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Better focuser for an SCT?


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Hello fellow astro-geeks! :D

I have a Nexstar 8se, and jolly nice it is too.

However, it wobbles when I focus it.

I don't want one of those electronic focuser gizmos. (Yet more wires? - No thanks!).

Is there an alternative focuser that would improve my observing experience?

An astro-mate of mine has a Moonlite dual-speed focuser and says it is super for fine focusing.

Would that be any good for my 8se?

Or would I still wobble? :)

Or are those two totally distinct issues?! ;)

Thanks in advance.

(No apologies for my ignorance, though!;))

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Does the wobble come from actually touching the focuser Lulu, or is it the mirror shifting when you change focus direction?

If the former, then an electric one would be the answer, if the latter, then a Moonlite would be good, the mirror would need to be positioned in an optimum position, so that the Moonlite can focus the full range of eyepieces you possess.

I'm not sure if your scope has a mirror lock, which prevents mirror flop when orientated in certain attitudes according to the angle the scope is positioned. If it has such a lock, then it should be activated when you find the optimum position for the Moonlite.

Ron.

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Lulu,

I use 8", 10" and a 12" SCT; mine are Meade so I can use a mirror lock to hold the primary mirror is place ( I don't think that's an option with the Celestron?)

I use the electronic versions... yes I know more wires... but it works for me; so I have a "rough" focus with a drive on the focuser knob and a Crayford "zero shift" on the rear of the cell.

The Crayford doesn't allow me to focus everything, very limited travel so I still have to use the knob...

The depth of focus ie the amount you can move the eyepiece without really seeing any real difference, on a f10 system is quite large, about 0.22mm so you don't really need a very sophisticated focuser. On my spectroscope I use a helical Baader focuser which has about 5mm of travel ( connects to a T thread -#BA2458125 costs about 35 euro) which does the job very, very well.

Think about it... there are a few cost effective options....

Ken

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Well, Ron, I rather think the former is true...(sigh)...I was hoping to keep to one wire only (but maybe I'm expecting too much!).

As for a mirror lock...I'd have to fish out the instructions again...

I may be gone for some time! :):D

Thanks for the advice, guys.

Edited by lulu57
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And what's the current retail $$$$$$ for a Moonlite focuser???

Lulu....Lulu..... are you there....the Celestron doesn't have mirror lock.....Lulu....come home all is forgiven.....Lulu.....

Edited by Merlin66
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And what's the current retail $$$$$$ for a Moonlite focuser???

.

Lots...

but we all know.... that especially in this game.. you get what you pay for!

I would however only recommend the moonlite if planning to put an imaging/camera load on the back...

Steve

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Moonlite are the absolute 'dogs danglers'...

rock solid, can take a load, smooth, and they look good..

Oh! and for you ladies.. they come in a choice of colours...

Only matched by Feathertouch R&P.

Steve

A CHOICE OF COLOURS???!!!!

Well! Why didn't you say so before?!

(I hope there's a red one!:))

Edit: Just seen that they do an orange one! Perfect match for my orange tube!

(But I'd have to do without new boots, make up, gas or food for a week or two....)

Edited by lulu57
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On an 8SE the clearance between the diagonal and the mount is very limited when pointing anywhere near the zenith. There isn't really enough room to use an SCT type crayford focuser.

When I had my 8SE I used a FeatherTouch microfocuser that replaces the standard focuser knob. Works brilliantly and no clearance problems. It only takes a light touch with a fingertip to move the fine focusing knob which virtually eliminates shaking while focusing.

John

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When I had my 8SE I used a FeatherTouch microfocuser that replaces the standard focuser knob. Works brilliantly and no clearance problems. It only takes a light touch with a fingertip to move the fine focusing knob which virtually eliminates shaking while focusing.

John

That looks very practical.

(Puts sensible hat back on).

Thank you, John.

I am now suffering from what the French call an "embarras du choix". Think I'll sleep on it.

Goodnight, gentlemen.

And thanks for the opinions.

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Not sure how the SE attaches to the mount (isn't it by dovetail....is there room to move it back and forth) but when i added a Crayford to the LX50 i opened a whole new can of worms. As already mentioned, the Crayford snags the fork base which prevents views towards the zenith. And then there is the balance issues, which then leads to even more expense with 3D balance rails etc. A heavy Crayford and 2" diagonal is a lot of weight to try and counter. I solved it by ripping the scope off the forks and making it an OTA for the EQ5.

But that aside, the focusing is no longer an issue, Crayford works a dream. :)

John's suggestion of the Feathertouch knob is an excellent one. Keeps everything simple and light.

Russ

Edited by russ
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I concur about the FeatherTouch microfocuser. I have one on my C8 and although it doesn't completely eliminate mirror slop like an external Crayford would, it really is feather touch. Paticularly, the 10x adjustment, it just needs the brush of a finger to move it slightly.

Also, make sure your tripod hinges and legs etc are all tight and rigid to avoid any movement in that.

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Yeah, I'd gop with the feathertouch knob too. Or probably an electronic focuser of some sort (JMI do one for about £150). In fact, I reckon the electronic focuser would be the best as the SE mount isn't the sturdiest with even the 6 SCT on it.

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I guess one of the things that is not in the scope spec is the back focal distance, and adjustment range, from the SCT thread. Something I keep meaning to measure! I have quite a lot of stuff hanging off the back of the C8 when in photgraphic mode, I suppose the eventual focal plane is about 160mm from the SCT thread. If I added a Moonlite it could be pushed out even further, beyond the focus range?

This may not be as much a problem if you are using dedicated astro CCD cameras, since the CCD plane is much closer to the mount thread in the camera tube. You still have to account for the filter wheel though. Also doesn't seem to be a problem visually through the diagonal.

It would be useful though if the scope manf published the back focal plane limits (or maybe they do and I haven't found it!). I'd hate to spend $$$$$ on a Moonlite and then find I can't focus on my DSLR!

(Also, while I'm at it, some guidelines from the manf on max field of view of the scope would be useful. Is it really +/- atan(h/f)? h - EP stop radius or CCD 'radius' {how far can you go?}, f - scope focal length)

post-16060-133877421633_thumb.jpg

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SCT's have huuuuge focus ranges. I use my mirror focus to get the image within fine focus adjustment and then the WO 10:1 go get final focus. This normally means I touch the mirror focus once in a session and then the crayford does the rest for the remainder of the session (changing eyepieces, cameras etc...)

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Not sure how the SE attaches to the mount (isn't it by dovetail....is there room to move it back and forth) but when i added a Crayford to the LX50 i opened a whole new can of worms. As already mentioned, the Crayford snags the fork base which prevents views towards the zenith. And then there is the balance issues, which then leads to even more expense with 3D balance rails etc. A heavy Crayford and 2" diagonal is a lot of weight to try and counter. I solved it by ripping the scope off the forks and making it an OTA for the EQ5.

But that aside, the focusing is no longer an issue, Crayford works a dream. :)

Russ

What I found with the 8SE was that even with the OTA moved as far forward as possible, clearance between the diagonal and the top of the mount was very limited. To gain a bit more clearance I used a WO 1.25" diagonal. This photo gives an idea of the minimal clearance. The FeatherTouch focuser knob just barely clears the top of the mount.

John

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FeatherTouch microfocuser that replaces the standard focuser knob. Works brilliantly and no clearance problems. It only takes a light touch with a fingertip to move the fine focusing knob which virtually eliminates shaking while focusing.

Yes. Seems to reduce mirror flop too.

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The Moonlite Crayford is fabulous but is also total overkill for visual use. One clever thing it does is allow the focal reducer, if used, to move with the drawtube to keep the backfocus correct for imaging. On my 10 inch Meade I just have a Revelation 2 speed Crayford bought second hand on this forum. It is superb. Even new they are not that expensive. Telescope House, I thnk. I was using it to look at Mars last night and the focus is very subtle.

Olly

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The Moonlite Crayford is fabulous but is also total overkill for visual use.

Agreed, and an electric focuser can be a big help when imaging planets at long focal lengths, but you can easily do without for visual work.

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