Mirror lock on the Celestron edge

[pyrasanth]

The Celestron edge telescopes are fitted with 2 mirror lock screws which allegedly prevent primary mirror wobble when the telescope slews. I think this is a good idea but the locks need to be electronic & not mechanical as I will explain unless anybody knows anything that I don't about this system.

Automated focusing software includes tools to refocus because of temperature differences in the expansion or contraction of the primary mirrors. I know on my images when taking long subs that there is a focus shift as well as the mirror flop issue when slewing to a new target. This can be substantial and has the potential to ruin an imaging session if not caught. How can temperature detecting auto focus routines work if the mirror is locked or is it locked loosely enough for the mirror to still move without damaging something. I simply don't know but if the lock is rigid you could ruin your auto focuser as it tries to move a mirror that is locked.

This is why I've ordered an on axis guider which monitors the focus in real time & so can dynamically correct minute changes in focus as opposed to a big temperature change where the damage could already be done to your images before the focus is corrected. If the lock was electronic it could be turned off before attempting refocus then re-engage after the re focus had been completed.

I don't see why you would lock a mirror if you cant refocus because of temperature changes and if you did you'd have to stay up all night & monitor as you'd need to do this manually which sucks!

[knobby]

Cost

[pyrasanth]

As an after thought you would need to lock the mirror then use a crayford or similar focuser to adjust for dynamic changes. This is expensive & sort of confirms that the celestron solution sucks.

[pyrasanth]

Cost

For who- the customer or Celestron?

[knobby]

Both ... It would add too much to the cost of the scope , making it less competitive.

Only my guess though, I'm no marketing expert

[pyrasanth]

If you look at the logic of this mechanism- it is nothing more than marketing. I did some more research and found that this feature will damage the focus screw & lock clutches if focus is attempted whilst the mirror is locked so it's a useless feature for automated imaging runs. I will need to lock the mirror at the current focus point using my existing electronic solution then have an additional crayford style focuser to tackle the issue of refocusing whilst imaging- this is a pain in the bum caused by the rubbish focus mechanism deployed by celestron. Moving the whole primary mirror to focus is well......just wrong!

[IanL]

It is inherent in the design and the advantage is a wide range of back focus. Might as well complain about buying four tyres for your car instead of buying a motorbike!

Typically you use the mirror movement to achieve basic focus depending on what imaging or visual train you have on the scope. You then lock the mirror manually and use a secondary focuser which might be manual or electronic for fine focus. This will usually be a Crayford type. Assuming you don't change the setup much other than filters, the secondary focuser will have more than enough motion for any filter or temperature change and also eliminates any mirror flop issues to boot since the main mirror is locked. Also enables you to collimate withe the mirror locked prior to imaging as flop will affect collimation.

In other words you're trying to solve the problem the wrong way.

The main problem with secondary focusers is that they may be too long to pass through the fork mount with a camera attached. Depends on model and not a problem on GEM mounts of course.

[pyrasanth]

It is inherent in the design and the advantage is a wide range of back focus. Might as well complain about buying four tyres for your car instead of buying a motorbike!

Typically you use the mirror movement to achieve basic focus depending on what imaging or visual train you have on the scope. You then lock the mirror manually and use a secondary focuser which might be manual or electronic for fine focus. This will usually be a Crayford type. Assuming you don't change the setup much other than filters, the secondary focuser will have more than enough motion for any filter or temperature change and also eliminates any mirror flop issues to boot since the main mirror is locked. Also enables you to collimate withe the mirror locked prior to imaging as flop will affect collimation.

In other words you're trying to solve the problem the wrong way.

The main problem with secondary focusers is that they may be too long to pass through the fork mount with a camera attached. Depends on model and not a problem on GEM mounts of course.

I accept all that you say. I spent my money on the primary focuser electronics so it looks like that may well take a secondary role as I need to buy a focuser stepper motor for the Crayford focuser which I have which is a moonlite so its a really good focuser. 

[ollypenrice]

You raise a very good point and it becomes more than merely 'very good' when we move to considering the Hyperstar and Rowe Ackermann, one of which I have sitting here eagerly awaiting a suitable camera for an Astronomy Now review. These ultra fast systems (fast is good) have ultra narrow focal depth (this is not good if you can't deal with it) and, you are right, the mirror locks prevent automated refocus. The RA F2.2 cannot be OAG guided but the focal length might well not require that.

The real crux of the matter is, 'Do we really need the mirror locks or will the mirror remain pretty stable without them?' Or even, 'Would a good compromise be a hint of mirror lock and autofuxs applied?' If anyone knows I'd love to hear from them.

Olly

[dave_galera]

The problem with the Edge HD design is that the distance from the backplate to camera is a fixed distance (from what I understand) and this negates the use of a secondary focuser, so focusing the mirror is the only option you have. But yes it is a problem having to slacken of the mirror locks and I also found that when the mirror locks are tightened the focus shifts a little.

To answer Olly's question I have found that just slightly tightening the locks will hold the mirror in place and still allow focusing with an electronic focus motor (Lakeside in my case) without breaking anything.

Dave

[Chris]

The problem with the Edge HD design is that the distance from the backplate to camera is a fixed distance (from what I understand) and this negates the use of a secondary focuser, 

Dave

I hadn't thought of that, no Crayford for me then

Are you sure its not just a fixed distance when used with the reducer, so as to keep the field flat? If you were adding a Crayford I'm not sure a reducer would fit anyway. It might work with a crayford when using the scope natively perhaps? 

[dave_galera]

I hadn't thought of that, no Crayford for me then

Are you sure its not just a fixed distance when used with the reducer, so as to keep the field flat? If you were adding a Crayford I'm not sure a reducer would fit anyway. It might work with a crayford when using the scope natively perhaps? 

Chris, It has a a couple of lenses stuck inside focus support tube which dictates the back focus distance with/without the reducer. This is fully explained in the document:

http://www.celestron.com/media/520893/EdgeHD_WhitePaper_FINAL.pdf

Hope this helps.

Dave

[dave_galera]

For the 800:

   Without reducer it is 135.35mm +- 0.5mm

   With reducer it is 146.05mm +- 0.5mm

So the necessary tolerance is quite tight, I had to get special spacers mad by PreciseParts in the USA

Dave

[Chris]

Chris, It has a a couple of lenses stuck inside focus support tube which dictates the back focus distance with/without the reducer. This is fully explained in the document:

http://www.celestron.com/media/520893/EdgeHD_WhitePaper_FINAL.pdf

Hope this helps.

Dave

Doh! I knew about the corrector inside the tube and it completely slipped my mind when I was thinking this through

Thanks for the reminder Yes I totally agree, there is no hope of a Crayford on this scope then  

[knobby]

Blazar uses a moonlite with no problems , admittedly he's mainly a Lunar nut .

It will work , but you won't be getting Flat fields to the 'edge'

........ Also , don't try sticking a Lymax cat cooler in it

[Chris]

Blazar uses a moonlite with no problems , admittedly he's mainly a Lunar nut .

It will work , but you won't be getting Flat fields to the 'edge'

........ Also , don't try sticking a Lymax cat cooler in it

lol Lymax cooler, that wouldn't be good, Edge HD to standard SCT in 2 seconds flat! 

I personally wouldn't wan't to lose my nice flat field, it is very nice!

[Gina]

I'm going off this scope

[Stu]

I've never understood why the mirror flop can't be designed out on these scopes.

My old OMC200 had a brilliant system with MicroFocuser which had zero image shift even at very high powers. When I took it apart it was actually a very simple arrangement but highly effective

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