Problems Setting Up OAG on 8" R-C

rcolman

We are trying to get the OAG working on the following setup:

main train: AT8RC ... tilt plate ... OAG ... filter slider .... ASI294MC
guide train: OAG ... helical focuser ....ASI290

Since the scope is designed for 10" backfocus, my main assmpotion is that both the 294 masin sensor and 290 guide sensor must both be at 10" out, regardless of stuff in the middle. And that once the main camera is focused, that the helical focuser can then be used to touch up as needed,

But, so far, i cant sdee anythiung in the guide camera. What am I doing wrong?

w7ay

rcolman But, so far, i cant sdee anythiung in the guide camera. What am I doing wrong?

Is the prism facing the correct direction? Remove the guide camera, and look down the hole. You should see daylight coming from the OTA. If you see the camera sensor instead, remove the stalk, turn it around and reinsert.

Chen

rcolman

That was not the probem, but thank you.

Both camera sensors are rectangular, the prism is square, does the alignment of the three elements make a difference?

rcolman

There is something wrong with this whole setup.
These are all ZWO COMPONENTS: THE asi294, assi290, ASI HELICAL FOCUSER AND asi oag. i CAN GET A FOCUSED IMAGE ON THE MAIN CAMERA, but all I see on the guide cameras is a dull grey background. Distances have been carefully measured, but nothing seems to matter. I am sure the prism is facing the right direction.
I tried swapping the cameras in case the guide camera had failed, but I get the same effect with either camera.
I am stumped and about to give up and go back to my glideslope setup.
I know a lot about telescopes and this should not be sd difficult
???????????????????
As a side note, the design for setting prism position could be improvedr. To move the prism, both set screw and positgion screw must be released, so it flops around without a reliable way to systematically movre from one position to asnother, so you are always starting over again each tiime.

stevesp

Did you move the prism a bit out?
there are a couple of screws that lock it radially
if you cannot get focus even extracting the camera (manually) the focus is too inside and you need to extract the prism

koichi

I was asked a similar question on my AstronBin page. If the cause of your issue is the same as that, the comments in the following link may help you:

https://www.astrobin.com/umbv5i/

rcolman

HERE IS WHAT i AM SEEING. WITH A REVERSE CONFIGURATION , I.E. 294 AS GUIDE CAMERAZ AND 290 AS MAIN CAMERA
(FOR TESTING ONLY). Using a close target (200 yards) when the prism is moved into the tube, over the center of the main sensor, everything suddenly comes into focus on the guide camera,
What does this mean?

stevesp

The more you extract the prism the more you extract the focus, but the less you intercept the light from the telescope

Tech@ZWO

Loosen this screw and move the prism up to try.

rcolman

thank you. will do.

rcolman

The solution ... I replaced the ASI290 with a ASI174 mini and now everything seems to work ok.
i am, however US$500 poorer and still not sure why the "pancake version" would not focus properly??

philippe

Helllo,

That's curious, I experienced something quite similar but probably not for the same reasons !

I'm currently doing my best to collimate my brand new TS RC 8, and initially, I had the same problem you detailed.

I've got the same cameras, and also the same Zwo standard OAG, and even though I initially set the highest gain for my 290 MM (with the risk of guiding on dead pixel or artefacts), I had nothing, blank screen, yet, swapping cameras in the ASIair App as you did, the 290MM, I could see stars !

I moved along and finetuning my colliamtion, and I'm now getting very close to a decent image, just the corners I've to work on as I'm probably dealing with a slight tilt and/or BF, not quite sure.

Now, with with a better collimation, I can get a few stars for my guiding camera, but it's very dim !

With my newton I used to get oval shaped stars, but always enough of them, now I get a sort of "blob" very faint, and what is normal, I get fewer stars.

I also improrved the quality simply enabling Bin2 for my guidng camera, what I didn't need to do with my newton 200/800 of course !

When the sky is clear I can guide reasonably well, I managed to get a good 0.40/0.5 last week during 1 hour, but 2 days ago, due to high altitude cloud I guess, it was harder and closer to 1" !

I set my prism so as to position it parallel to the sensor and slightly above when you are looking the front part of your OAG, but it doesn't overlap the sensor, what I've controlled with my flats.

On top of the guiding part, I've also had to struggle with step size for the EAF and find the right setting, it was quite long and iterative, but now it's ok.

The point is that with 1 624 mm of focal length and above for some other telescopes, guiding with OAG is getting a really hard job !

Philippe

stevesp

Setting OAG focus is very difficult during the night
I did it during daylight with a target

rcolman

So the conclusion to this story ,,,

The configuration that was "focusable" was:
The ZWO OAG coupled to a ZWO helical focuser using a ZWO ASI174 imager through a UVIR cut filter.
The other side is a ZWO ASI294 OSC.We were not able to use my existing ASI290 pancake imager because we could not get it positioned prperly with ther focuser, etc. apparently.

Being new to OAG guiding, although with lots of guidescope experience; the resulting guiding image looked dull, and few stars were visible. The ASI174 larger sensor could probably use a larger prism within the OAG.

I have spent a lot of money to gear up for off-axis guiding, but at 1620mm I suspect that differential flexure is not much of a problem and guiding performance might not be better than my 400mm guidescope. I am currently guidescope tracking at under 0.5" RMS total error.

but, I cant say yet since the clouds have not liofted in southern ca for two weeks straight.