First attempt to use an off access guider - questions

[g__day]

Well between showers tonight I tried to use the Lumicon OAG. Pointed it at Rigel Kentarus and got the Canon DSLR focused, but had rather more trouble focusing a Meade DSI.

So I swapped out the DSI and used a 22mm Vixen LVW through a 1.25" diagonal - and I still couldn't even see the star in the guider. No matter how I moved the mount around, how I changed the position of the OAG, nor bringing the eye-piece in or out I couldn't get a star, let alone a focused star.

I removed the eye-piece and looked into the prism and could see a star doughnut of light, but it looked to be off centre to the back of the eye-piece - not centred. I tried moving the mount but couldn't get it to be centred in the guide cell.

Any idea of what I am doing wrong. The image train goes SCT -> Meade Motorfocuser -> OAG -> DSLR on the main path and 1.25" diagonal -> Meade DSI on the other path.

Could it be the prism is slightly adjusted wrong, the image path having the Meade motorfocuser is screwing things up or does anyone with knowlegde of how to set OAG up correct able to offer suggestions please!

Many thanks,

Matthew

Picture of rear of OTA attached - before I added Canon DSLR and 1.25" diagonal

[Merlin66]

Looking at the photo one thing stands out.. I need at least a 2" extension between the OAG and the guide eyepiece/ camera.
I use a prism diagonal in this section to minimise the distance ( over hang)
(Must be a prism type not the mirror type - to reduce the optical path length)
This should get you into a focussed guide star image.

To get the set-up "tuned" the easiest way is to position the OAG pick off prism in the RA direction. That way, when you position a bright star you can allow it to drift in RA until it hits the prism, makes it much easier to check things out.

Light train SCT -> Meade Motorfocuser -> OAG -> T ring + DSLR .
Guide: -> 2" extension ( and /or prism diagonal) -> Meade DSI.
I assume the diagonal you mentioned in the image train was for visual checking only!

[Kokatha man]

Looking at your attached image, the answer is quite simple Matthew - your equipment is suffering from a massive overdose of optics: is that 5 or 6 scopes clustered there?!?

No wonder you could only see a single donut of light - that many scopes are just soaking up too many photons!!!

[g__day]

Well I must say Lumicon tech support are very good at getting back to you with substantive advice!

* * * *

Sorry for the inconvenience. The following are ideas, starting with the easiest to check.

The first thing to check is to be sure that the prism is mounted correctly. See attachment. The “flat” side should be facing the “upstream” telescope side. The 45º slope of the prism should be facing “downstream” toward the camera. This is most likely correct but not impossible to be wrong. You can see how easy it would be to switch. Just remove two screws and turn the assembly 180º then reinstall it.

Your camera target is one thing but you know the off axis guider is made to not guide off the target object but guide off a nearby star in the same field. The 360º rotation is to help you find a bright enough guide star in the nearby field of your target.

Focusing the DSI: I am not sure but I suspect the focus distance for the DSI is close to what is needed for an eyepiece. So this would mean you need the diagonal (or an extension tube) to focus the DSI. This is different than shown in the picture you sent. I heard that the depth of focus is almost nil for a DSI so it would be harder to find.

Most troubling and puzzling is you say moving the telescope did not change the doughnut. This points to something very wrong. You should be able to move the telescope and get a full doughnut in the off axis tube. OR is it not a full doughnut because it is off axis? And it will always be less than a symmetric doughnut no matter where you point it? The off axis image will be good once it comes into correct focus? Is it a non symmetrical doughnut because it is picking up some coma? Is it too far off axis? Some daytime test might be necessary just to figure out where focus of the off axis is.

The first thing to check is the prism direction.

You are the first person to tell us about using the OAG with an electric focuser and a CCD camera with a OAG.


Let me know what you find out.



* * * *


Well I just tried daytime focusing on a tree about 150 metres away - could see the leaves and bugs on them easily! I managed to get the OAG guider to focus in a 22mm eye piece by moving it outside the diagonal - so I guess I need just a tad more extension on the DSI (which I can probably to short term with a barolw I have for the DSI).


Next problem is to try and find a 3" extender tube with diammeter 1.25" (versus teh common 2" ones).


Merlin - when you say move the Guide Port on the OAG so its in the RA direction - what orientation is that - 12 noon, 3 o'clock, 6 o'clock or 9 o'clock?


Many thanks,


Matthew

[gbeal]

Matthew,
if I guess Merlins answer correctly it will be what I like to do, orientate the camera in the same line as the RA axis, and or the dec axis. Not necessarily 9, 12, 3, or whatever o'clock though. I usually just do it by eyeball.
Gary

[Merlin66]

That's right.
If the pick off prism is positioned such that a star when in the centre of the camera can be allowed to "drift" (ie with either the RA motor off, or with fine slow motion control) it will end up on the prism. You should be able to do the same during daylight, focus on a definate object and then slightly slew until it shows in the guide. Good practise!!

[g__day]

So if I read that right for a SCT - its put the prisim at the top of the tube, or is it two light bounces so put the prism at the bottom of the Guider tube? A picture would be easier for me to comprehend guys! Once I have focus - it will be easier to follow your advice of course!

[Merlin66]

Depends on the mounting. If its an equatorial then the position may be at 9 o'clock.

[gbeal]

I'm guessing 9 o'clock or 3 o'clock. Try it during the day.

[g__day]

Taa - thanks!

I was seriously pleased, very seriously pleased with my first imaging attempts even without a guider - as the OAG has a focal reducer, bringing the optics down to probably F6.5.

I targeted Eta Carina, M42, M46 then M47. Shots looked more than fine out to 6 minutes unguided. Then I did 10 and finally 20 minute guided shots of M47. It can be hard to find a guide star with a DSI - probably need a DSI II Pro, but jeeze this was fun and lovely to see pin point stars!

Well quit after 4 or 5 of the most enjoyable hours of imaging, wherein:

1. Every slew (80% remote) was spot on - and I mean dead centre of the CCD
2. Even with high cloud cover - every shot looked great, was focused (not yet colour adjusted - but hey captured RAW so do that later)
3. Tracking was great all round
4. On 80% of shots a star was captured in the OAG - and this meant tracking out to 20 minutes gave pin point stars - no trails whatsoever (but the core was blown out)
5. First time use of a focal reducer was brilliant!