Does a RASA corrector plate actually correct anything?

[Malpi12]

53 minutes ago, davidc135 said:

  an 8'' sct

But I presume not a rasa with its special lens group ? 

[900SL]

not wishing to sidetrack the thread, but this got me to thinking about Newtonians. Can you use a plain glass disk to support the secondary instead of spider vanes, to eliminate diffraction? Akin to a Mak Newt but without the front corrector profile?

[Malpi12]

49 minutes ago, symmetal said:

how long it takes for the replacement

Now I am conflicted : between wishing you a speedy new corrector and , , long enough to conclude the experiment for the sake of human understanding

 

[Peter Drew]

1 hour ago, 900SL said:

not wishing to sidetrack the thread, but this got me to thinking about Newtonians. Can you use a plain glass disk to support the secondary instead of spider vanes, to eliminate diffraction? Akin to a Mak Newt but without the front corrector profile?

Yes indeed you can.  I have made several in the past, I have a 13" "optical window" from one at present.  They do enclose the tube of a Newtonian, keep the dust etc away from the optics, reduce thermals and of course eliminate spider diffraction.  Their downsides are significant increase in weight, cost and susceptibility to dewing up.     🙂 

Edited December 29, 2022 by Peter Drew
typo

[davidc135]

1 hour ago, symmetal said:

Thanks David. To my untrained eye it just looks out of focus. What features of the image indicates it's SA? 🤔

Alan

I'm not sure as they are similar. The scope with massive SA would have a huge depth of 'focus' where each zone in turn would attempt to form an image surrounded by a wide halo of unfocused fuzz. For instance, in an f10 sct there would be a kind of recognisable image over 50mm of eyepiece travel. So, easy to tell visually.

With images, the optic with extreme SA perhaps combines a certain retained resolution with a loss of contrast over a wide area. An unfocused image with similar reduced resolution would lose contrast just close to the feature. I'm trying to imagine it.

David

[davidc135]

2 hours ago, Malpi12 said:

But I presume not a rasa with its special lens group ? 

No, not a RASA. But now the needed correction (with 8'' f2 primary) has increased to over 22 waves, without the sct's secondary. I'd expect the corrector again to deal with the bulk of that.

David

[Malpi12]

55 minutes ago, davidc135 said:

expect the corrector again to deal with the bulk of that.

I think that is what we are all expecting, but it was the question asked and it is an opportunity to find out ! You havn't got a spider you could lend him  ? 

And setting up the experiment would take his mind off the pain of what just happened ! 😢
 

[symmetal]

In Celestron's RASA White Paper 2020, Rowe stated that a more powerful corrector plate was required compared to the SCTs but Celestron wanted it based on the 11" SCT corrector plate to keep costs down, so a longer radius of curvature mirror is used instead compared to the SCT, (using the same blanks) with a standard 11" SCT corrector plate followed by the 4 element lens group to create a flat field and to maintain focus over the full visual spectrum. Whether this includes further SA correction it doesn't say.

Alan

[davidc135]

11 hours ago, Malpi12 said:

I think that is what we are all expecting, but it was the question asked and it is an opportunity to find out ! You havn't got a spider you could lend him  ? 

And setting up the experiment would take his mind off the pain of what just happened ! 😢
 

I knocked up a spider whilst working on this 6'' corrector which could be popped on and off.

David

Edited December 30, 2022 by davidc135

[davidc135]

9 hours ago, symmetal said:

In Celestron's RASA White Paper 2020, Rowe stated that a more powerful corrector plate was required compared to the SCTs but Celestron wanted it based on the 11" SCT corrector plate to keep costs down, so a longer radius of curvature mirror is used instead compared to the SCT, (using the same blanks) with a standard 11" SCT corrector plate followed by the 4 element lens group to create a flat field and to maintain focus over the full visual spectrum. Whether this includes further SA correction it doesn't say.

Alan

Thanks for the link.  David

[spacedobsonian]

On 28/12/2022 at 16:12, ollypenrice said:

The curve on a Schmidt plate is interesting. Bernhard Schmidt, who blew off his arm as a child chemistry experimenter, claimed that he could get comatose drunk, pass out for a long time and then come round with the solution he'd been looking for. He had conceived of the curve required in principle on a lens to reduce coma on a reflector but it was, apparently, impossible to grind. Then, perhaps after one of his bibulous escapades, he went and asked Walter Baader about the physics that would apply to the distortion of a glass blank if it were placed over the top of a cylinder from which the air could be evacuated.  Baader (one of the few people whom Schmidt liked) said he didn't know but pointed him to the right book. Schmidt's hunch proved to be correct: if you put the blank over the top of a cyclinder, paritally evacuate the air beneath it and then grind the vacuum-distorted blank flat again, it will, when released, have the desired shape. This is still how they are made. Since the primary of an SCT is spherical and the corrector can be made in this relatively simple way, they are an attractive commercial proposition.

Olly