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SCT COLLIMATION - Much more than collimat'n screws

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#1 bilgebay

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Posted 20 December 2011 - 02:19 PM

This will be a long story but I hope some fellow SCT owners will benefit from it.

Everything started after my visit to NEAIC and NEAF last April.

I was terrified at NEAIC when I heard Alan Holmes, telling us, during an experiment he conducted, even touching the tube with his hand for a very brief period of time, would result in a recovery period like 5-10 minutes from the effects of that. The subject of the seminar was " Tube Currents in SC Telescopes"

I have decided to actively cool the scope to get rid of the tube currents and at the same time to insulate the tube from inside and outside to eliminate the effects of the tube temperature creating new tube currents. This would also help avoiding quick focus changes. I've started and finished this project last August and I am very satisfied with the results so far. I will continue the thread after I do some more testing this winter when temperatures will fluctuate drastically. For those who are interested the thread is here .

The reason I mention the cooling project is, during that, I messed up and gone through a painful and difficult research period regarding the corrector orientation. It took some time but I got some clues from Celestron support. You can read about this here. A greater help came from Dean of Starizona. He was giving me the information I needed as follows:

" The critical rotational orientation is not the corrector but rather the secondary mirror. The critical X and Y (left and right and up and down) is the placement of the secondary opening over the primary. Have you seen this video? http://starizona.com... Kit/Video.html

We make a super soft gasket from sorbothane that is self sticking to the corrector glass and the front of the conversion kit."

...but at the same time he was showing me the guide lines of my next project :) which is the subject of this post. Centering the secondary/secondary housing to the primary.

I will show you the result first and then continue with the details of the procedure. Below is a (uncalibrated, unprocesed) photo I have taken with my Hyperstar lens after I have done the "mechanical" collimation. Hyperstar users will appreciate the roundness of the stars on the sides and corners. While shooting this photo, all push screws on HS collimation collar is taken off and the lens assy is fuly seated into the shoulder flat and fixed in that position. This means "no collimation" was done on the Hyperstar.

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Hires photo is here .

#2 bilgebay

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Posted 20 December 2011 - 02:56 PM

Unless your primary and secondary are precisely on the same line you will never get a perfectly flat FOV no matter what you do with your collimation screws.

In the Starizona video mentioned above ( http://starizona.com... Kit/Video.html ) it is recommended to make this alignment with your eyes. Probably, Dean and other guys in Starizona are able to do this since this is their expertise. However, I cannot trust my eyes to that extent. From August to November I kept postponing this alignment job. I am sure, somewhere in my brain, the gears were running fast to produce a solution to this situation. At the beginning of November, the alignment tool was ready in mind and I made a simple sketch to produce it.

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Sorry, it is not a nice drawing and descriptions are in Turkish. Hopefuly, tomorrow I will be posting a nicer drawing when my daughter finishes drawing it in Autocad.

Here is the real thing though:

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#3 bilgebay

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Posted 20 December 2011 - 03:15 PM

When I designed the tool, I assumed that the mirror of my C11 Edge HD was precisely centered on the baffle tube. Luckily it was :) I have also verified whether the center cavity of the baffle tube was concentric with the outer diameter. Positive again.

Then I have measured the position of the in-baffle corrector lenses very carefuly. I have covered the tip of the measuring tape with felt completely. I measured this distance to be approx 15 centimeters from the front of the baffle and limited the rear end of the tool with a pin so that it would go into the baffle tube only 13 centimeters.

Long story short, all the crucial dimensions are on the drawing.

#4 bilgebay

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Posted 20 December 2011 - 03:35 PM

Before taking my corrector off for replacing the gaskets of the secondary holder with the sorbothane gaskets Dean sent to me, I wanted to see if my secondary was centered or not. It was off by approx 2 milimeters.

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In this photo, you are seeing the shaft fully inserted into the baffle tube. There was some very little play and I got rid of this by putting some sellotape (0.03 mm thick) on the shaft which eliminated the extra clearence I had there. The shaft extends all the way out of the secondary holder. I then insert the secondary holder centering tool on the shaft. If the secondary holder is precisely centered to the baffle tube, the tool should go all the way in to the secondary register without any resistance. In this photo, this wasn't case. I know it is not showing the defect as I was seeing it but please take my word for it.

#5 bilgebay

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Posted 20 December 2011 - 03:53 PM

I have then took the tool out, dismantled the corrector, dismantled the secondary holder and replaced the gaskets on the secondary holder.

In the photo attached, you are seeing the sorbothane gasket installed. This gasket is soft and self sticking thus avoids the accidental rotation of the secondary holder when switching between Hyperstar and visual sessions. This was a major problem for me and it is over after this mod.

You are also seeing that I am putting on some felt material to reduce the play of the secondary holder within the corrector plate. The secondary holder had over 1mm play in this hole. The felt was just the right thickness I needed.

#6 bilgebay

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Posted 20 December 2011 - 04:07 PM

After this step, I didn't forget to spray the threads of the secondary holder with McLube Sailkote. This is a very interesting material, a dry lubricant, we are using in the marine field. The reason I applied it was to facilitate the assembly of Hyperstar there. If you heard me putting the Hyperstar on these threads you would believe I was scratching my corrector or something :) The threads are too loose, probably conical. They only get to normal tolerances right before the final turn. For the time being Sailkote helped greatly.

#7 bilgebay

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Posted 20 December 2011 - 04:24 PM

Following this, I assembled the secondary holder to the corrector. Before tightening the secondary baffle, double checked that it was precisely centered relative to the outer diameter of the corrector. I was also careful to align the secondary mirror slot with the mark and etching on the corrector plate.

I, then, put the corrector on the front cell, etching on the corrector and the secondary slot positioned at 3 o'clock position. The photo below shows this step, with the tool inserted. I am starting the alignment procedure by playing with the set screws centering the corrector. One word of caution here; I believe you should be very gentle with these screws and be careful not to overtighten them. This can stress your corrector. This is my gut feeling. These screws are there just to help with the alignment. They are not holding the corrector in place.

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#8 multiweb

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Posted 20 December 2011 - 04:55 PM

Clever way to center mechanically.

#9 GeneT

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Posted 20 December 2011 - 05:05 PM

Very interesting report!

#10 Steven

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Posted 20 December 2011 - 06:31 PM

Clever way to center mechanically.


+1

So it is confirmed that the corrector center bore instead of the edge should be eccentric to the optical axis?


Steven

#11 DesertRat

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Posted 20 December 2011 - 09:56 PM

Interesting work Sedat. The use of a mechanical extension is clever. Seems gravity might be a problem, was the centering done vertically?

I recommend backing off the corrector centering set screws after the retaining ring is installed. It does'nt take much to stress the corrector plate.

Glenn

#12 MrJones

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Posted 20 December 2011 - 11:44 PM

Thanks for the interesting report. I would so love to be able to take my secondary apart from the corrector but there are two large gobs of epoxy on it. Who would do such a thing?

I still wonder how good mechanical alignment is and how important this all is especially regarding variations in manufacturing and testing. If you rotate your corrector and secondary can you actually see a difference? Anyway I came up with a simple idea to check optical alignment:

1) Collimate on a bright star.
2) Rotate the corrector, thereby rotating the secondary.
3) In theory this should result in no change to collimation if the secondary, primary and eyepiece are aligned right?

What I actually observed was a noticeable but not severe change which I was able to correct for with a small twist of one collimation screw. So I probably have something a little off. Oddly enough the diffraction/Fresnel pattern looked a little better to me so I left my secondary rotated 180 deg. from the factory setting ever since.

That's my story!

#13 multiweb

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Posted 21 December 2011 - 12:51 AM

Maybe your primary and secondary are very close to a spherical profile with minimal hand figuring. In my system if I rotate the secondary/corrector assembly just 10 degrees the scope won't focus properly. I get terrible astigmatism even on axis. Otherwise moving the corrector sideways or even tilting it slightly didn't cause much degradation in the image. Mechanical misalignment can be compensated by proper tilting of the various components IMHO. Sometimes you can't rely 100% on the tube and baffle.

#14 bilgebay

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Posted 21 December 2011 - 12:51 AM

Sorry guys, it appears I have fallen asleep on the sofa :)

We are not finished yet. I will reply the comments first.

#15 bilgebay

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Posted 21 December 2011 - 12:58 AM

Thank you Marc.

This is way too easy compared to what you have done with your mirror holder. But you can double check your system with this method. Of course, the tilting of the mirror will change a lot of things, including the squareness of the corrector to the primary etc. If I were you, I would re-machine the rear cell to make the baffle tube centered and square, parallel to the designed optical axis. When you tilt the mirror, even the shadow of the baffle tube can be a problem.

#16 bilgebay

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Posted 21 December 2011 - 01:04 AM


So it is confirmed that the corrector center bore instead of the edge should be eccentric to the optical axis?


Steven


Thank you Steven.

Actually no. In my case, all is concentric now. Before installing the secondary holder, I measured center bore and it was perfectly centered relative to the outer diameter of the corrector. The only issue was it was too big for the secondary. I sorted this out by wrapping flocking paper around my secondary.

#17 bilgebay

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Posted 21 December 2011 - 01:05 AM

Hi Gene, thank you.

#18 bilgebay

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Posted 21 December 2011 - 01:17 AM


Interesting work Sedat. The use of a mechanical extension is clever. Seems gravity might be a problem, was the centering done vertically?



Thank you Glenn. No, the gravity wasn't a problem, I have checked that. The tool + baffle tube assy was flexing in the order of 0.1 mm in total. However, what you are suggesting is very logical. If we do this operation when the scope is looking to zenith the lateral loads on the baffle tube will be almost zero. I thought of this in fact, and checked the alignment vertically once again but saw no difference.

I recommend backing off the corrector centering set screws after the retaining ring is installed. It does'nt take much to stress the corrector plate.



This is another good advice. I guess we should turn all 4 screws by 1/8 turn counterclockwise after tightening the retaining ring screws.

Thank again Glenn.

#19 bilgebay

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Posted 21 December 2011 - 01:37 AM



Thank you Mr. Jones.

I still wonder how good mechanical alignment is and how important this all is especially regarding variations in manufacturing and testing.



This made a HUGE difference. That's why I have posted the sub I took with Hyperstar first. This is very hard to achieve with Hyperstar normally.

Of course, the method and tooling I am sharing with you guys may not be applicable to all the scopes or you may not be a hands on guy to do the task but at least some of you will benefit and some others will have a better understanding as to the cause of problems they cannot sort out by adjusting the collimation screws.

The regular collimation method is good but unless your secondary is perfectly on the same axis with your primary you can only correct on axis problems. This is why we are centering the collimation star at every step of collimation.

I hope experts like Eddgie is following this thread and can chime in to shed more light onto this subject.

#20 bilgebay

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Posted 21 December 2011 - 01:39 AM

By the way, English is not my native tongue, so if something is not clear to you please make sure you ask me again and again until you are satisfied.

#21 bilgebay

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Posted 21 December 2011 - 02:25 AM

Let's continue from where we left.

The adjustment is finished now and I am showing how the secondary holder centering piece is smoothly being seated in to the holder:

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Glenn, as you can see, I am verifying the alignment with the scope in vertical position.

If you are wondering the white stuff between the collimation piece and the shaft, it is a delrin bushing. The clearance between these two parts is so little that if I let the metal move on metal they would soon start seizing. I could have used bronze or similar bushing material but opted for delrin as it was what I had at the time of production.

#22 bilgebay

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Posted 21 December 2011 - 03:14 AM

Upto here, I can say that this procedure is an indispensable part of SCT collimation. I will soon make the tooling for my C9.25 and C5 as well.

The following steps can be a fun and easy way of completing your collimation upto 98 % precision without taking your telescope under the stars at all. However, this second part cannot be done without completing part 1, described above, first. Some of you can find this part completely unnecessary or stupid to do :) As I am an experimental guy here is how it goes:

First, I made sure my secondary was clean by dusting off and checking visually very carefuly. I then placed it on a thick pile of Kleenex tissues.

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#23 bilgebay

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Posted 21 December 2011 - 03:18 AM

Then I took the cover off of the mirror back plate.

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#24 bilgebay

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Posted 21 December 2011 - 03:23 AM

This is how the inside of the cover plate is

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#25 bilgebay

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Posted 21 December 2011 - 03:28 AM

..

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