Importance of corrector centering on SCTs

[multiweb (Marc)]

First time I have actually been in a position to make a quantititve comparison with the offset of the corrector plate on a C11 SCT. These pictures will hopefully help other understand how critical the axial offset of this big lens in front of your primary is. In both test original conditions were as follow:

1_ Both shots were taken with an hyperstar so there is no secondary to compensate axial error via collimation.

2_ The hyperstar was not tilted or collimated in anyway. It sits flat on the corrector.

3_ Both photos were taken in the same conditions. Seeing, transparency, temperature, camera, etc...

In the first shot the corrector was shifted 1mm towards 2 o'clock facing the front. In the second shot the corrector was centered in the aperture. Both shots are taken in focus. The two small schematics show the corrector offset direction.

In the first case (offset) you can see stars flaring. The degradation in the picture is massive. The flaring in and out of focus will shift by 90 degrees, typical of astigmatism. The scope could not come in sharp focus. Even trying to collimate with the secondary in place would show oval airy patterns in and out of focus.

We are not talking about rotational shift here, just a 1mm axial shift. Rotational shift of the corrector would cause the telescope to not focus at all. Even a 5 degree rotational offset will make the SCT unusable.

I read that the corrector placement is critical before but these two shots illustrate it quite well.

I have a higher resolution pic here. For those interested in the whole field there is a flicking slideshow between the two here. [5.9MB]

[Barrykgerdes]

Well said Marc

I have been saying this for years. The corrector plates primary task is to correct spherical aberation that occurs quite severely with the fast mirrors used in SCT's. To correct this problem the corrector plate progressively from the centre alters the angle of incident wave on the edge of the mirror to make the mirror work like a parabola.

If it is not precisely located (centered) it won't do its job.

Any other corrector device used in the optical path must also be correctly positioned at its design location to be effective.

Barry

[Astro_Bot]

Thanks for that demo. I agree that corrector centring is vital for image quality. That's some really useful evidence of the principle.

Originally Posted by multiweb

Rotational shift of the corrector would cause the telescope to not focus at all. Even a 5 degree rotational offset will make the SCT unusable.

I'm not sure about the rotational alignment, though. Isn't the corrector (and primary and secondary for that matter) symmetric? I've read reports that say they are, and a few others that claim the corrector and secondary are hand-touched for defects in the primary, but I find it hard to believe, in a mass-produced SCTs like Celestron and Meade, that there's hand-touched anything given the prices.

I've fiddled with corrector centring in my SCT, and visually noticed a difference (soft images and, when examined closely, lop-sided astigmatism - at least that's what I've been calling it) but I couldn't see any difference with respect to rotational alignment rotating the corrector (with secondary) through 90 deg or secondary (alone) through 120 deg.

[multiweb (Marc)]

Originally Posted by Astro_Bot

I'm not sure about the rotational alignment, though. Isn't the corrector (and primary and secondary for that matter) symmetric? I've read reports that say they are, and a few others that claim the corrector and secondary are hand-touched for defects in the primary, but I find it hard to believe, in a mass-produced SCTs like Celestron and Meade, that there's hand-touched anything given the prices.

Celestron do the final figuring on the secondary because it is the smallest surface to touch up. But the primary and corrector are made as a pair so they're matched before the secondary is hand figured.

From what I've read I don't believe Meade hands figure anything. They have more generic parts. So I suspect rotating the secondary or corrector on a meade would have less effect on the system.

Originally Posted by Astro_Bot

I've fiddled with corrector centring in my SCT, and visually noticed a difference (soft images and, when examined closely, lop-sided astigmatism - at least that's what I've been calling it) but I couldn't see any difference with respect to rotational alignment rotating the corrector (with secondary) through 90 deg or secondary (alone) through 120 deg.

I guess different systems are affected differently. Do you have a Meade or Celestron?

My secondary is hand figured. Somehow the rotational position of my corrector is critical also. I don't know if the primary was hand figured to match the corrector's orientation or vice versa. There are registration marks on the primary, etched into the corrector glass and at the back of the secondary. All point to the 3'oclock position facing the scope which is opposite the focuser. There is also a mark to show which side of the corrector faces out. I read that the schmidt profile can be inside or outside the scope. It varies from one scope to the other even in the same range but it doesn't mean that it's ok to flip the glass obviously.

[Astro_Bot]

Originally Posted by multiweb

Do you have a Meade or Celestron?

Meade. There's a physical indentation to orient the secondary onto the secondary holder (I suspect to ensure the holes line up before inserting bolts), but no other marks anywhere in the optical system.

(FWIW, my corrector/secondary assembly is centred +/- 0.08mm and, taking an average of many measurements around the front corrector cell, the cell is "round" to within +/- 0.05mm, so I have high confidence I'm getting centring right. Corrector/secondary centring was out of whack by ~1mm when I got it - it definitely makes a difference!).

[multiweb (Marc)]

Originally Posted by Astro_Bot

Meade. There's a physical indentation to orient the secondary onto the secondary holder (I suspect to ensure the holes line up before inserting bolts), but no other marks anywhere in the optical system.

(FWIW, my corrector/secondary assembly is centred +/- 0.08mm and, taking an average of many measurements around the front corrector holder, the holder is "round" to within +/- 0.05mm, so I have high confidence I'm getting centring right. Corrector/secondary centring was out of whack by ~1mm when I got it - it definitely makes a difference!).

Wow! Your measurement tolerances are pretty tight, I don't think I can measure or even place anything that precisely on my system. Cork shims, cast corrector cell and glass thermal expansion would be greater than any of these figures.

[Astro_Bot]

Kincrome digital vernier caliper at Bunnings for ~$70. One of my better purchases.

It took many hours of trial and error, believe me!

[Poita (Peter)]

So what is your process for making sure it is centred?

That reminds me, I need to get a new washer/shim thing for behind the removable secondary on the 9.25 HD, otherise it turns when tightening it up when removing for the hyperstar...

Is this the vernier you have?
http://www.bunnings.com.au/products_..._P5660320.aspx

[Astro_Bot]

Is this the vernier you have?

Looks like it.

Originally Posted by Poita

So what is your process for making sure it is centred?

I briefly thought about writing it up for an IIS article, but it's not rocket science - it's finicky, very time consuming and error prone, and I figured I'd get many complaints from people expecting miracle cures or instant results. My method works for me, but I don't guarantee it will work for anyone else.

It took many hours trial and error and painstaking care.

I first measured the machined secondary holder outer diameter and found it to be "round" to within the limit of measurement error. All other measurements were for the distance from the inner edge of the corrector cell to the outer edge of the secondary holder.

The key points are (IIRC):

• Have the OTA in a stable, comfortable working position - I had it in my EQ6 Pro, oriented to point vertically so that the corrector cell was at chest height, and levelled.
• Take many measurements and take mean and RMS error (ignoring obvious outliers), or I suppose you could calculate standard deviation in lieu of RMS error - this is, IMHO, the best way to reduce method, observational and instrumentation error, though the Vernier was quite reliable - I think it lives up to it's stated single measurement error margin of +/- 0.01mm (<50mm) or +/- 0.03mm (>50mm).
• For roundness, use measurements at 30 deg. intervals around the cell - add opposite measurements to calculate a "diameter".
• Complete each entire process more than once to improve/verify technique.
• Measure "roundness"; calculate centre position; centre corrector assembly; re-measure centring; re-centre corrector; re-measure centring ... until satisfied that no better precision can be achieved.

Physically, I loosened corrector cell retaining screws and lightly tapped the secondary holder to achieve movement.

After tightening the corrector cell screws, I took a final set of measurements to confirm no unintentional movement.

Of note, I found that the secondary holder did not measurably move relative to the corrector.

[multiweb (Marc)]

Originally Posted by Astro_Bot

Physically, I loosened corrector cell retaining screws and lightly tapped the secondary holder to achieve movement.

After tightening the corrector cell screws, I took a final set of measurements to confirm no unintentional movement.

Of note, I found that the secondary holder did not measurably move relative to the corrector.

You have to be very careful with the corrector retaining ring. It's touch only. No pressure. It's very easy to apply a lot of pressure with 8 screws or more over the circumference. It's like a flange. Only a small torque on each screw can add up to an incredible amount of pressure on the lens. Ideally the shims should be the only things stopping the corrector from moving sideways. If you apply too much pressure with the retaining ring you risk cracking the lens with thermal contraction/expansion if it's got nowhere to go.

[Poita (Peter)]

Thanks, I am going to have to check mine now!

[Astro_Bot]

Thought I'd refresh this after some more work.

I had cause to clean the corrector in my Meade LX200-ACF 10", so pulled it out and cleaned the OTA interior (but not the primary mirror, which was/is quite clean anyway) and secondary while I was at it.

After pulling the corrector out (first time I'd done it) I did find a maker's mark - I didn't see it the first time because it had largely worn off (probably for reasons explained below) and I only had the retaining ring off momentarily last time around ... and the fact that I got my first pair of reading glasses last week, and, oh my God, I can see ultra-fine detail close-up again ... just like when I was a youngster!

The cork shims were in a poor state - one missing altogether, and the seven remaining all compressed. Now I can see why there was 1.1mm of lateral movement in the corrector! That permissable movement against the corrector retaining ring, back before I owned this OTA, is probably what wore off the maker's mark.

With the corrector unmoved (I think - I've been very gentle with it) from my aligned/collimated position of a few months ago, I dug out the digital vernier caliper and measured separation from the corrector's outer edge to the corrector holder's edge. It was surprisingly uniform, with variation of only +/- 0.07mm or thereabouts - the average was around 1.8mm. The cork shims had shrunk or compressed to about 1.3mm.

I set about finding replacement shims - there are a few possibilities online - but as it turned out, I had a sturdy rubber strip in my toolbox that was 1.75mm +/- 0.05 thick (according to the caliper). Just goes to show that keeping those little things around can pay off in the long run. I cut that into suitable small strips and inserted the new shims into the gap - it was fiddly ... no, actually it was downright farnarckling ... but it worked - using a clean paddle-pop stick (they're 1.5mm thick) to push the shims into place.

The corrector now doesn't budge at all and is aligned perfectly with the maker's mark, what's left of it. By the way, the shims do come out again - using said paddle-pop stick and a bit of perserverance.

I re-measured the centring of the corrector using my original method (well, not quite as pedantically, as I have more confidence in the caliper's accuracy now) and found the variation to be about +/- 0.12mm, which is not quite as good as the original +/- 0.08mm, but close enough.

Now to re-collimate the secondary and test ... when the weather clears.

Here's a photo of the compressed cork shim - see the gap? - the gap actually looks smaller in the photo due to the camera angle. And, just above it, the new rubber shim.

You can also see some felt-tip writing on the outer edge of the corrector. It may refer to some specific alignment info, but didn't make any sense to me. I'm still mystified as to why the maker's mark is there, since I couldn't see any difference when I rotated the corrector 90 deg, the last time I aligned it. Oh, well - it's rotationally aligned, just in case.