SCT corrector plate replacement

[Westmoorland]

Revelation Picostar Artifical Star - Widescreen Centre (widescreen-centre.co.uk)

 

The Widescreen Centre said the hubble artificial star is rubbish and suggested this--which they just happen to stock. Not sure which way to go.

[davidc135]

 

I haven't used the Hubble so I'd be guided by the Widescreen Centre. My artificial star is by Geoptic and cost quite a bit more- you get what you pay for, I guess. You could also use a ball bearing plus torch for next to nothing but it's  more awkward and I use my Geoptic a lot.

In rotating the corrector/2ndry you won't be affecting spherical aberration which is variation of focal length with radius. For instance under-correction is the outer zone of an optic focusing shorter than the more central zones. 

Astigmatism is variation of focus length across two axes at 90degrees to each other. Ie The faulty surface(s) can't be figures of revolution. If there are two faulty components they could be rotated relative to each other such that the astig is reduced or annulled in a way that can't happen with SA if the surfaces are figures of revolution,.

Hopefully modern production will produce figures of revolution, or near, on all surfaces in the scope and only collimation will be required.

With the articial star setup it would be interesting if you rotated the cracked corrector by 45degrees and compared views after collimation. I could easily be wrong but I'm guessing that the image quality will be the same.

David

Edited October 26, 2022 by davidc135

[Westmoorland]

On 26/10/2022 at 19:52, davidc135 said:

 

I haven't used the Hubble so I'd be guided by the Widescreen Centre. My artificial star is by Geoptic and cost quite a bit more- you get what you pay for, I guess. You could also use a ball bearing plus torch for next to nothing but it's  more awkward and I use my Geoptic a lot.

In rotating the corrector/2ndry you won't be affecting spherical aberration which is variation of focal length with radius. For instance under-correction is the outer zone of an optic focusing shorter than the more central zones. 

Astigmatism is variation of focus length across two axes at 90degrees to each other. Ie The faulty surface(s) can't be figures of revolution. If there are two faulty components they could be rotated relative to each other such that the astig is reduced or annulled in a way that can't happen with SA if the surfaces are figures of revolution,.

Hopefully modern production will produce figures of revolution, or near, on all surfaces in the scope and only collimation will be required.

With the articial star setup it would be interesting if you rotated the cracked corrector by 45degrees and compared views after collimation. I could easily be wrong but I'm guessing that the image quality will be the same.

David

Haven't got the artificial star yet but the new plate arrived from Celestron. I had a look through the cracked plate for the first earlier this morning and besides two possible faint diffraction spikes coming out of Mars, I couldn't tell there was anything wrong with it: This considering I put the plate back in randomly after the first crack causing removal.  I also got my best ever view of the Orion nebula at 168x mag. 

 

I'm thinking of installing the new plate and forgoing the star test.  As I'm typing, I'm trying to think of how much the secondary might have rotated or what the 'B''s drawn on the side of it denote. I don't know what to do, but need peace of mind my scope isn't impaired. I was going to test on a few more stars but the fog rolled in. I don't want to lose resolution if that's what spikes do. I can't recall if I saw them before the crack. I don't know. 

[davidc135]

1 hour ago, Westmoorland said:

Haven't got the artificial star yet but the new plate arrived from Celestron. I had a look through the cracked plate for the first earlier this morning and besides two possible faint diffraction spikes coming out of Mars, I couldn't tell there was anything wrong with it: This considering I put the plate back in randomly after the first crack causing removal.  I also got my best ever view of the Orion nebula at 168x mag. 

 

I'm thinking of installing the new plate and forgoing the star test.  As I'm typing, I'm trying to think of how much the secondary might have rotated or what the 'B''s drawn on the side of it denote. I don't know what to do, but need peace of mind my scope isn't impaired. I was going to test on a few more stars but the fog rolled in. I don't want to lose resolution if that's what spikes do. I can't recall if I saw them before the crack. I don't know. 

Whilst there's the opportunity, it's interesting to see the effects of the cracks on star images and whether they amount to much more than the faint diffraction spikes.

It might be worth setting up a ball bearing or anything similar outside with a torch that could be a point source of light; water droplets, xmas tree decorations etc. It's convenient not to have to rely on the weather if adjustments are to take any time.

If, during this time with the existing cracked corrector, you rotate the 2ndry (but not the corrector) by 45 deg, you'll have an opportunity to find out whether or not it matters. If you see no difference then there's that potential problem solved when it comes to fitting the new corrector. My guess is that it won't, but...

David

Edited November 4, 2022 by davidc135

[SthBohemia]

@Westmoorland How badly cracked is the original plate? I ask because I purchased a 9.25" Celestron umpteen years ago while within Oz for a pittance due to it having a corrector in 8 pieces 🙂 Wanted the CPC fork and the heavy duty tripod for the Meade 10" SCT OTA.

I glued the broken plate with E6000 gunk on a relatively flat lathe bed, have not bothered to try the performance on stellar objects, only test performed was resting it on a windowsill as a terrestrial scope, which gave ok results.

Be interested to read the difference between performance of the original plate and the replacement.....

[Westmoorland]

10 hours ago, SthBohemia said:

@Westmoorland How badly cracked is the original plate? I ask because I purchased a 9.25" Celestron umpteen years ago while within Oz for a pittance due to it having a corrector in 8 pieces 🙂 Wanted the CPC fork and the heavy duty tripod for the Meade 10" SCT OTA.

I glued the broken plate with E6000 gunk on a relatively flat lathe bed, have not bothered to try the performance on stellar objects, only test performed was resting it on a windowsill as a terrestrial scope, which gave ok results.

Be interested to read the difference between performance of the original plate and the replacement.....

You can see the actual crack on my profile picture. 

[Westmoorland]

11 hours ago, davidc135 said:

Whilst there's the opportunity, it's interesting to see the effects of the cracks on star images and whether they amount to much more than the faint diffraction spikes.

It might be worth setting up a ball bearing or anything similar outside with a torch that could be a point source of light; water droplets, xmas tree decorations etc. It's convenient not to have to rely on the weather if adjustments are to take any time.

If, during this time with the existing cracked corrector, you rotate the 2ndry (but not the corrector) by 45 deg, you'll have an opportunity to find out whether or not it matters. If you see no difference then there's that potential problem solved when it comes to fitting the new corrector. My guess is that it won't, but...

David

Could I keep the scope indoors but pointing out the back door into the garden for this? I was set to buy the Hubble thing last week but will have to wait a few more days now.  I have a pic before the crack showing the position of the 'fastar' logo on the secondary housing relative to the retaining ring so I can put that back to where it was.  Not sure my power tank torch will be able to focus enough light on a ball bearing to make it 'star-like' enough?

[SthBohemia]

11 hours ago, Westmoorland said:

ou can see the actual crack on my profile picture. 

As that's the total extent of the damage,  within your position I would not be overly concerned. Would simply have covered the crack with ultra thin PCB masking tape or painted over it with black lacquer/paint. Image below is of the one I described previously! If you have any intentions of selling your single cracked plate please let me know.

Cheers Rob.

[RT65CB-SWL]

On 26/10/2022 at 13:25, Westmoorland said:

The Widescreen Centre said the hubble artificial star is rubbish and suggested this--which they just happen to stock. Not sure which way to go.

Ouch... that’s a bit harsh! - Though you can’t beat using a ‘real star’ for the final test, (assuming the atmosphere is still and cloud free).

I have one, and at £19.00GBP + P&P + batteries, it will do and I will continue to use it. So I don’t lose the small ‘magnetic’ mask/shield thingy, (when not in use), I put a vinyl pipe/tube end cap over the end of it... and remove the batteries.

 

Edited November 5, 2022 by Philip R

[davidc135]

13 hours ago, Westmoorland said:

Could I keep the scope indoors but pointing out the back door into the garden for this? I was set to buy the Hubble thing last week but will have to wait a few more days now.  I have a pic before the crack showing the position of the 'fastar' logo on the secondary housing relative to the retaining ring so I can put that back to where it was.  Not sure my power tank torch will be able to focus enough light on a ball bearing to make it 'star-like' enough?

I've often done the same although the room I use is unheated and I leave the double doors open for a while to reduce any thermals.

Any small torch will do set up close to the ball bearing and making a small angle with the line of sight. The torch doesn't need to 'focus' on the ball bearing. But I expect the Hubble will be fine used at its smallest aperture.

David

[Westmoorland]

On 05/11/2022 at 08:34, SthBohemia said:

As that's the total extent of the damage,  within your position I would not be overly concerned. Would simply have covered the crack with ultra thin PCB masking tape or painted over it with black lacquer/paint. Image below is of the one I described previously! If you have any intentions of selling your single cracked plate please let me know.

Cheers Rob.

I admire scopes too much, even as ornaments, to tolerate even a small crack. Besides, it reminds me of my incompetence and carelessness in causing it so my replacement will go on once I'm ready. Not sure what you'd think it's worth?

[Westmoorland]

On 05/11/2022 at 10:13, davidc135 said:

I've often done the same although the room I use is unheated and I leave the double doors open for a while to reduce any thermals.

Any small torch will do set up close to the ball bearing and making a small angle with the line of sight. The torch doesn't need to 'focus' on the ball bearing. But I expect the Hubble will be fine used at its smallest aperture.

David

I actually don't think I have a ball bearing at hand. My scopes cooling down outside; I'm going to have one last look through the cracked plate to see if there's any aberration. Tomorrow I'll put the secondary back to it's position before the crack and hopefully all will be well enough to put the new corrector on. If you think I should wait for the Hubble test perhaps I will. I'm not confident about judging the optics myself. I might check again for anyone in my area who can do it. I don't want to crate it off to Orion optics etc...  Thanks for your advice and sorry I'm not very clear. A bit fed up with the situation.

[davidc135]

You might try putting a pinhole in aluminium foil wrapped in front of a torch. Place the foil on a glass or similar flat surface and tap it lightly with the tip of a needle and then inspect with a (reversed) eyepiece. It probably won't be very circular but if small enough and far enough away it should work OK to collimate the scope.

The simplest outcome is that, with your best preparations, the installed secondary only requires collimation to give, at high power, at focus and in centre of field, an Airy disc surrounded by circular concentric diffraction rings. An artificial star in hopefully stable air will enable a forensic look at the star image.

David

[SthBohemia]

13 hours ago, Westmoorland said:

Not sure what you'd think it's worth?

Leave things on the backburner until such time as you have sorted out the collimation issues with the new plate.

[Westmoorland]

On 09/11/2022 at 20:41, davidc135 said:

You might try putting a pinhole in aluminium foil wrapped in front of a torch. Place the foil on a glass or similar flat surface and tap it lightly with the tip of a needle and then inspect with a (reversed) eyepiece. It probably won't be very circular but if small enough and far enough away it should work OK to collimate the scope.

The simplest outcome is that, with your best preparations, the installed secondary only requires collimation to give, at high power, at focus and in centre of field, an Airy disc surrounded by circular concentric diffraction rings. An artificial star in hopefully stable air will enable a forensic look at the star image.

David

Looks like I'll be sending it off to the pro's after all. The cracked plate seems like it might be glued to the secondary baffle/housing tube thing, and even if it isn't, I daren't twist it in case the crack gets worse. It never ends...

 

[SthBohemia]

18 minutes ago, Westmoorland said:

It never ends...

Not unusual for the gaskets to get a bit stuck both where the secondary attaches to the lens and where the lens attaches to the tube rim. Just a bit of acetone or another solvent fixes the problem.  You are almost there not time to  throw the towel in yet...

[SthBohemia]

@Westmoorland try not to let disassembling SCT's intimidate you. If the secondary assembly is VERY obstinate, fill a glass or some container with petrol/solvent to the brim and place the holder assembly on top of it and leave to soak for an hour or two. Spray the thread with WD-40 or equivalent and then remove the retaining ring. With brute force if necessary! The fibre washer may get damaged but easy enough to make a new one with some thin cardboard and a scalpel.

Never use any substance such as 'locktite' when re-assembling, for that matter don't even bother to lubricate the threads when putting all back together.

[davidc135]

Good advice above. You could reinforce the glass around the crack(s) with plenty of tape to help save the corrector. But it's not worth a huge amount.

David

[Westmoorland]

On 14/11/2022 at 10:19, davidc135 said:

Good advice above. You could reinforce the glass around the crack(s) with plenty of tape to help save the corrector. But it's not worth a huge amount.

David

Removing the tube would be one thing, but fixing it to the new plate afterwards would be another for me. Orion Optics time, I think.