Off axis guiding

[Newforestgimp]

Hi all,

is it normal to have 1/2 moon shaped stars when using OAG with a large dia RC ?

getting focus is busting my balls.

[vlaiv]

6 minutes ago, Newforestgimp said:

Hi all,

is it normal to have 1/2 moon shaped stars when using OAG with a large dia RC ?

getting focus is busting my balls.

Never saw such stars, but it is not uncommon to have distorted stars at the edge of the field where OAG operates.

Star to the right in above screen shot are closer to optical axis and still "concentrated" while those on the left are starting to show signs of field curvature and astigmatism.

That is OAG fairly close to optical axis on 8" RC (about 10mm away from optical axis or so - it is part of FOV next to shorter side of 17.7 x 13.4 mm sensor).

[Newforestgimp]

4 minutes ago, vlaiv said:

Never saw such stars, but it is not uncommon to have distorted stars at the edge of the field where OAG operates.

Star to the right in above screen shot are closer to optical axis and still "concentrated" while those on the left are starting to show signs of field curvature and astigmatism.

That is OAG fairly close to optical axis on 8" RC (about 10mm away from optical axis or so - it is part of FOV next to shorter side of 17.7 x 13.4 mm sensor).

Hmmm looks like something isn’t right with mine, seem to struggle getting stars to focus on let alone guide on.

maybe I’m a bit conservative how far the prism is pushed into the light path ?

[Jonk]

You don't need round stars for guiding - I have half moons, seagulls, pint glass shaped, all sorts on my OAG - as long as the guiding software can calculate the centroid, you'll be fine.

It doesn't even need to be in good focus - when I change filter, the slight difference in focus means a slightly different shaped guide star - never had a problem.

 

[vlaiv]

4 minutes ago, Newforestgimp said:

Hmmm looks like something isn’t right with mine, seem to struggle getting stars to focus on let alone guide on.

maybe I’m a bit conservative how far the prism is pushed into the light path ?

I place mine so that it almost starts to cast shadow on sensor.

I sometimes see edge of it starting to intrude on flats.

Here are flats on two different occasions - shadow creeping in from the bottom in both cases is shadow of prism. It also shows where I place prism - along longer edge of the sensor - that gives me more room for it to be pushed towards optical axis.

[Newforestgimp]

Thanks chaps I shall try this tomorrow, it’s hard to get it set up when you can’t get any stars in view !!!

Centred scope on vega and it only appeared as a moon shape at edge of frame on OAG, I literally just guessed it might still be vega and tried to focus on it.

any help with gain and exposure ? I’m using an ASI120MM, I’m finding it quite demanding stepping up from a refractor.

[michael8554]

13 hours ago, Newforestgimp said:

any help with gain and exposure ? I’m using an ASI120MM

In PHD2 they found the optimum gain for the ASI120MM was 48, and they made that the default.

Expose 2 to 3 secs.

To check guidecam focus, first focus the imaging camera.

Then temporarily lower the prism as low as possible, to aid finding a star with the guidecam.

Focus the guidecam (don't alter the imaging camera focus)

Then raise the prism back to the position where it isn't shading the imaging camera, and finetune the guidecam focus.

Odd shaped stars are not unusual with OAG, PHD2 seems to cope.

Michael

[Jonk]

I expose for 4 seconds, 2 seconds might be too much. I think you just need to experiment until you get reliable, repeatable results.

Don't forget, the atmosphere varies wildly so after a time, you will learn what's best.

[alacant]

On 17/11/2021 at 23:01, Newforestgimp said:

I’m using an ASI120MM

Hi

Bin 2x2 at gain 95 looping 3s.

Works for us with the t7m, 120 clone under INDI. Tested up to 1200mm.

Align the prism along the centre of the long edge. Push it in gradually taking flat frames as you go until you see its shadow. Then pull away until the shadow just disappears.

Focus the main camera on a well lit object -it doesn't have to be distant- during the day, lock its focus and bring the guide camera to focus. You may need a tiny adjustment under the stars. A helical guide focuser makes the procedure much easier. Use somewhere rich in stars. Cassiopeia is good ATM.

Cheers and HTH