Making a Dynamax DX-6 corrector plate

[davidc135]

I've nearly finished figuring a replacement corrector plate for my Dynamax 6 sct.

Two 156mm discs of 5.5mm B270 glass were fine ground and then polished on a vertical spindle running at 35 rpm. Polishing and figuring took place at the same time and needed around 6 hours per side. The glass was worked back and forth using short strokes and the desired Schmidt curve was gradually produced. 

The knife edge image shows a diagonal symmetry probably due to non alignment coma. Some slight zones can be seen.

The large obstruction is from the 8'' sct used to produce collimated light.

David

 

[davidc135]

Contact interference fringes form against an optical flat.  Edit.  The corrector surface is slightly concave which pushed the neutral zone further in from 0.71r

Figuring in the later stages was helped by a petal lap.

The corrector plate was propped up on a plywood jig during testing.

David

Edited November 28, 2023 by davidc135

[Glasspusher]

Very interesting post David.  I have never attempted a Schmidt corrector plate, you make it sound very straight forward.

John

[davidc135]

Thanks John. 

The key for me was using a vertical spindle. It was a stroke of good luck that a simple technique naturally wanted to produce a Schmidt curve or something close.

In my case I am right handed and the spindle spun clockwise. The short stroke begins with the workpiece on top and centre over centre. It is pushed forwards and returned to the same centre over centre position all the while with most downward pressure being applied to the edge of the glass at 2-3 o'clock position. The left hand merely steadies. Sometimes the work is displaced slightly to the right.This I repeat a few times and then allow the glass to rotate a little in my hands before the process is repeated and so on.

For mild aspherics as in say a Schmidt Newt just average hardness pitch would be fine from start to finish but deeper aspherics require a softer pitch that might need to be poured on a hard rubber mat which in turn is pitched to the base. I use tractor tyre inner tube. Surfaces with shallow aspherics should be polished out using ordinary technique before being aspherised.

So straightforward enough. It takes a similar time to any other surface although repeatedly remaking the softer pitch lap towards the end of this job stretched it out.

This corrector plate or one for a C-8 would be at the limit for this technique. The other limitation is the flexibility of glass in the thicknesses that commercial scts can accomodate which is about 6mm max. This can cause irregularity of polishing which shows as astigmatism although this could be fixable.

To do a good job some figuring is needed towards the end to get a decent result but probably no more than with other surfaces. In the Foucault knife edge image above there is a slight raised zone towards the edge best seen at 1 and 7 O'clock positions. I'll cautiously use a ring lap to wear it down but it's easy to get local polishers wrong and make things worse.

David

Edited November 28, 2023 by davidc135

[Glasspusher]

Thanks for that explanation of your technique David.

John

[jimsshed]

I have no idea how to get started at making a corrector plate but wish I could have a go.

Are there any books on this topic?

I have a broken 9.25 plate and Celestron won't sell me one in Australia.

[davidc135]

7 hours ago, jimsshed said:

I have no idea how to get started at making a corrector plate but wish I could have a go.

Are there any books on this topic?

I have a broken 9.25 plate and Celestron won't sell me one in Australia.

You could try Grovers Optics in the UK, email: hello@grovers.biz.

At one time they supplied Celestron made corrector plates to customers in the UK and might send them overseas. Prices were moderate.

Corrector plates would be a very tough challenge for a beginner.

1.  'Making Schmidt Cassegrain Telescope Optics' by Robert Piekel. Uses petal shaped facets to fine grind the complex curve.

2.  There should be stuff on the net about Bernhardt Schmidt's original method using a partially evacuated pan to support the edge of the plate as it's ground and polished.

3.  My spinning method requires a machine to be made.

The 9.25 has a slower primary which very much eases the amount of asphericity needed on the corrector. However...

Unless the maker has some experience with producing mirrors from scratch I definitely wouldn't advise tackling a corrector plate, regardless of which method was chosen.

David

Edited December 29, 2023 by davidc135

[davidc135]

An update.

I'd assumed that the surfaces of the B270 float glass were sufficiently parallel and hadn't checked. A disappointment to see Airy discs as little spectra!

What can I say?

Both corrector discs had around 125 microns of wedge where 25 should be the limit and so there was nothing for it except to regrind one side of each plate.

It's likely that the nearly finished corrector will suffer some astigmatism due to irregular polishing. Avoiding pressing down too hard will help whilst interference fringes will show likely problems. The pattern around the neutral zone should be circular with any variation being improved by selective pressure. 

On this earlier 4'' c.p extra time spent pressing down at 10 and 4 O'clock evened the fringes out. The gram weights were to centre the fringes.

There's still a little astigmatism in my DX-6 but it gave sharp images at x370 during spells of good seeing in the garden, on high contrast objects.

David

Edited March 5 by davidc135