more testing before i can start to rule things out.

I need to following testing :

• TS-OAG - Without Coma corrector (need to buy TS 2" nose piece)
  

• Guide Scope - Main camera with Coma Corrector (Need to buy adapter to connect coma corrector to filter wheel.
  

• Guide Scope - Main camera with no coma corrector.
  

The reason i want to also do test with guide scope Vs OAG is to make sure there aren't any issues with my mount.

I Check my mount is properly polar aligned with using Eqascom alignment polar alignment tool.

Rich.

[barkis]

I made the diagram a bit more contrasty Ken. Hope you don't mind.

Ron.

[Merlin66]

Rich,

Those are all factors which will impact on the final quality of the image, but I'm not sure that any of them would impact on the relative movements/ differences between the guide camera/ imaging camera. If the OAG/guide camera is doing its job, the the imaging results should be tight.

[ribuck]

Well I had the OAG locked down tight, so nothing was movng. I'll do more testing on Wed as that looks to be the first clear night i'm going to get.

Rich.