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LukeSkywatcher

Checking collimation in a SCT?

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Collimation is not something i do often. I have done it once on my Heritage 130P since i bought it and it is still holding firm.

Now i am just wondering if anyone can tell me how to check it on a SCT?

Basically all i am looking for is either an image or a description of what an out of focus star should look like in a well collimated SCT (call it a star test).

Last night when i was observing i could focus the scope on any star and it was pinpoint sharp. When i defocused on the same star it was round with a dark shadow in the dead centre (the secondary mirror?).

Is this what i should be seeing?

I just want to know. I have no reason to think the collimation is out on the scope because it was packaged so well for transit. The views are amazing.

I'm new to SCT's so this is why i ask.

Is there something different i should be looking for in the collimation of an SCT compared to a reflector?

Thanks

Paul

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Paul

There is a really simple method on the sct, using a simply constructed mask and you need to get three lines to meet in the middle. I'll try and find the thread where this was discussed for you.

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Do you mean the Duncan Mask Thread?

http://stargazerslounge.com/discussions-scopes-whole-setups/166087-collimation-cpc-800-a.html

Here's a link to an an "offsite" article on constructing and using a Duncan mask...

http://www.astrovox.gr/forum/download.php?id=17658&sid=646453d99bbd6eef6845f3d3254b6ff9

Peter...

Edited by Psychobilly

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I used to defocus mine and then adjust it to make sure i could see evenly spaced concentric rings either side of focus.

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I used to defocus mine and then adjust it to make sure i could see evenly spaced concentric rings either side of focus.

Yep thats the check I used to use...

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They do hold it well Paul, the amount of times I have moved the 925 over the years shows how well it does hold. I have not seen a drop in performance after all this time, no doubt it could do with a slight tweak but I would rather leave it until it does definitely need collimation. I did ask about this when I first bought it..the answer given was just leave it :o:(

I now understand what the guy meant :p

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I used to defocus mine and then adjust it to make sure i could see evenly spaced concentric rings either side of focus.

I've read time and time again that this is how a well collimated scope will show an out of focus star. Honestly i dont know if it is my eyesight or my scopes are woefully under collimated but out of focus stars to me never have concentric rings either side of focus. The simply appear as big round circles with a dark spot dead centre (the secondary mirror).

I'm in no way worried or bothered if my scopes are badly collimated (i dont think they are) because the views i get are stunning (IMHO). I am a firm believer in "If it aint broke dont fix it".

In my Heritage i have it so when i look through a colli cap............everything is almost perfectly centered. It might be out ever so slightly but thats down to my eyesight and judegment of what is perfectly centered.

I was just curious if the SCT is similar in how things should look when collimated, to my Heritage.

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They do hold it well Paul, the amount of times I have moved the 925 over the years shows how well it does hold. I have not seen a drop in performance after all this time, no doubt it could do with a slight tweak but I would rather leave it until it does definitely need collimation. I did ask about this when I first bought it..the answer given was just leave it :o:(

I now understand what the guy meant :p

Its a robust beast and i am only moving it maybe 20 feet from where i store it.

I am blown away with the views, so i am not going to even bother thinking about tweaking it (if it even needs it).

But a link to how to actually collimate an SCT (for if and when) is much appreciated.

I know the screws for the secondary are behind the Faststar badge in the center of the corrector plate. I havent even seen then yet.

Edited by LukeSkywatcher

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Paul, I bought my CPC800 off Brantuk last year, he dropped it off at Worcester, then later on in the year I took it to Peterborough for it to be transported via truck to Crete. After all that the collimation was perfect, they are tough as boots. The only reason mine needed re-collimating was when I checked what type threads on the screws on the secondary mirror. (I have bought a set of Bobs Knobs for it)

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I bought my 10" LX200 in August 2004 and have never collimated it.

I have noticed recently that stars do not seem as sharp as they used to be so I got some of those bobs *** things and installed them but have never had an occasion yet to try them.

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Even I was able to do it, and if I can do it, anyone can, believe me
that's reassuring to read Demon, must admit it would be handy to go through this process with some experienced hands just to see how things work out.

Edit..this looks wrong, I am not suggesting you are in anyway unable to follow how to Collimate Demon just that if one has never tried to do it it can be daunting prospect when messing about with the optics of a telescope.

Edited by Nexus 6

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Yup, that's the one I meant, Peter.

Even I was able to do it, and if I can do it, anyone can, believe me

I made a quick Duncan mask up to see how my SCT was collimated, I'd previously set it using an artificial star and fine tuned on a clear night using a real one.

Was pleased to see the mask worked and showed a good collimation.

I printed the mask on card and cut it out so it was next to the corrector plate, to seal any light from around the secondary and to keep the mask in place I used a scunci (hair elastic band) around the secondary.

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Sorry i asked now. I simply cant be trusted with scissors and cardboard/plastic.

Nothing good ever comes of it.

LOL.

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Hi I'm Dunc and new to the SGL but a long time member of the Orpington Astronomical Society,

The Duncan Mask document was drawn up by me based on something I read on the web, I used to have great difficulty collimating my 8" LX90. I still do as the dratted thing needs collimating every time it's used. I tried out of focus stars etc. but they were too imprecise especially given my far from perfect eyes. It takes 10 minutes, scissors and a piece of card to make the mask and it will save you hours *and* you will be certain your SCT is collimated correctly.

My next astro kit project is to dismantle and reassemble the secondary holder to find out why it won't hold collimation. :icon_scratch: Should have done it years ago!

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I've read time and time again that this is how a well collimated scope will show an out of focus star. Honestly i dont know if it is my eyesight or my scopes are woefully under collimated but out of focus stars to me never have concentric rings either side of focus. The simply appear as big round circles with a dark spot dead centre (the secondary mirror).

you might be de-focussing too far or might not be using high enough mags or might not have steady enough skies. de-focussing until you just see a big doughnut with a hole in the middle will only tell you if your secondary is in the middle. tbh, I'm kind of sceptical of the "star test" - if it looks good through the cheshire and the views are good, that's what counts.:icon_scratch:

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Knic..................i am sure i am de-focusing too much,and not using a high enough mag and sure as hell dont have steady skies.

All i see when i de-focus is a do-nut with my secondary slap bang in the middle.

Honestly when i focus in on stars etc with my scopes, everything it tack sharpe in my opinion, and the views are great...................

So why even bother with collimation?

I dont see any reason to screw around with it if the views i am getting are great.

I have bought 2 scopes (Heritage and SCT) that could need collimation, but they were collimated in the factory and from what i cazn see/tell...............they are both holding perfect collimation.

I also have 2 refrac's and they simply dont call for collimation.

I have 4 scopes now and the views in each are great. I simply am NOT gonna worry about collimation in any of them any more.

Edited by LukeSkywatcher

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Hi I'm Dunc and new to the SGL but a long time member of the Orpington Astronomical Society,

The Duncan Mask document was drawn up by me based on something I read on the web, I used to have great difficulty collimating my 8" LX90. I still do as the dratted thing needs collimating every time it's used. I tried out of focus stars etc. but they were too imprecise especially given my far from perfect eyes. It takes 10 minutes, scissors and a piece of card to make the mask and it will save you hours *and* you will be certain your SCT is collimated correctly.

My next astro kit project is to dismantle and reassemble the secondary holder to find out why it won't hold collimation. :icon_scratch: Should have done it years ago!

Welcome to SGL and my thanks for the shared method for easy collimation - especially useful when cloudy.

Do you have any more info on the template, my SCT is a 6" so I resized the image from the PDF to overlay a 6" circle (seemed to work). I wondered whether it would be worth seeing if the bahtinov maker(s) could create this in a more permanent material for old fuss pots like me :D

Thanks

Steve

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Honestly when i focus in on stars etc with my scopes, everything it tack sharpe in my opinion, and the views are great...................

So why even bother with collimation?

I dont see any reason to screw around with it if the views i am getting are great.

Totally agree with the 'if it ain't broken don't fix it' approach. But the Duncan mask makes it so easy to check that it is still in collimation it is worth doing once in a while. That way you can pick up on any 'drift' that you might not notice during normal viewing.

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Totally agree with the 'if it ain't broken don't fix it' approach. But the Duncan mask makes it so easy to check that it is still in collimation it is worth doing once in a while. That way you can pick up on any 'drift' that you might not notice during normal viewing.

I can't disagree, but for me to construct a duncan mask would take six months planning and getting in the frame of mind to do it and then another 3 weeks construction.

I am seriously that USELESS with scissors,card,plastic,glue etc.

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If you are happy with the views, then why bother? You say that the stars are sharp, so it sounds like you are getting a bit anxious over nothing, if you know what I mean?

Is 100% collimation needed if you are visual only? I guess if you are imaging, then you want everything as perfect as possible (and some planetary imagers re-collimate on near the object to be imaged to ensure that the mirror sagging slightly is not affecting things)

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If you have a webcam, download a copy of MetaGuide (free) which has a very nice collimation function.

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If you are happy with the views, then why bother? You say that the stars are sharp, so it sounds like you are getting a bit anxious over nothing, if you know what I mean?

Is 100% collimation needed if you are visual only? I guess if you are imaging, then you want everything as perfect as possible (and some planetary imagers re-collimate on near the object to be imaged to ensure that the mirror sagging slightly is not affecting things)

I totally agree.

If your collimation is off - even by a relatively small amount - you are degrading the performance of the scope by some way... but if you are happy then leave as is.

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Hi paul I have a nexstar 5 which I carry on my back for about half a mile to my observing position. it appears to hold collimation pretty well so I doubt that yours needs collimatimg at all. like demonperformer I do occasionally use duncan's mask and it is a snap to use especially with bobs knobs on but to be frank It rarely needs adjusting yours is probably alright.

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