Does a RASA corrector plate actually correct anything?

[symmetal]

Looking at the broken corrector plate of my RASA 11, I was surprised that it appears to just be a flat piece of glass. Laying a steel rule across it there is no visible curvature on either side and calipers measure 6.16mm thickness at all places from the outside to the centre.

There are no shims to adjust its orientation to the mirror, as it just lies directly against a fixed metal flange on the scope front housing and is pushed against it by the front mounting ring screws. Four nylon tipped grub screws push against the plate edges from the side, to centre the front quad lens assembly which does the actual correction I imagine. I'd have thought there would be a ring of softer material betwen the glass and the metal though. No wonder the plate broke when the scope was dropped. There is some grey fibrous material each side of the plate where it touches the lens holder. The ring below the plate screws into the central ring and has two holes on the bottom to fit an adjustable lens assembly tool which arrived today from Amazon. 🙂

I may as well try fitting a new 'corrector' plate myself, when Celestron can supply one, as there are several Youtube videos on hyperstar corrector plate replacement and adjustment. It just looks like I need to get it central over the mirror as there are separate tilt adjustments on the lens assembly used for collimation. Here's the broken plate edge on, with the lens assembly removed. The lens assembly comes in from the bottom and engages with threads on the inside of the top ring on the picture. The push/pull tilt screws are just visible below the top ring.

Alan

[DaveS]

A Schmidt corrector plate is very nearly plane parallel, but not quite. On side will have a very slight, very complex aspherical surface only a few wavelengths deep. Roughly it's the difference between a sphere and a parabola of long focal length.

[symmetal]

@DaveS Thanks for clarifying that. I'd always assumed they were quite curvy. 🙂

Alan

[Paul M]

Very intriguing. You'd think that such a thin plate of that size would distort more than that amount of figuring under the weight of a camera hanging off it at a right angle.

It's a conspiracy!!!

[symmetal]

1 hour ago, Paul M said:

Very intriguing. You'd think that such a thin plate of that size would distort more than that amount of figuring under the weight of a camera hanging off it at a right angle.

It's a conspiracy!!!

Actually the weight of the lens assembly hanging off the back of the corrector does balance the camera weight to some degree when horizontal but then doubles when the scope is pointing straight up so it must sag to some degree. Maybe not enough to cause a problem if it's thick enough, but then its own weight would add to the sagging effect. 🤔

Alan

[ollypenrice]

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

[Peter Drew]

Current corrector plates are produced by evacuating parallel sided discs against a former that has the opposite curve to that of the finished corrector rather than a cylinder, the rest of the process remains the same.  When I made Schmidt-Cassegrains and Schmidt  cameras in my early professional career the optics were made by Jim Muirden.  Jim hand figured the difficult correctors from scratch!.  The figuring for Schmidt camera correctors was interesting, Jim introduced a thin wire into the field of the test apparatus of a thickness that when the image of the artificial star could be completely occulted the star image was smaller than the resolution potential of the film available at the time.     🙂  

[JeremyS]

2 hours ago, ollypenrice said:

. 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.

Never worked for me 🤔

[ollypenrice]

36 minutes ago, JeremyS said:

Never worked for me 🤔

No, your avatar does seem to show two arms....

Olly

[Louis D]

2 hours ago, JeremyS said:

Never worked for me 🤔

You probably didn't have a vintage chemistry set with potassium nitrate in it.  It was included for making a smoke bomb.  Of course, it's also the main ingredient in gunpowder.

I'm thinking they need to sell the Breaking Bad chemistry set to put the fun back into childhood chemistry: 😉

[JeremyS]

5 minutes ago, Louis D said:

You probably didn't have a vintage chemistry set with potassium nitrate in it.  It was included for making a smoke bomb.  Of course, it's also the main ingredient in gunpowder.

I'm thinking they need to sell the Breaking Bad chemistry set to put the fun back into childhood chemistry: 😉

Actually I did have a complete chemistry set, Louis. I used to make gunpowder. This was for burning rather than blowing things up. Unfortunately one of my chums did the same but committed the cardinal error of compressing when try to put in a used Sparklets spent CO2 bulb. It resulted in him losing two fingers.

Nevertheless, the whole chemistry set experience led me to a life in chemistry which continues to this day.

The was a discussion on vintage chemistry sets recently on SGL.

[JeremyS]

2 hours ago, ollypenrice said:

No, your avatar does seem to show two arms....

Olly

But it doesn’t show my right hand 😳

[Louis D]

5 hours ago, JeremyS said:

But it doesn’t show my right hand 😳

So that's why you keep it in your pocket!  We won't tell. 🤫

[Louis D]

From my online reading, the RASA does indeed use the standard SCT Schmidt corrector plate which is very nearly plane flat at the macro level.

[Malpi12]

On 27/12/2022 at 19:26, symmetal said:

broken corrector plate of my RASA

Oh dear ! or similar words.

Would it be considered a heresy if I were to suggest making some vanes for it, to support the lens assembly pro tem ? Only as a scientific curiosity of course just to see how much correcting it /is/was doing  ?

 

 

Edited December 28, 2022 by Malpi12

[Louis D]

It would have the spherical aberration of an uncorrected f/2.2 spherical primary, which is to say a lot.  I've looked through an f/4 spherical primary Newtonian (Celestron FirstScope 76), and only the central region of a 20mm Plossl was barely usable.  f/2.2 is going to be way worse.  I'm not sure how much the camera corrector lenses would help with the SA.

[symmetal]

4 hours ago, Malpi12 said:

Oh dear ! or similar words.

Would it be considered a heresy if I were to suggest making some vanes for it, to support the lens assembly pro tem ? Only as a scientific curiosity of course just to see how much correcting it /is/was doing  ?

An interesting thought. I could 3D print an outer and inner circle with vanes linking them but it would likely sag with the weight, introducing tilt and be a nightmare to maintain any semblance of focus. 😬

1 hour ago, Louis D said:

It would have the spherical aberration of an uncorrected f/2.2 spherical primary, which is to say a lot.  I've looked through an f/4 spherical primary Newtonian (Celestron FirstScope 76), and only the central region of a 20mm Plossl was barely usable.  f/2.2 is going to be way worse.  I'm not sure how much the camera corrector lenses would help with the SA.

My initial thought in posting was whether the lens assembly could be made to correct the SA instead of the corrector plate, but it may be simpler having a separate large corrector plate doing it as it's a tried and tested solution. 🙂

Alan

[Rusted]

Does household contents insurance cover the accidental dropping of an OTA?

I remember in my youth making a so-called f3.8 "lenseless" Schmidt.
Field stop at twice the focal length. Curved 35mm film holder at focus.
I went on to parabolise the 8.5" mirror as a rich field for visual.
Massive but lightweight, stressed skin, altazimuth fork mount and aluminium truss tube.
It was displayed at the city reference library. I eventually sold it via the Exchange&Mart.
An early form of eBay but printed on news paper.

[ollypenrice]

3 hours ago, Rusted said:

An early form of eBay but printed on news paper.

Really? Is that made of goat skins?

Olly

[JeremyS]

3 hours ago, Rusted said:

Does household contents insurance cover the accidental dropping of an OTA?

I remember in my youth making a so-called f3.8 "lenseless" Schmidt.
Field stop at twice the focal length. Curved 35mm film holder at focus.
I went on to parabolise the 8.5" mirror as a rich field for visual.
Massive but lightweight, stressed skin, altazimuth fork mount and aluminium truss tube.
It was displayed at the city reference library. I eventually sold it via the Exchange&Mart.
An early form of eBay but printed on news paper.

I bought my 2nd and 3rd telescopes via Exchange & Mart in ca 1974 and 77

I hear @Peter Drew modified one of the first Caxton presses to advertise his scopes

[Peter Drew]

I think I could claim that Exchange and Mart effectively launched my career.  In the mid 1960's I was making telescopes as a hobby and advertising them in E&M.  If more than one wished to purchase a telescope I made more to supply the demand.  Eventually this became so time consuming I decided to go full time professional.  The rest, as with Caxton, is history.    🙂

[Malpi12]

11 hours ago, symmetal said:

likely sag with the weight

When I first had the thought my concern was how to support/attach the vanes without resort to drilling holes for bolts !!  If 3d circles could be printed that is good - could the vanes be made of stiff material (conventional metal) and be included in the 3d plastic ? Perhaps like rebar.

No one (I imagine !) would want to take apart a rasa just for this, but here we have an interesting possibility, I am sure the rasa will get over the insult to its dignity when the new corrector arrives !

 

12 hours ago, Louis D said:

I'm not sure how much the camera corrector lenses would help with the SA

Yes, exactly, and without needing to bend the sensor like we did with film in days of yore 🤣  One would think that SA was still easier to correct in a large dia. front optical. But who knows ?! with modern tech and clever design ?  There was a time when manufacturing tech needed correctors matched to their primaries, but now I read that machine tolerances are good enough that that it is not needed any more. A bit like the change in threads from in-house specials to Whitworth

 

[davidc135]

22 hours ago, Malpi12 said:

Oh dear ! or similar words.

Would it be considered a heresy if I were to suggest making some vanes for it, to support the lens assembly pro tem ? Only as a scientific curiosity of course just to see how much correcting it /is/was doing  ?

 

 

Below are images of graph paper taken with an 8'' sct, with and without the corrector plate in place. The effects of 16ish waves of spherical aberration can be seen.

The graph paper was placed at the focus of a Meade LXD55 and the image picked up by a second sct whose corrector could be removed, the 2ndry being supported by vanes as mentioned.

David

[symmetal]

15 hours ago, Rusted said:

Does household contents insurance cover the accidental dropping of an OTA?

I believe mine covers accidental damage to any personal items in your possession at or away from home up to around I think £5000 in value.

6 hours ago, Malpi12 said:

When I first had the thought my concern was how to support/attach the vanes without resort to drilling holes for bolts !!  If 3d circles could be printed that is good - could the vanes be made of stiff material (conventional metal) and be included in the 3d plastic ? Perhaps like rebar.

No one (I imagine !) would want to take apart a rasa just for this, but here we have an interesting possibility, I am sure the rasa will get over the insult to its dignity when the new corrector arrives !

Good suggestion. I'll see how easy it is to fabricate something, and depending how long it takes for the replacement plate to arrive. 🙂

Alan

[symmetal]

42 minutes ago, davidc135 said:

Below are images of graph paper taken with an 8'' sct, with and without the corrector plate in place. The effects of 16ish waves of spherical aberration can be seen.

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

Alan