stretched stars, new guidescope worse guiding?

[MarkInSpace (Mark)]

I have a CEM70 and I've just mounted an edgeDH8 on it+reducer.
Plate solving says the EdheHD + reducer is 1450mm - sounds about right.
My old guidescope is an orion 50mm f3.5 - plate solving says it is 162mm
However, the stars are stretched in spite of good alignment. The guider reports it is guiding at approx 0.5" to 0.8" (ASIAIR).
So today I tried an Orion 70mm f4.5 guidescope.
After re-calibration the guiding numbers are worse!! 0.8" to 1.2"
Does this make any sense?
I have guide speed at 0.25X. I tried 0.5X and the numbers were still worse.
Any thoughts?

[MarkInSpace (Mark)]

I've have an OAG, but have not been able to get it set up correctly. maybe someday I can meet someone in real life and they can give me a pointer in that regard.
Until then, I will need to use a guidescope.

[Drac0 (Mark)]

First off, don't get fixated on the guiding figures, it's the image results that matter. My recent results have shown this to me - just started using a new mount & while my guiding figures are much worse (I use PHD2), even double what I used to get, my image results are much better even taking much longer subs and the wind blowing.

What was the weather like, any wind around? While the CEM70 is a hefty mount, it can still suffer from the effects of the weather. And this can of course vary from night to night. And the guide speed should be set higher in the mount settings, not lower as that makes the mount 'slower' to react. Try 0.8x or 0.9x instead.

OAG's really aren't that hard, just confusing. Basically, both the imaging camera & the guide camera need to be at the same focus distance. With the Celestron reducer the back focus is 105mm, so both cameras sensors need to be 105mm from the reducer.

Easiest way is to put the OAG in the image train & set the imaging camera to the correct back focus distance and check it can focus correctly. Then measure the distance from the prism in the OAG to the imaging camera sensor - this is the approximate distance the sensor of the guide camera also needs to be from the OAG prism. Add/remove/change extenders as required to get the correct distance for the guide camera. I find it easier to have the OAG closer to the telescope than the imaging camera - put the camera too close & you may not have enough distance for the guide camera to achieve focus.

Cheers,
Mark

[Nikolas (Nik)]

Originally Posted by MarkInSpace

I have a CEM70 and I've just mounted an edgeDH8 on it+reducer.
Plate solving says the EdheHD + reducer is 1450mm - sounds about right.
My old guidescope is an orion 50mm f3.5 - plate solving says it is 162mm
However, the stars are stretched in spite of good alignment. The guider reports it is guiding at approx 0.5" to 0.8" (ASIAIR).
So today I tried an Orion 70mm f4.5 guidescope.
After re-calibration the guiding numbers are worse!! 0.8" to 1.2"
Does this make any sense?
I have guide speed at 0.25X. I tried 0.5X and the numbers were still worse.
Any thoughts?

CEM from what I have read in the asiair forums is tracking at 0.75x

[Craig_]

Originally Posted by Drac0

Easiest way is to put the OAG in the image train & set the imaging camera to the correct back focus distance and check it can focus correctly. Then measure the distance from the prism in the OAG to the imaging camera sensor - this is the approximate distance the sensor of the guide camera also needs to be from the OAG prism. Add/remove/change extenders as required to get the correct distance for the guide camera. I find it easier to have the OAG closer to the telescope than the imaging camera - put the camera too close & you may not have enough distance for the guide camera to achieve focus.

Cheers,
Mark

The easiest way to set up an OAG in my experience is to do it during the day. Get your gear out, focus your OTA on a (very) distant target such that it's focus position is going to be largely similar to focusing on stars. When the main camera is in focus, start moving the guide body in and out of the OAG slowly until you can see focus starting to come together. Then fine tune it. Once that is done, leave it in place, do not disassemble it after each session.

For my OAG the guide camera basically needs to be pushed as far down as it will possibly go to reach focus.

[multiweb (Marc)]

Originally Posted by MarkInSpace

I have a CEM70 and I've just mounted an edgeDH8 on it+reducer.
Plate solving says the EdheHD + reducer is 1450mm - sounds about right.
My old guidescope is an orion 50mm f3.5 - plate solving says it is 162mm
However, the stars are stretched in spite of good alignment. The guider reports it is guiding at approx 0.5" to 0.8" (ASIAIR).
So today I tried an Orion 70mm f4.5 guidescope.
After re-calibration the guiding numbers are worse!! 0.8" to 1.2"
Does this make any sense?
I have guide speed at 0.25X. I tried 0.5X and the numbers were still worse.
Any thoughts?

It's your primary shifting. Just use an OAG and you're good to go.