Off Axis Guider help needed!

[AstronomyWithClaire]

Hi everyone - I've been taking astro photos for a while now but recently obtained a Starlight XPress filter wheel with an OAG. I'm using an Altair 290C with it and cannot for the life of me get focus. 

I'm using NINA and a Pegasus Focus cube 2 to get Scope/Camera focus then I take the laptop outside, set the PHD2 refresh to 0.5 second and try to focus using the tiny grub screw and moving the camera / OAG body in and out of the tube. All I get is lines instead of stars.    /     <-- that sort of idea. If I move the prism in and out, the shape changes but not enough. I dont have the dexterity mental or otherwise to move both at the same time and lock off.

Help!?

Edited May 12, 2023 by AstronomyWithClaire

[knobby]

I'd try using a distant object in the day time, dont use PHD for rough focus as it will 'auto everything' to try to pull out a star !

Try sharpcap to see what going on. Also, roughly match the distance from scope to sensor with the main camera using a measure of some sort before you begin.

[AstronomyWithClaire]

1 hour ago, knobby said:

I'd try using a distant object in the day time, dont use PHD for rough focus as it will 'auto everything' to try to pull out a star !

Try sharpcap to see what going on. Also, roughly match the distance from scope to sensor with the main camera using a measure of some sort before you begin.

Hey So set up in the day, focus on a tree using the main camera/scope, then use sharpcap to focus the guide camera? How do you measure the distance from the scope to the main camera? I don't know where the sensor is on the Altair 26c. Thanks for any Pointers! 

[ONIKKINEN]

If daytime focusing is still difficult, maybe check that the prism is the right way in the OAG?

When i first installed my OAG i had my prism like in the left example above which made more sense to me but actually the prism needs to be oriented like in the right example.

[Elp]

You measure from the back of the last scope piece of equipment be it the visual back (sct) or a field flattener. The scope/flattener will have a specification for backfocus distance that the main camera needs to be set at in order for all stars to be flat across the field and focusable, measure from the back (if it's a flattener from the bottom most position of the screw thread to the physical stop) to your camera front face. Your camera should have a specification or a technical drawing showing the front of camera to sensor distance. You whole imaging train should be setup at this point including the OAG, filter draw etc. Add it all up. Your main camera should be at focus at this point and do not move it or the focus position. Then place your guide camera into the OAG at the same distance measuring from the centre of the prism approx (where the main scope light beam would hit the prism before it bends 90 degrees). Move the camera very very slightly in or out of the OAG mounting, if it's got a micro focuser even better, until your guide camera shows focus. You might need some extension rings for the guide camera but they usually achieve focus easily with standard OAG mounting designs. I wouldn't use auto equipment for this, do it all manually. You can achieve it during the day focusing on a far tree or pylon.

 

Edited May 12, 2023 by Elp

[michael8554]

Hi Claire

1. "set the PHD2 refresh to 0.5 second"

As knobby said, use SharpCap in video mode, auto exposure.

2.  "moving the camera / OAG body in and out of the tube"

The OAG "stalk" has two adjustments.

One allows the guidecam to move closer or further away from the prism, to get focus once the main camera is in focus.

The other lowers or raises the prism and camera without altering their spacing, which would alter focus.

You can roughly setup by measuring with a ruler on the outside of the OAG and cameras.

Measure from the middle of the OAG body to where the main camera sensor is.

All measurements from the prism, not from the back of the OTA !

Place the guidecam sensor this same distance from the prism.

Now focus the main camera on a distant landmark.

Then drop the OAG "stalk" all the way down, so that the prism is well into the light path.

Focus the guidecam by loosening the correct screw and by moving the guidecam closer or further away from the prism.

Take main camera Flats, adjusting the stalk height, not the focus adjustment, so that the prism is as low as possible, but not causing a shadow on the Flats.

Fine tune the OTA focus, then the guidecam focus, at night on the moon and then on stars.

Then in future only adjust the main camera focus with the OTA focuser, the guidecam will always be in focus too.

Michael

 

 

Edited May 13, 2023 by michael8554