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PST mod donor scope


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Thanks again Peter,I am slowly planning the project-taking time to understand the principles behind the conversion and Ha filters and telescopes.Things are starting to come together,merlins PST mod paper is becoming tattered!

How critical is placement of the ERF behind the objective(for distance)?and I think a tilt is needed,is the angle critical or is there a forgiving "window"

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Hi Gerry,

I used a Thorlabs Fixed Lens Mounts for Optics from Ø2" to Ø4" see here:-http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=1433 They have facilities in the States.

If you use the Baader 75mm ERF you need the LMR75(/M), M stands for metric. The mount is also the correct dimensions to hold the ERF in the center of the tube.

When you come to mount the ERF internally you want it to be positioned as far away from the telescopes focal point and also so that you do not cause vignetting. You need to calculate where the diameter of the light cone from the objective is 5-8mm less than the diameter of ERF.

Ian

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  • 1 month later...

Hi Gerry,

I used a Thorlabs Fixed Lens Mounts for Optics from Ø2" to Ø4" see here:-http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=1433 They have facilities in the States.

If you use the Baader 75mm ERF you need the LMR75(/M), M stands for metric. The mount is also the correct dimensions to hold the ERF in the center of the tube.

When you come to mount the ERF internally you want it to be positioned as far away from the telescopes focal point and also so that you do not cause vignetting. You need to calculate where the diameter of the light cone from the objective is 5-8mm less than the diameter of ERF.

Ian

Was this distance every worked out? Also, are there any problems having the derf mounted behind the front objective instead in front of?
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Just use a bit of trigonometry based on the objective diameter and the F/L of the scope. It is best to have the ERF as far from the focal point (this is the same as having the ERF mounted as close to the objective) as possible without the ERF interfering with the light cone, thereby causing vignetting.

Ian

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