Help understanding Celestron Cassergrains

[bluesilver]

Hi, I am hoping i can find out a bit of information in regards to the Celestron line of scopes,  mainly the C14 XLT ( the non edge version )

I have heard that a lot of people are saying to install a grayford focuser on these scopes.

So i kind of have a few questions that i am struggling to understand and was hoping someone might be able to help me out a bit here.

1.  I see that they have their own knob type focuser on them that i am guessing adjusts the primary mirror to get focus?

But by doing this, wouldn't it always be putting the scope out of collimation?

2.   By adding a Crayford focuser, would this only be for very minor adjustments?

3.  There is talk of mirror flop as the scope moves around,  I take that is why the scope apparently get shipped with the mirror locked down with bolts?

So are they designed to say collimate or focus on a star, then re install the bolts to stop the mirror moving?

It just all seams a tad odd.

I have only had refractors and Dobsonians,  so just trying to find out what  am in for if i get a C14 scope,  what if anything needs to be improved and why people install grayford focusers on them if you could just put an electronic focuser on the original focusing nob?

I hpoe all that kind of makes sense,

Any advise or information would be appreciated.

Cheers.

 

 

 

 

 

[PeterCPC]

Firstly a crayford focuser will enable much finer adjustments to be made.

Some do get mirror flop - maybe I'm lucky but my C9.25 has never had this issue.

You do indeed move the primary but it does not throw out the collimation.

[Mr Spock]

2 hours ago, PeterCPC said:

Some do get mirror flop - maybe I'm lucky but my C9.25 has never had this issue.

My C9.25 was exactly the same - zero flop.

[Oldfort]

The mirror is locked down to protect it during shipping.  For ordinary day to day movement it is not needed, and I think only the C14 has the mirror locking screws.h However I think all the Meade SCTs have locking screws, but again only for shipping.

[michael8554]

1.  I see that they have their own knob type focuser on them that i am guessing adjusts the primary mirror to get focus?

Yes

But by doing this, wouldn't it always be putting the scope out of collimation?

Maybe, if your OTA has a lot of mirror flop.

2.   By adding a Crayford focuser, would this only be for very minor adjustments?

Yes

3.  There is talk of mirror flop as the scope moves around,  I take that is why the scope apparently get shipped with the mirror locked down with bolts?

So are they designed to say collimate or focus on a star, then re install the bolts to stop the mirror moving?

The mirror is locked during shipping to prevent the heavy primary mirror coming free of the somewhat lightweight focusing arrangement during the usual rough handling.

If you were imaging with just one setup, ideally you'd collimate and lock the mirror, and from then on only use the Crayford, to make fine focus.

Visual use involves changes of eyepieces which may not all focus with the limited Crayford range.

Michael

[Same old newbie alert]

They install Crayford so the mirror stays in a fixed position and so the drawtube of the Crayford can move to achieve focus rather than the mirror going up or down the draw tube

1.. why would that put it it of collimation as collimation is aligning the mirrors, nothing to do with focus

2.. crayfords have 10-1 fine focus

3..simple , mirrors weigh a fair bit and combined with gravity you can get mirror shift and flop as the mirror shifts around as the mount slews to different positions.. 

If you have platesloving software you can see it as you solve a image.. only slightly but it's there and will disturb your focus.. that's why they say to focus on a star high up so gravity is at its maximum, focus in a positive direction if using the standard focus knob

Hope this helps

[bluesilver]

Thanks heaps for those replies, very much appreciated and has explain a lot of things.

Just one other thing if someone doesn't mid me asking,

Instead of installing a crayford focuser,  wouldn't you be better of installing a ZWO electronic focuser?

These have very small adjustments as well as the course adjustments.

Seams like it would be a easier solution,

Just a thought.

 

 

[jacko61]

11 hours ago, bluesilver said:

Instead of installing a crayford focuser,  wouldn't you be better of installing a ZWO electronic focuser?

 

Celestron make their own focuser which I have fitted to my C8. It works very well once calibrated and has very fine adjustment.  I did have a 10-1 rack and pinion fitted (similar to a crayford but with less slip IMO) but couldn't get on with it. The Celestron motor focus has solved a lot of issues. 

Graeme

[Louis D]

Or ditch Crayfords altogether and get a Bellerophon Clement focuser for the C14.  It is well reviewed here on a C11.

[Same old newbie alert]

21 hours ago, bluesilver said:

Thanks heaps for those replies, very much appreciated and has explain a lot of things.

Just one other thing if someone doesn't mid me asking,

Instead of installing a crayford focuser,  wouldn't you be better of installing a ZWO electronic focuser?

These have very small adjustments as well as the course adjustments.

Seams like it would be a easier solution,

Just a thought.

 

 

It would still have the same image shift/flop issues as that would still move the mirror up or down.. it isn't a focus issue it's a mirror movement caused by gravity on the mirror

[Merlin66]

Even with a rear crayford etc, the mirror is not fully restrained and can still flop.

 

[Same old newbie alert]

12 hours ago, Merlin66 said:

Even with a rear crayford etc, the mirror is not fully restrained and can still flop.

 

While this is true using the standard focuser method will have more of a shift because of the mesh of the threads... That's why you should focus in one direction, the + on the focuser shows which way to go

[Cosmic Geoff]

Don't assume that accessories that some people have fitted to their telescopes will be needed on yours.  I have had a CPC800 for several years, used it for visual, planetary imaging and some deep-sky imaging, and so far have not been troubled by mirror flop or felt the need to invest in a Crayford or an electronic focuser.

[inFINNity Deck]

Just to make things clearer: mirror-shift and mirror-flop are two different things. The former is the effect of the native focuser pushing against the primary mirror and by that causing it to tilt. Mirror-shift is defined by Celestron as being less that 30" and hardly affects collimation. Mirror-flop is caused by poor mounting of the primary mirror in combination with gravity. Mirror-flop can have a significant effect on collimation. I had a C11 XLT carbon and still have a C11 XLT EdgeHD, both of which show severe mirror-flop. The EdgeHD has been returned to Celestron in order to get the mirror-flop fixed, but it came back as poor as it was before, so apparently this is how a C11 should behave. Celestron does not specify how much mirror-flop a SCT may have, so I have no way to check if transporting my C11 back may have caused the issue.

More info about mirror-flop, and Celestron's response to questions about it, in this thread on a Dutch forum (opening it in Chrome should provide a reasonable translation). I only use the C11 for solar and planetary imaging. The way I deal with mirror-flop now in planetary imaging, is to collimate the C11 prior to imaging on a nearby star and to repeat that collimation after the meridian flip.

Nicolàs

 

[Merlin66]

The slop can be reduced by running the mirror up and down the baffle - about thirty turns of the focus knob.

This redistributes the grease and reduces the gap between the mirror sled and the baffle.

Doing this every month or so definitely improves the situation.

works for both Celestron and Meade SCTs.

 

[Merlin66]

Nicolas,

I thought the HD version had the mirror lock screws to prevent movement?

 

[inFINNity Deck]

  

18 minutes ago, Merlin66 said:

Nicolas,

I thought the HD version had the mirror lock screws to prevent movement?

 

That is why I changed to an EdgeHD in the first place. Sadly enough they fail to overcome the issue, at least in my C11...

Nicolàs

 

Edited March 10, 2022 by inFINNity Deck