Collimating a long image train

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DavidP
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Collimating a long image train

Post by DavidP »

I am using an achro with a long imaging train which includes a Solar Spectrum etalon and telecentric converter. I know I have some sag that I want to adjust and compensate for. I suppose I could buy a bunch of extensions and put a laser at the end, but I remember there is a trick to getting a central red reflection from the etalon, but for the life of me I can not recreate it. I have taken off the energy reflection filter placed before the etalon, but I am just seeing a mirrored reflection that I can not center. I seem to remember Christian making some suggestion.
The image below is one I took some time ago that allowed my to collimate.
Any suggestions?


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Re: Collimating a long image train

Post by DavidP »

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Re: Collimating a long image train

Post by marktownley »

What's the focuser like on the scope?


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Re: Collimating a long image train

Post by DavidP »

It’s a new Baader Steeltrack. Adjustable. I just can’t remember how I got that central red reflection from the etalon.


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Re: Collimating a long image train

Post by marktownley »

Does it need to be pointing at the sun to get the red reflection?


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Re: Collimating a long image train

Post by AndiesHandyHandies »

Hi

Auto-collimation on the bare focusser with 1.25" exit. I assume its firmly attached.

Punch a hole in a piece of card and draw a 1/4 inch black cross on one side.

Stand 20 feet in front of the OTA, look through the back of the card with cross facing the OTA.

The cross should be centred and when you move back and forwards the image of the exit 1.25" should centre on the front aperture.

Adjust the main lens mirror to suit.

Its very accurate as you are using the OTA as an optical lever.

Cheers. Andrew.


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Re: Collimating a long image train

Post by marktownley »

Can you adjust the fixing of the focuser? How is it fixed to the ota?


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Re: Collimating a long image train

Post by mdwmark »

OK,
if you have the setup all set up with the telecentric( your above F28) and the mirror side of the filter facing towards the objective.
1) Take off the ERF. This will not hurt the filter in any way. Point the scope at the sun. When it is centered in your eyepiece. Take a look at the front reflection coming back at the objective. If it is in the center, then you are probably aligned up OK.
If there is any sagging , you will need to increase the set point. Any tilt movers the bandpass towards the blue, but also widens the HW. The narrower the filter the more effect of broadening you will see.
2) make sure your spacing is correct for the telecentric. If not, the filter may not be working as well as it should be.
3) put your ERF back on to keep the energy out of the scope.
Mark Wagner


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Re: Collimating a long image train

Post by MalVeauX »

Hi,

A tilt adapter will help. Your system doesn't have to be perfectly collimated, which is really hard without a dovetail that spans the scope and its imaging train to anchor to. That would be ideal.

I have no problems with things not being orthogonal and I don't need a tilt adapter with a cheap linear bearing GSO focuser. And my imaging train is pretty ridiculous (two air spaced etalons, telecentric amp, two focusers, etc). For example, this is a 150mm F8 and its imaging train is pretty long.

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Very best,


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