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Greymouser

Crayford focusser for a SCT?

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I have been told it is worth buying, but am unsure, one of these: https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sct-telescopes.html

I have no idea if the gain will be sufficient to justify the cost, ( especially considering visual only at he moment, ) particularly if I get a better, more expensive version. For now I use my Evolution for visual only, but am considering imaging. Is there any point bothering, or should I save the money and either save it for a future Hyperstar purchase, or other imaging set up?

Thanks.

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One of the main advantages of adding a focuser is that it adds fine focusing. Also as the fine focus knob can be moved with a finger tip it doesn’t cause the scope to shake.

I use the Baader Steeltrack but it is a lot more expensive however the one linked tois fine for visual use.

Edited by johninderby
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Don't forget the extra light path you will be adding to your telescope...shrinking your aperture and lengthening your focal length...no idea why someone would want to put a crayford on an sct...people really should read more.

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I use aftermarket Crayford focusers on all my SCT's, the advantages of the better focusing easily outweigh any minor compromises.    😀

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If you're unsure, I'd say don't buy it.

I do have a secondary focuser on my C9.25 (though it's a rack and pinion model) and I added it specifically because I wanted wobble-free focusing when I am imaging planets.  If I weren't imaging then I'm not sure I'd bother at all (though I might motorise the standard focuser).  I think that's something you tend to work out with use.  If you're comfortable with how your OTA works at the moment then I'd suggest sticking with what you have.  If you try imaging and aren't happy with the performance of the stock focuser, then look at what you might do to improve things.  If you find shifting of the primary to be a problem when you're just visual, perhaps it's worth consideration.

James

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59 minutes ago, Permafrost said:

Don't forget the extra light path you will be adding to your telescope...shrinking your aperture and lengthening your focal length...no idea why someone would want to put a crayford on an sct...people really should read more.

Unless someone can enlighten me I think that every single utterance in this post is incorrect. 

Olly

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Hmmm

Well, adding a rear focuser (or any instrument which requires a significant back focus i.e. spectroscope) actually does increase the focal length/ ratio...in my C11 adding the spectroscope makes it an f11 system.

(This is due to the changing SCT amplification factor, primary/ secondary, when the primary mirror is moved to achieve the new focal point.)

 

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6 hours ago, Merlin66 said:

Hmmm

Well, adding a rear focuser (or any instrument which requires a significant back focus i.e. spectroscope) actually does increase the focal length/ ratio...in my C11 adding the spectroscope makes it an f11 system.

(This is due to the changing SCT amplification factor, primary/ secondary, when the primary mirror is moved to achieve the new focal point.)

 

Yes, true, but the addition of a compact focuser has a minimal effect of the FL (mine adds 60mm or 1.7% backfocus). And how can adding a focuser at the rear change the aperture at the front?  You might introduce vignetting by putting things into the light path but adding a focuser of the right size won't do this. And why add a draw tube focuser to an SCT? Primarily in order to achieve fine focus more easily and, above all, to prevent image shift by leaving the mirror locked throughout the operation. 

8 hours ago, paulastro said:

Don't hold back Olly, you just say how you see it  🤣.

Yes, sorry, a bit blunt. But it was the phrase, 'People really should read more,' which led to this bluntness. Apologies.

Olly

Edited by ollypenrice
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It’s the real world experiences of people using  an add on focuser vs the theoretical disadvantages that are what count here. I’ve found the advantages greatly outweigh any disadvantages. 

Anyway that’s been my experience.  🙂

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11 minutes ago, johninderby said:

It’s the real world experiences of people using  an add on focuser vs the theoretical disadvantages that are what count here. I’ve found the advantages greatly outweigh any disadvantages. 

Anyway that’s been my experience.  🙂

Mine too. Even for visual observing I find the moving mirror a pest. Anyone doing planetary imaging will find it worse than a pest!

Olly

Edit: There's also a slight risk in using the moving mirror if you also use the lock. It is easy in the dead of night to forget to unlock the mirror before trying to move it.

Edited by ollypenrice
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I have one of these exact ones and they are fine for moderate use.  You probably wouldn't want to hang a large CCD and filter wheel on the back, but for planetary video imaging and visual use, ideal.  I actually use a motorised Moonlite on my C8 now, which I only use for planetary/lunar imaging, and it works absolutely great.

As noted above the benefits, for me at least, by far outweigh any down sides.

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You should read my comments on this topic as related to my Maksutov. The focal length will be increased but its marginal, especially as regards viewing.  The light path reduction by vignetting can easily be tested.  Take a look here

http://www.astro-baby.com/reviews/Skymax 180/Skymax 180 Review.htm

the light path test is described about halfway down the page.

image shift can be a massive pain at high powers and like others I find the strictly theoretical issues about FL and light cone clipping are as nothing compared to the stability of focusing.

A wobbly or irritating scope to use is annoying no matter how short its light path might be ............😍 peace out

 

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12 minutes ago, Astro_Baby said:

You should read my comments on this topic as related to my Maksutov. The focal length will be increased but its marginal, especially as regards viewing.  The light path reduction by vignetting can easily be tested.  Take a look here

http://www.astro-baby.com/reviews/Skymax 180/Skymax 180 Review.htm

the light path test is described about halfway down the page.

image shift can be a massive pain at high powers and like others I find the strictly theoretical issues about FL and light cone clipping are as nothing compared to the stability of focusing.

A wobbly or irritating scope to use is annoying no matter how short its light path might be ............😍 peace out

 

Excellent read and welcome back ! Not seen your posts for a long time 😉

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55 minutes ago, knobby said:

Excellent read and welcome back ! Not seen your posts for a long time 😉

Quite so :)

James

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Thank you everyone, this has been a very informative thread for me, I didn't even know you could lock the mirror in place! Plenty to read and consider, not least which direction I want to go in. I will read some more via the above suggestions and not pull the trigger on buying one yet, but I think I will be doing so at some point.

Still no one has answered my question about the point, if I go the Hyperstar route, which either means not many use that, or the answer is obvious: Don't bother! The camera is then at the other end of the scope I guess! 🙂

I will have other questions soon enough, but need to think on this for a bit. 🤔

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I use a Crayford focuser on an sct and on a maksutov. It makes a big improvement in focusing accuracy and avoids mirror shift.

A trade off is adding weight which isn't a problem if your mount is solid enough.

It does mean you have to change the mirror position to push the focal point back and this lengthens the effective focal length of the system but I think this doesn't matter unless you are measuring things.

I was measuring moon features a while back whilst I had a filter wheel on my Maksutov and that resulted in the same effect due to the increased distance to the eyepiece.  I measured the effective focal length of my scope to be about 2100mm with the filter wheel in place when the scope is normally 1900mm. 

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6 hours ago, Greymouser said:

 

Still no one has answered my question about the point, if I go the Hyperstar route, which either means not many use that, or the answer is obvious: Don't bother! The camera is then at the other end of the scope I guess! 🙂

 

In researching the Hyperstar option I would advise you to ignore many of the claims on the Starizona website and particularly any which suggest the instrument is easy. Nobody who's grappled with deep sky imaging for any length of time is going to say that ulltra-fast systems are easy. They are quite the opposite. Collimation and mechanical orthogonality have to be near perfect. The depth of field of critical focus is incredibly shallow and the moving mirror focuser is not very precise. A bad combination.

Have a good look at published Hyperstar images, noting whether they are narrowband or broadband, and paying more attention to whichever you are intending to try yourself. Look at star size in particular.

If I believed I could work at 500mm focal length and F2 and get the same results as I get with my F5 refractor in a fraction of the time then I'd be using a Hyperstar. So would thousands of other people. But I'm still using my refractor, and so are they...

Olly

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