Help with off axis guider.

[HunterHarling]

Hi, my Orion thin off axis guider has some issue that I cannot get to the bottom of. I was able to focus on the moon and it looked okay but when I slewed to Alnitak and tried to focus on it, it was split into four different parts and was really distorted.

I tried moving the prism farther down but it didn't help.

Also, the stars that where in the field of view where too large to be guide able. 

I'm thinking that maybe the prism is bent at an angle slightly off of 90 degrees?

Is this even adjustable? 

Any ideas?

 

[wimvb]

Have you tried focusing on a less brighter star than Alnitak? Alnitak may be so bright that you are picking up diffractions, which are caused by anything in the light path, including mirror/lens clips or the oag prism.

An image of the problem would help.

[alacant]

Hi. Tell us more. The optical train, the cameras, a photo of the setup...

4 hours ago, HunterHarling said:

stars that where in the field of view where too large

...suggesting they are not in focus? Is the prisim turret inserted correctly (not at 180º)?

HTH.

[HunterHarling]

Well the prism definitely was looking in the wrong direction. The screws holding it came loose. I adjusted it and it is definitely better but the out of focus stars look like half of a donut!

The telescope is an 8in sct and the guide camera is an asi120mc s.

I'll get an image of one of the weird half donut stars so you can see what it looks like.

Thanks!

 

[ChrisEll]

I've seen banana stars in my OAG, the focus needs a tweak.

[wimvb]

Yes, most likely the shadow of the oag prism. An image should confirm.

[newbie alert]

2 hours ago, HunterHarling said:

adjusted it and it is definitely better but the out of focus stars look like half of a donut!

Definately a focus issue then..

I'm about to test my oag tonight on my sct..fingers and toes crossed

[alacant]

27 minutes ago, wimvb said:

shadow of the oag prism

? I think the OP is referring to the image in his guide camera.

Where is the prism? E.g. this will produce bananas:

   

This one doesn't:

 

Both will guide fine if focused. HTH.

[wimvb]

1 hour ago, alacant said:

I think the OP is referring to the image in his guide camera.

Of course. What was I thinking?

[lukebl]

I had no end of aggro sorting out my OAG. All the guidance told me to have the prism as far inward as possible. However, I could never find any stars and gave up for a while and reverted to a guidescope.

I revisited it, and put the prism right at the edge as shown here, against conventional wisdom and not even at the same orientation as the sensor, and finally managed to focus on some stars to guide on. Odd shaped ones, but my guiding is now effortless. So it's worth experimenting. As others have said, funny-shaped stars aren't a problem for PHD.

[RayD]

The prism just needs to be in the light cone.  How far it can go in depends on your filter and sensor size.  If you have a large sensor and 2" filters like my 16200 then you really need to consider OAG's with large apertures like the Atik one.  With smaller sensors and 1 1/2 filters you will be fine taking the prism all the way to the filter's edge as is demonstrated very well in the images posted above by @alacant

Don't forget moving the prism up and down doesn't change the OAG focus.  Only moving the camera up and down from the prism changes focus.  Once the prism height is set this will never change unless you change filters or camera.

Well worth persevering with it, especially on an SCT, as OAG will give you best performance, but don't beat yourself up achieving perfect focus as Craig Stark (PHD2 author) notes that slightly out of focus is better as it provides a better basis for the centroid calculation.

[ChrisEll]

Moving the prism as far in as possible is worth doing as it will give a brighter image.