Schmidt-Cassegrain Collimation Issues

Hey fellow Schmidt-Cassegrain owners. I have done much research on all types of telescopes and own refractors and Schmidt-Cassegrains. I have seen many collimation problems on these that I said this is a problem area. Well I have found the cure and it is worth the trouble. First save your money don't buy a tool they don't work on Cassegrain telescopes that well due to the folded optics and optical alignment problems both mechanical and optical. Lets assume you have good optics mirror / secondary and corrector plate (meniscus). I'm using a 9.25 Celestron..

First I found that when I collimate by eye looking at the front from say 8-9 feet the shadow of secondary and ring look good BUT the outer ring is a little off with the housing tube itself. Star test good but could be better I think? So here we go

Alignment of secondary housing to main mirror not the secondary mirror but the holder in the corrector plate. Great care must be taken here to avoid mistakes. First get a caliper - digital is best but slider works fine. Measure the distance of the secondary holder from the outside edge to the inside edge of housing ring [the flange holds the corrector and secondary]. Measure at 12 o'clock 6 and 3 and 9 write down your findings. Now if it is off allot like mine was 0.15 at one side 3 to 9 and 0.8 at 12 and 6 O'clock Take a felt pen and mark the corrector plate with a dot at the edge after removing the outer retainer ring. It will be out of view under the ring so you can leave it there for reference.

Now inspect the outer edge of the corrector while in place. You will see some small cork spacers. Mark the position with dot. Set scope tube on side with pillow or blanket. Do this before removing retainer ring. Now lift off corrector plate IT MIGHT BE STUCK so be gentle. Once off, unscrew secondary housing after marking its clock position. All the markings are to put every thing back where it was if the results are bad .

Now I noticed that the hole in the corrector was bigger than the housing by a lot. Remember don't touch any mirrors and put your cover over the tube while you work on the corrector unit. I took some teflon tape and put one layer around the housing where the plate rides. There are two paper or other type of gaskets between the top and bottom of glass so keep them handy. Next put secondary back on glass - remember clock positions and dot side up or facing out side when assembling. Don't over tighten, just snug and make sure it is all the way flush around the housing don't forget the gasket on both sides. Now the secondary is centered in the glass and won't shift.

Now place glass back in scope housing after removing pads [cork spacers in my case]. There were only two so I cut them in half and made four. Now make sure you are clean underside of glass and set glass [corrector] back on tube. Get caliper and measure again The 12-oclock 6/3/9 positions center the secondary and put spacers [pads] in the proper position to hold it centered. The glass is smaller than the housing so there is room to move it in all directions. The last pad might be snug to get in but a slight wedge on glass with a popsicle stick or a small screwdriver. If you dare (could chip glass edge) will be fine to get last pad in place. Put retainer ring on tighten snugly and evenly and you are done. Collimate secondary as normal and star test away .

THIS is risky business so take caution. Don't over tighten any thing when working with glass to much or it will be deformed or worse, break. This exercise is not for the average person but the results can impressive indeed IF you suffer from missaligned optics. For me it was a night and day . This 9.25 absolutely KICKS PHOTON BUTT!

Feel free to Email me for help on refractors and cassegrains. I know many tricks of the optics trade and also EQ mounts can be made very well improved with little or no money just time and patience. Thanks and happy Photon busting (STAR GAZING)

PJSVIOLIN@aol.com

Addendum

ONE great way to collimate better than collimater tools including laser which are soso for schmidt cass scopes. Because of the design of the optics on cass (including maks) perfect collimation is best done with eyes and star test

Start with 250x to 350x mag. Defocus to a doughnut about 1/3rd the field and start getting a perfect center on secondary mirror shadow in center on both sides of focus. Use your best diagonal as this will be used to view. Some say do it at the back of the tube with no diagonal. But that is not accurate unless your diagonal and tube on scope to secondary is perfectly square on optical axis which is almost impossible. Diagonals and mounting hardware are seldom true to axis so use diagonal and collimate the way the scope will be used. Also don't use focal/corrector reducers for collimation as the whole optical physics has been greatly compromised as to normal scope f ratio and focal length. Most of the time the reducers work well with normal scope collimation but for photography some tweeking may be useful for best performance.

Now back to work. After using 250-350x mag move up to 450 to 600x mag and start all over again . Adjust the doughnut hole to center but this time work with as small a defocused image you can see the doughnut center and outer diffraction rings. Remember to check both sides of focus. After you have got your best image rack the focus back and forth just past normal focus. Look for a tiny flare or spikes just before the image makes a circle or out of focus pinpoint. Remember which way is was flaring or spiking then defocus about 1 to 1 1/2 wave and use your hand in front of the main lens [corrector plate]. This way you can move around the defocused image and see where your hand is pointing. Motor drives are a must as the image moves very quickly out of field at high powers. Can be done without but a real pain!

When you have the spot located with your hand shadow tweek in that direction ever so slightly. The screw must only turn about a fraction of a tweek, barely able to notice a turn or movement of the screw . I'm talking almost nothing. Most people don"t realize how much change is in a tiny tiny tweek. At very high powers and good seeing the pinpoint stars can go from diamonds on black velvet to fuzzy blurs . It is pain staking work to get it absolutely dead on but it is worth it.

Many mass produced scopes have optics that are good to great but near perfect is rare. Mine is one of those near perfect ones and I got very lucky. On average most will not be able to get all the spikes or tiny flare just off of focus down to say 1/7 to 1/8 wave. Those scopes are far and few between. Also I have seen that many have a tiny bit of astigmatism, not bad but it is there. More common are turned edges on primary mirror. Not much can be done for them but getting the best out of what you got makes a scope that is exciting and fun to use allot. To one that is used sometimes and good on planets and soso on deep sky.

Take time to dial it in to the max you will be amazed at the images the scope can produce if it of good quality. My 9.25 is something you have to see to believe. It has a very very slight turned edge but not a tiny trace of astigmatism which my 4.7"refractor has a tad and no coma at all and no zones. I would like to do a ronchi test just to see how good these optics really are. From doughnuts to pinpoint at a near perfect crossover from intra focus to extra focus on both sides. I am impressed and I don"t impress easy. Thanks and happy star Gazing