Help needed with OAG on QSI

[Paul Haese]

Hi all,

I am looking for some help and advice with my off axis guider on the QSI.

When I try to focus on stars I get two things happening. One is that the stars just will not focus to points. In fact they are about 2mm long by about 1 mm or so wide. This is with the focus ring fully inward. This is despite the idea that the Star Shoot Autoguider is supposed to be ideal as a guider.

My question is; will the shape of the stars matter to guiding (my guiding seems really erratic in PHD with big peaks and valleys) and it seems that the stars are eggy. What about focus? I had heard it is better if the stars are slightly out of focus for guiding, but these stars seem a bit too out of focus.

I have used an OAG in the past and the stars were oval shaped but I never got great results then and I am just trying to work things out. I contacted QSI and they have been really helpful. They suggested that the RC is most likely the cause of the stars being that shape and I should use a flattener. I have at this stage not used the flattener. I had to resort to using the guide scope the other night just to get some sense out of the guiding.

I would really appreciate so advice.

[gbeal]

I'm guessing coma off axis Paul, and yes, the flattener would be the berries I fear.
If you have successfully used an OAG in the past then you are step ahead of me, and many others I fear.
Proof obviously is in the pudding. If the erratic guiding works, then great if not, (and especially if your separate guide-scope did) then I reckon you may be forced to use the flattener.
Is there any way that you can move the pick off prism more on axis, without impinging of course on the image?
Gary

[multiweb]

As long as the star is not saturated and PHD can work out a centroid I think the shape of the star doesn't matter. I did guide quite accurately on my C11 at F/10 with an OAG and the stars were blurry seagulls on the edges.

[Bassnut]

Does the QSI allow raising or lowering the pick off mirror?, this can affect asigmatism and focus.......... (oops, as Gary mentions). If not, and the guide cam is too close, can you just pull the guide cam out a bit and reclamp. If not, make a spacer?. I have also found, out of focus guide stars are fine, but being eggy is a worry.

[Paul Haese]

Fred I don't think the mirror can be raised or lowered. It seems fixed into position but there are three screws holding it down. These might be for tile and tip I guess.

The guide cam does not have to pull out to reach focus, it needs to go in to reach focus. Well to make it clear I had to wind down to move close to focus. When I was fully in I still had to move further in. If I put a spacer in this will just make the guide camera further away from focus? Isn't this right?

Kevin from QSI seemed to think mis-shapen stars are not a problem. What are your thoughts Fred?

[Bassnut]

I dont remember guideing with eggy stars, Marc may well be right that as long as the centroid can be calculated itll be all right, I certainly dont have a problem with out of focus stars. Can you post a pic?. Still leaves you with why guiding is jumpy. There are so many things to tweak in the guide software its hard to tell where to start, I found the best way is to have the guide error graph up and carefully watch it, tweaking stuff to see what effect each tweak has. Play with min and max moves, agressiveness and binning. Can you post a pic of the guide graph?.

[g__day]

Paul,

Can you try moving the guide star around the full frame of your guide program and informing us if the shape of the eggy guide star changes?

I have an OAG with a DSI II Pro Mono - stars were eggy on the full half of the frame that was closer to the OAG and round on the half that was further from the central light path of the OTA.

I should have seen the opposite - which revealed to me it wasn't coma - it was mis-alignment of the optics! CCD Inspector confirmed this - I had massive tilt - turned out one of the screws of the OAG was still tight but the whole OAG had shifted down about 2mm out of true

Maybe try the 30 day evaluation of CCDWares CCD Inspector v2.1.6 and see what it thinks of your main imager and your OAG alignment.

Lastly you can guide successfully with eggy stars - but its hit and miss (at best a 50 - 50) affair in my experience. Not something you want to put up with. Definitely diagnose the source of the issue and get it sorted if you can.

Matt

[Paul Haese]

Fred, I don't have an image and the sky has been raining cats and dogs. I often have the graph going, but I am using PHD at present rather than maxim. I might give maxim a try next time and see if I can get any sense out of that. The guiding on my Lagoon shot was the same but the stars seemed nice and round.

Matt, the stars are the same axis all the way through the shot, elongated up and down and same width side to side. I have CCD inspector and it tells me I have some where around 4" miscollimation. Something that needs fixing. However different shots show collimation to be near 10" and then back to 3 or 4 again on the following frame. Seeing induced. I think that I will address this on the next clear night down there.

I am also going to use the flattener too. It should flattener out the curvature of the optics.

I really appreciate everyones thoughts. It makes for sorting the problem much easier. Guiding seems to be the one bug bear that will not go away.

[wysiwyg]

Paul,

Having similar problems with my OAG.

I have now come to an understanding that the problem of oval stars is the prism is picking off light right from the edge of the imaging circle, hence especially when not using a flattener, the stars will always look oval.

I was able to get focus with my eyepiece and the stars were nice and round. Howevere when using the Rmote Guide Head I was getting lemon shaped stars. I now know that the reason they were lemon shaped was because I was not in focus. Apparently they do turn nice and round when in focus.

So if you can work out how to get your guide camera closer to the prism, you might solve this problem.

I still need to confirm all of this in a real test, but from what I have gathered through various forums, it should work.

Fingers crossed!
Mark

[Paul Haese]

Thanks mate, well hopefully when the next new moon get together at Clayton happens we will have this completely sorted.