Any examples of movable primary mirrors?

[furrysocks2]

I'm building a dedicated video/imaging scope with a camera at prime focus. I will likely use a non rotating helical micro focuser for the camera but travel is typically <10mm. It was suggested that I might make the primary moveable for focusing, assuming it keeps colimation. I am anticipating needing >10mm travel when changing between focal reducers, barlows, adding a filter drawer, etc. I should have adjustable truss rods all being well so a degree of adjustability there, but looking at options.

Are there any examples around? I've had a google but nothing turned up yet.

[Peter Drew]

You could consider effectively reversing the system used on SCT's. With the sliding components behind the mirror backplate the mirror would not need to be perforated.

[furrysocks2]

6 minutes ago, Peter Drew said:

You could consider effectively reversing the system used on SCT's. With the sliding components behind the mirror backplate the mirror would not need to be perforated.

Thanks - I was not aware they did that!

I imagine at least three points of contact would be needed in my case, as widely spaced as practical?

 

My current thought was to use three 16mm threaded posts anchored to the backplate, with a gear housing below containing coupling nuts (47mm). The nuts could be driven synchronously by chain, belt or wooden gears. In this configuration a handwheel on the rear of the scope (or a stepper) could be used to move the primary mirror and adjust focus.

The same could perhaps be achieved using 3 linear bearings running again on posts anchored to the backplate, necessitating only one threaded component. This is perhaps more like the SCT configuration. A benefit of this approach may be increased spacing of the rods, as pulley wheels or cogs would not be required on each post.

[furrysocks2]

http://ben.davies.net/Cell.htm

[ollypenrice]

Worth remembering that the primary of an SCT is spherical and so not terribly sensitive to alignment. You'd need a more accurate system, I presume, to move a parabolic mirror.

Olly

[furrysocks2]

6 minutes ago, ollypenrice said:

Worth remembering that the primary of an SCT is spherical and so not terribly sensitive to alignment. You'd need a more accurate system, I presume, to move a parabolic mirror.

Olly

Thanks.

I've no idea what profile my mirrors have - does focal length affect the ability to tolerate error in alignment, or just the profile?

Matt

[ollypenrice]

42 minutes ago, furrysocks2 said:

Thanks.

I've no idea what profile my mirrors have - does focal length affect the ability to tolerate error in alignment, or just the profile?

Matt

If the scope is fast it is sure to be parabolic and, therefore, more sensitive.

Olly

[andrew s]

I would strongly advise against moving the primary mirror as it is a heavy component and sensitive to misalignment. If I were doing your project I would look at a single spider design attached to a linear slide (with or without your helical focuser). You would need to provide a means of squaring the slide on to allow it to be collimated as you shifted to different configurations.

Some professional old film based Schmidt camera's did move the mirror for fine focus using calibrated micrometer screws but not over a significant range. 

 

Regards Andrew

[hughgilhespie]

Hi Matt,

Your brain is working overtime!!

Purely my own opinion, but if it were me, I would go for a 'conventional' fixed mirror cell and have the focus adjustment at the camera end. Moving a heavy old mirror up and down seems likely to be hard to do with any accuracy / stability. With the camera, probably less than half a kilo in total, you can design something easier. If you aren't too worried about camera orientation, then I would think about putting the whole camera assembly (camera, filter housing, whatever) on the end of a threaded rod, something fairly meaty, say M16 fine. Then fit a threaded bush to the top of your spider and you can easily have +/- 50 mm travel that should be plenty to accommodate all scenarios.

You could also have a 'fine' three screw adjustment fitted between the camera and threaded rod to allow you to tweak the camera to be orthogonal to the optical axis without changing the focus.

Just my 2 pence worth.

Regards, Hugh

 

Beaten to it by Andrew!!

[furrysocks2]

Thanks, both.

I appreciate the comments and tend to agree that it may not be worth the effort for a potentially poor result. I'm certainly not ignoring your advice, just thinking it through...

 

My understanding is that In essence, longer primary collimation bolts could achieve the same thing albeit without synchronous motion of the three at once and also subject to spring length and rate. The greater the range of primary focus travel, the more difficult it would be to keep the weight of primary and cell (~5kg in my case) from racking or otherwise deforming the support, particularly at lower altitudes.

In an effort to overthink things, I've tried to calculate how sensitive such an arrangement might be with three M16 rods on a 200mm PCD - I think it would need the three rods to within 94% of each other (equivalent to 1/16th of a turn) to keep the focal point on a 1/3" sensor. I'm measuring about 0.15mm play in a coupling nut which is already outwith the accuracy required though this could be reduced with PTFE tape and further reduced by gravity. However, none of that accounts of deformation under load.

For me, it's mostly about whether such an arrangement would hold collimation through the vertical range of motion - if I were to have a movable primary for configuration changes, I'd likely be checking collimation anyway as a matter of course, though obviously a pain if there was significant movement every time. I would like to try to keep camera rotation independent of all other adjustments. I have a few rough ideas how to adjust collimation and alignment at the top end simply enough, but for gross focus changes nothing I've envisaged yet seems as elegant as a hand crank on the bottom of the scope. Another possibility if I ditch the dustbin, might be outboard supports (as illustrated in this youtube video) which appears to me to maximise separation and keep the weight between the supports, both of these things seem beneficial.

 

I still expect my scope to begin with a fixed primary and a spider with collimation adjustment and microfocuser, with gross focusing achieved by adjusting the truss rods and recollimating.

[michael8554]

Meade made the RCX series of SCT's for a while.

These had a fixed primary mirror and moved the corrector plate (with attached secondary) by three motors with encoders, for focusing and for collimation. 

But I would keep it simple:

1)  Fixed primary, one helical focuser, setup for your longest image config, with the helical focuser mid-range.

2) For shorter imaging configs add spacers to give rough focus with 5mm of adjustment either way with the helical focuser.

Michael

 

[cjdawson]

Just jumping in late on this one.  My Meade LX-90 has a primary mirror that move for focussing.  This works ok, however there is a problem known as "Mirror flop" that all SCT's suffer from.  The RCX method was ment to solve that issue.

The mirror flop issue means that the scope primary mirror can shift about 4mm when the scope rotates enough.  This will knock out the focus.  Also the design leads to having some backlash in the mechanism.  The focusser is a threaded rod with a pin on the end.  The pin hooks into a hole on the back of the primary.  In order to be able to locate pin in the hole, they oversized the hole by a couple of mm, this is the main source of the backlash.  The rest of it comes from the primary being able to rotate slightly on it's mountings.  The amount of play in the system can end up being quite horrible when trying to fine focus (the work around is to pull back from the focal point, allow the backlash and mirror flop to sort itself out, then start to approach focus, again you need to work the backlash and mirror flop out of the system before attempting to reach focus.   If you don't, you'll have a hard time getting the scope to stay on that sweet focus spot.