OAG Focus help

[AstroBogan]

Hi all,

after spending all afternoon/night trying different setups, I cannot for the life of me get my ZWO asi 120mm-s guide camera to show any stars through my ZWO OAG, and it is just a fuzzy grey screen.

My image train is attached in the picture but it goes like this:

ZWO ASI1600mm > ZWO EFWMINI > 10mm spacer > ZWO OAG > 16.5mm spacer > CCDT67 > Moonlight focuser > Collimation ring > 50mm extension tube > GSO RC8 CF

I can get focus fine with the main imaging camera, but I cannot seem to get anything from the guide camera through the OAG.

Any help please would be appreciated as I've spent too much time trying to figure it out and there isn't too much on the net :(

[speach]

looks like you need spacer rings between the camera and the OAG. Had the same problem.

[AstroBogan]

I already tried using as many spacers as I had, adding on each one as I went, which even at 100mm+ didn't do anything

[Joshua Bunn]

Hi Jacob.
You need to put the pictured setup on the scope in the focused, rack the focuser till you reach focus on the main camera. Make a not of that position.
Next, rack the focuser till you reach focus with the guide camera however much this takes... note that position. Now, the difference between the two positions that you noted is the amount you need to space or remove spacers from one of the cameras.
Does this make sense?


Josh

[Merlin66]

+1 for Josh's suggestion.
You could try to focus the guide camera first, then refocus for the imaging camera.
Have you had the guide camera working previously????

[AstroBogan]

Yeah i've had it working with a guidescope previously, but with such a long FL on the RC8 i want to drop the guidescope for the OAG..

Should I try and add another 25mm extension infront of the focuser maybe?



That's another problem. I had the focuser racked all the way in and out and still could not get any focus in the guide camera. Still a grey fuzzy screen. I read somewhere that someone else had an issue where their ZWO OAG was not close enough to the end of the imaging train, but I physically cannot move mine closer to the focuser.. Not sure if that applies here but it was so frustrating wasting 5-6 hours trying to get it to work.

[Merlin66]

Jacob,
I assume you've read the ZWO manual?
https://astronomy-imaging-camera.com/manuals/OAG%20Manual%20EN.pdf

In infers when you add the filter wheel and spacer to the imaging camera you need to add a similar length spacer to the guide camera .

[billdan]

I would check that the prism is actually in the light path, you should adjust the prism stalk so it sits just above the long edge of the ASI-1600 chip.

Probably not related to your problem, but my prism fell out of its stalk and when I put back in, I installed it back to front and obviously got nothing. That took me an hour to work out what had happened.

[AstroBogan]

I have, and unfortunately nothing is working :(

I've tried that combination and unfortunately got no image at all. I have the prism facing the correct way. I've tried with the focuser racked all the way in and moved it all the way out checking as I went. This is bloody annoying lol.

[beren]

Using the Moon at a late phase I found makes dialing in a OAG focus easier. Get the main camera in focus centred on the lunar surface,then manuiplating exposure times on the guide camera watch the screen and you”ll see variations in brightness and then add spacers/OAG focus until you can see a clear image

[AstroBogan]

Hmm, I tried using all different combinations of spacers on the Venus before it set and then the moon last night and the screen was just fuzzy and grey. Once I take the camera off of the OAG it shows that it is definitely picking up light.

Can anyone pick up any faults on the image train in my first post photo?

[Imme]

I'm with Merlin....get the guide camera focused first. At this stage it sounds as though you are not even sure if it is working in this setup (it may have previously in the guidescope)

It could even be your exposure time is too long causing a 'grey out'

[Merlin66]

I use the equivalent of an "on axis guider" with the reflective slit in the spectrograph. Used on a C11 with Lodestar guider. No issues, no drama.

A few years ago I sketched out the OAG arrangement to show why in 99% of the cases many guide stars looked crappy! and "seagulls" etc. The problem is that the prism needs to be tilted to bring the optical axis closer to the guide camera optical axis - this is commonly overlooked, hence the bad performance....

OK. Don't waste any more time under the stars. Set it up in daylight.
The attached images (not mine) show a typical set up and comparative FOV's.

[LewisM]

If I simply use my fingers as a set of pseudo-dividers, I believe that your guide cam is actually spaced too far out, literally by the length of that spacer you have there.

When I used an OAG, I used an helical eyepiece holder as my spacer - just move the guide cam up and down till focus, then lock it in. Makes the job 1000x easier than trying to guess a fixed spacer.

[Bart]

Just a thought, the main camera and the guide cam will not see the same thing.

I use the full moon as a guider to dial it all in. Aim at the moon and focus your main camera, adjusting exposure etc to achieve this.

Next, flick over to the guide camera screen and move the telescope a few degrees in each direction until you can see the brightness of the moon, moving it to cover the guide cam chip and keep adjusting focus of the guide cam and exposure to get an image of the moon with out moving the telescope focuser. You may already know which direction to move the scope to get the guide cam to see the moon.

Once you get a good moon focus in each camera, you can move to a bright star with a Bahtinov mask and repeat the process to refine the focus.

I think the key is to understand they are hard to set up initially and the cameras will not see the same thing.

Good luck!

[Lee]

How are you trying to focus the OAG after getting focus with the main camera?

[AstroBogan]

Thanks for the info and the pics! i'll have a better look into those when im home.



I might have a look into investing one, bloody money sink :P



I tried that in a way, so I'll definitely give it a go when the moon is nice and bright and move to a star.



I've been marking the point that the main camera gets into focus, and then I move the focuser in and out until I can see something in the guide camera, but I get nothing at all out of it. It has to be the position of the prism or the spacing between the guide cam and the OAG.

I tried with the moon last night and there was absolutely nothing no matter how far I tried moving the focuser. I also tried moving the focus of the OAG up and down to no avail.

[Joshua Bunn]

I think you should go back to basics.
Make sure the camera works, work out the backfocus of the scope by measuring the distance that the imager comes to focus, then get out some measuring calipers to work out where the guide camera needs to be placed.

[Lee]

Assuming the camera is working, and the prism is pointing towards the scope objective, it then has to be distance between the guide cam CCD and the mirror in the OAG..... this distance has to be equal to the distance from the mirror in the OAG to the imaging cameras CCD plane.

I just pointed the scope into the middle of the milky way, with the guide cam positioned at approx the right distance, focus the imaging cam, then adjust the distance of the guide cam along its stalk on the OAG rig.

[AstroBogan]

Thanks for all the help everyone.

Just another curious question. I have a 80x400 guide scope to use in the meantime whilst I figure the OAG out. Will guiding with such a different FL to the main scope affect the guiding performance dramatically?

[Merlin66]

I would try to get the guide camera on the OAG working in daylight.
Once set up it’s good for ever and in the end will give better results.
Persevere.......

[Joshua Bunn]

With basically a 4:1 ratio of imager to guider FL, that should be ok.


Josh.

[AstroBogan]

Ok so I need clarification..

I was able to reach focus on the guide cam tonight.

To get focus on the main imaging camera I needed to rack the focuser further in by about 10mm. Does that mean i need to add 10mm to the OAG for the guide cam to become focused at the same time or do I need to take 10mm away from the main imaging camera before the OAG?

[Lee]

I think you need to either remove approx 10mm from in between the OAG and guide cam, or add 10mm in between OAG and the imaging camera.....

[Joshua Bunn]

Lee is correct, of course Lee's second option will depend on whether you have enough backfocus to do this.


Josh

[Lee]

This is still what you should do - I think you are getting there, but doing it the hard way.... remember the imaging and guide CCD planes need to be the same distance from the centre of the pick of prism. Get it close with calipers, focus imaging cam then fine tune the guide CCD distance.....

[RobF]

Sounds like you're getting there Jacob, but whenever you're lost for focus it can be helpful (and morale building!) to stick an eyepiece in and check you're in the ballpark for focus. Once you know within a cm or so where your main focus and spacings have to be, it becomes a matter of tweaking.

Until you get "in the ballpark" though, there's nothing much more frustrating than tweaking focus and spacings but only see unchanging grey/static on your guide cam screen.