DIY - OAG

spaceboy · November 24, 2011

A true work of art Francis

fwm891 · November 24, 2011

Many thanks Spaceboy.

Just managed to find a 10 min gap in the cloud cover to try out the tilt/rotate adjustments - they work.

Initial focusing gave me a short linear star and rotating the prism (apart from shifting the said star off screen:eek:) brought the star back to round again. This also brightened the star image and brought some fainter stars in the background into view.

So the adjusters let me do two things:

Adjust the star shape

Move the star around the viewing screen without having to reframe the main subject (had a few occasions where the best guide star is right at the edge of the camera FoV). In extremes this will alter the star shape but small movements of the prism quite quickly traverse the star from one side to the other anyway:D

Need a bigger patch of clear stuff now to check the guiding with the belt drives

Probably do some in-situ photos tomorrow.

perrin6 · November 24, 2011

wot's an OAG ?

m.tweedy · November 24, 2011

off axis guider

http://www.mapug-astronomy.net/ragreiner/OAG.html

perrin6 · November 24, 2011

off axis guider
Off Axis Guiders

Ah-ha! thanks!

fwm891 · November 25, 2011

Hi, I added some photos and an early schematic of how I thought the OAG layout would look based on actual measurements of the QHY5, 36mm filters the 8300 chip and the 10x10 prism.

Details have changed as the build progressed (be boring otherwise).

The photos have been taken at the telescope but with the filter wheel removed for clarity.

Apologies for the state of the chemically blackened parts (pressure springs & mounting flange) these didn't coat well at all and will be given a coat of paint at some stage

That about wraps this up but feel free to comment, I will answer more questions etc...

Thanks for the interest

yesyes · November 25, 2011

A true work of art. It's even got a little crayford focuser... ;-)

fwm891 · November 25, 2011

Thanks Chris - and this little Crayford works!

Merlin66 · November 25, 2011

I'm very interested in the effects of the prism rotation....I've never seen any measurable results before.

Can you supply a guide image with no prism rotation v's the results after rotating the prism?

fwm891 · November 25, 2011

Hi Merlin66,

Will do. I can put the old OAG in and the replacement and do screen grabs of both.

Be later in the week unless we get some clear skies tomorrow...

Francis

Gina · November 26, 2011

Yep! A beautiful job, that's for sure An example of fine engineering.

Merlin66 · December 3, 2011

Any success in getting some guider images with/ without the prism rotation?

fwm891 · December 3, 2011

Tried tonight but after getting the scope aligned the cloud rolled in - manage a few shots of the moon through cloud just so I could say I'd done something!

Not forgotten

duncan.p · December 3, 2011

A true work of art Francis, well done.

yesyes · December 3, 2011

Tried tonight but after getting the scope aligned the cloud rolled in - manage a few shots of the moon through cloud just so I could say I'd done something!
Not forgotten

Capturing some AVIs for Registax?

fwm891 · December 5, 2011

OK - Some images (2) showing effect of the prism rotation.

The angled image is at the maximum rotation of my new prism (sorry didn't have chance last night to put the old OAG into the system)

The straight is as it says prism aligned.

Both images are 100 x 100 pixel crops from small frame stacks which were captured using the QHY5 and QG video software supplied with the QHY5.

This shows image distortion with rotation of a quite well aligned prism. My old OAG never good as good as the angled image shown here!

Merlin66 · December 5, 2011

Thanks for that.

If I read it correctly - the tilted but straight prism is currently giving you much superior images than the previous OAG?

When you rotate the prism in your current design the star image appears elongated - but still better than the pervious OAG?

Can you confirm?

Do you know the relative off-axis position of the prism (in the old v's new)

fwm891 · December 5, 2011

Hi Merlin66,

If I read it correctly - the tilted but straight prism is currently giving you much superior images than the previous OAG?

Much better. I can only assume that a) the prism is of better quality, that the old OAG prism because It could not be properly adjusted always smeared the starlight to varying degrees.

When you rotate the prism in your current design the star image appears elongated - but still better than the pervious OAG?

Can you confirm?

Yes that's right. I can tilt (up/down) or rotate (left/right) the angle of the new prism or a combination of the two motions.

Do you know the relative off-axis position of the prism (in the old v's new)

The old prism sat at approximately the same radial distance from the OA as the centre of the new one.

Jessun · December 5, 2011

Impressive work! Must be a great feeling that it even outperforms the shelf ones!

Hats off!

/Jesper

Merlin66 · December 6, 2011

An excellent outcome!

Well done.

I believe most of the benefits you're seeing are coming from the ability to independently tilt the prism - this brings the off axis image closer to the centre of the guide tube and hence the guide camera.

fwm891 · December 6, 2011

An excellent outcome!
Well done.
I believe most of the benefits you're seeing are coming from the ability to independently tilt the prism - this brings the off axis image closer to the centre of the guide tube and hence the guide camera.

Thanks Merlin66.

I've attached a schematic side view of the OAG.

From that you will see that there is no axial movement when the prism is tilted.

Effects on the image are entirely due to the angle of the prism face to the light entering the prism from the telescope.

Focusing is undertaken first with any rotation/tilt adjustments made afterwards (the image needs to be focused to see if there are shape issues).

Francis

Merlin66 · December 6, 2011

Very nice!

The "optimum" prism angle will vary with the focal ratio of the scope and the distance from the optical axis....a neat refinement to have.

(I had similar adjustments for the beamsplitter plate I use for guiding in the spectroscope, but now it's just "plug and play")

michael.h.f.wilkinson · December 6, 2011

Amazing build, real work of art! :icon_salut:

fwm891 · December 6, 2011

Thanks for the comments Michael.

Merlin66 - spectroscopy sounds interesting...

Merlin66 · December 6, 2011

Francis,

With your obvious talents, building a spectroscope would be easy...

I design, build, and use a Littrow spectroscope "Spectra-L200"

Here it is mounted behind a Vixen flip mirror body, converted to a beamsplitter guider, on the C11.

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