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Ceph and Cass

Catadiptric Newtonian Secondary Mirror Position


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Hi folks, this is my first post, so hello!

After some time observing just with a pair of Binos, I've decided to move into the world of telescopes, and have just got my first.

It's a Catadioptric Newtonian, 114mm diameter, 1000mm focal length.

Now I've been through a whole host of collimation tutorials on many sites and forums, but I haven't seen anything addressing my particular issue.

It seems the first step in collimating these 'scopes is to correctly position the secondary mirror so that it appears centred and perfectly circular through the focuser.

This is where I'm falling over.

Looking through the focuser, through a collimation cap (just a dust cap with a central hole), my secondary is too close to the aperture.

This is with the secondary central adjustment screw fully unwound, at the end of its travel, only holding by its last thread.

I've tried slackening the screws holding the focuser to the tube, pushing the focuser toward the aperture end, and retightening.

So, I think I'm out of options without making some modifications...

Now, before I pull out a drill and make some new holes in the tube to move the spider further from the aperture, does anyone have any suggestions?...

Thanks,

Cam

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Do you have a makers name? Or any photos of the scope?

I had a similar problem on a brand new (ordinary) newtonian some years back and returned it to the (UK) manufacturer.

A new tube assembly later, things looked better.

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If it's a second hand scope - has the previous owner removed the secondary arrangement and refitted the spider upside down?

Are you sure the focuser is square to the tube? - It's not that the focuser is pointing down the tube making the secondary look too far up the tube?

Edited by haitch
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Thanks for the replies,

It is a Sky-Watcher SK114EQ1.

Can be seen at http://www.shopmania.co.za/optical-tools/p-sky-watcher-sk1141eq1-reflector-telescope-2228327

I've obtained it second-hand, and I doubt the guy I got it from has had the spider out for any reason (I imagine it comes from the factory assembled?). Unfortunately that rules out returning for a replacement.

From memory, I think the part of the spider that goes from the 'legs' toward the secondary mirror is larger than the part that goes toward the aperture, but this is certainly something I can check, good idea haitch, I'd much rather simply invert the spider than start making holes.

But if I'm remembering right, it won't do the trick. The central screw is counterbored in the spider, and I doubt this counterbore would be present on the 'wrong' side of the spider.

I suppose if I can get hold of a central screw and three fine adjusting screws with the right threads but longer, this would save me any drilling.

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I should probably also point out - I can adjust the secondary so that the entire primary is visible through the focuser racked full in or out, so this poor alignment of the secondary might not be too much of a problem on this scope?

I'm just concerned that having the secondary where it is will put my optical axis way off the centre of the focuser axis. (I believe this is what will be happening anyway, I'm very new to all of this!)

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Hi Cam,

Without photographs it is really difficult to make a "remote" diagnosis.

From your description of having to drop the secondary support to the limit of travel to "centre" the secondary using a collimation cap It sounds possible that the focuser draw tube is not level, or less likely, that the secondary support bolts have been replaced with accessory market versions that are too short.

I wouldn't start drilling holes in the tube just yet until you have exhausted all other possibilities!

I can recommend this book, "New Perspectives on Newtonian Collimation" by Vic Menard, £15 at F.L.O. and it is worth reading before you do anything else as the diagrams and text together are so much easier to understand than trying to follow the huge variation in on-line discussions on this subject!

Here is a link to the book:

http://www.firstligh...vic-menard.html

Having just joined the Newtonian owners club myself I found this book invaluable to help me set up my scope.

If you can take and post a few photographs of your problem, at least one picture looking straight down the focuser draw tube towards the secondary and another looking straight down the tube towards the primary then I,m sure there will be a few more experienced members here that can help with your problem.

P.S. according to the model number you stated and the link provided your telescope is just a normal Newtonian, not a Catadioptric...

William.

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Thanks Oddsocks, I'll see if I can get some decent pics tonight.

I had found Vic Menard's site in my searches - very good, clear information, his book could be a wise purchase.

Also very helpful have been Astro Baby's and Cloudy Nights' pages on collimation.

Unfortunately however, none of them have been much help with my own 'scope since I don't appear to have enough travel in the secondary to get past step 1!

You are correct about the description of the 'scope on the site I gave in my last post, and I can't see it described as catadioptric anywhere else either... however, there is certainly a lens present in the bottom of the focuser tube, and I believe that is the distinction.

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Hi Cam,

I did some digging as well and found this telescope is the same as the Celestron CG-2, made with a low cost spherical mirror instead of a parabolic and fitted with a non-removable tele-negative lens in the focuser to correct for spherical aberration.

Here is an extract from the F.L.O. site at this page :

http://www.firstligh...hawk-1145p.html

The Skywatcher Skyhawk 114 v 1145p

The more affordable Skyhawk 114 offers greater magnification from a cheaper to manufacture spherical mirror. Unfortunately to achieve this it uses a magnifying element in the focuser drawtube. The magnifying element is the 114's Achilles' heel and because it cannot be removed it restricts the telescope's maximum performance. The 1145p offered here costs only £10 more, has a regular focuser without the magnifying element and features a more sophisticated parabolic mirror. With the Skyhawk 1145p you can realistically upgrade the supplied-in-the-box eyepieces to achieve an even higher optical performance

And here is a link to a review of the scope at stargazing.og.uk:

http://www.stargazin...wk 114 EQ1.html

It seems from the review and other comments posted elsewhere that as optical performance is not this scopes strong point then aiming for collimation perfection is not going to yield a noticeable performance gain?

However go ahead and post the pics, I'm sure there will be other members on SGL who have first hand experience with this (or the CG-2) that can advise.

William.

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Oh dear!

Oh well, I can't complain, there's no better price than free...

Here are a few pics,

showing the focuser is square to the tube:

http://www.flickr.com/photos/ccmck/9296020320/

View straight down the tube

http://www.flickr.com/photos/ccmck/9296020032

Views straight down the focuser (or as straight as I could manage, not easy trying to line everything up!

http://www.flickr.com/photos/ccmck/9296019778

http://www.flickr.com/photos/ccmck/9293240261

http://www.flickr.com/photos/ccmck/9296019230

You know, looking at these photos I'm beginning to wonder if this secondary misalignment is all in my head! It is difficult to get camera pointed straight down the focuser though...

And in light of Oddsocks' last comment, it doesn't sound like it will make a great deal of difference!

Even so, any an all comments welcome

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Regarding the spider, inverting it wouldn't help as suggested in post #3, both sides are the same size.

Looking through the collimating cap rather than through a camera, the secondary alignment doesn't look too bad until you rack the focuser right in. Then the 'near' edge (left in my photos) of the secondary is 'occulted' by the focuser edge while a gap remains at the 'far' edge.

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Hi Cam,

I can see from the photo's that the secondary is almost in the correct position, another few millimetres down towards the primary would be ideal but as you say the secondary screws are at the maximum length.

From the photos I think you can be certain that the screws are the original ones fitted in the factory.

So you have a couple of options.

Use longer screws for the secondary support, not so easy to obtain being imperial (US) threads but I believe motorcycle specialist shops carry quite a few sizes, or go with your original plan and drill a new set of spider fixing holes in the tube.

Obviously you would need to be sure that there is enough thread length adjustment on the secondary screws to make the "upward" adjustment, back up towards the aperture from your new lower spider mounting.

New holes are free of course but I would try for longer screws myself as there is less chance of making a mistake.

Since the current secondary position is so close to centre there is no reason not to continue with the rest of the collimation procedure and then re-appraise the performance of the scope.

Reading between the lines of the various reviews for this telescope, the permanently fixed corrector in the focuser draw tube is not of a high quality, or rather, it is a match for the supplied eyepieces and while these are adequate for a starter telescope you would not be advised to buy better quality eyepieces as an upgrade because the inbuilt corrector is the limiting factor (as well as the non-parabolic primary).

Don't try to spend too much time, money and worry on improvements to a basic beginners scope but simply get as much experience and enjoyment from using it as it is!

Hopefully a collimation expert will pick up this thread and have a better solution than I can give.

William.

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Sound advice!

Everything points towards this being very much an entry level telescope, which is likely just the step up I need from the binoculars. I can certainly still learn through using it, the secondary mirror can stay where it is!

Thank you for taking the time.

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I collimated a similar design for a visitor last Saturday night as it was obviously not working too well. Using a well collimated laser I found that the "Barlow" lens fitted to the focuser was distorting and offsetting the laser spot negating any benefit of the laser. I finally improved the situation by judging a star image by eye, the result was not perfect but was probably as good as it was going to get without a great deal of fiddling. :smiley:

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I think I've managed to collimate about as well as I can now after a couple of nights of not-terrible seeing and a whole lot of twiddling.

Things look reasonably sharp with the 25mm kit eyepiece, but the sharpness drops off considerably with the 10mm. And the kit 2x barlow? Forget it!

None of the above was much of a surprise I suppose, but the performance with the 10mm eyepiece (100x) is a little disappointing.

I could buy some better eyepieces, but as pointed out by Oddsocks previously, the lens integral to the focuser is always going to be a limiting factor. I might try removing it just to see what difference it makes (besides halving the magnification).

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Hi Cam,

Just a thought but it might work, try making a field stop for the primary.

Your telescope's primary mirror suffers from spherical aberration, to make it useable the designers fitted the corrector (Barlow) permanently in the focuser barrel.

As you have found, trying to use the telescope with an additional Barlow fitted makes things a whole lot worse.

If you do go ahead and remove the makers permanent Barlow from the focuser the spherical aberration in the image will become much more noticeable and you would still have to fit the telescope with a Barlow of you own supply to make the telescope useable.

I read somewhere that one technique used by telescope makers for hundreds of years to reduce spherical aberration was to make a spherical mirror about one third bigger than would have been the case for a perfect parabolic shape and then stop it down with a diaphragm so that only the central portion of the mirror was used, where the mirror shape became naturally more parabolic.

So why not try making a diaphragm from black card or paper to fit over the primary and stop it down a bit, leaving a stopped down diameter of around 80mm or 90mm?

Try this before removing the factory fitted Barlow from the focuser and see if it makes any difference, then if you do remove the Barlow try it with just your eyepiece and no additional Barlow.

The image will be dimmer of course but should gain some definition.

You would need to fit the diaphragm directly over the mirror surface for it to be effective but it doesn't need to be centred with millimetre precision, just laid on top temporarily for testing and if successful then a permanent diaphragm cut from plastic and painted black might be an answer.

William.

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