Advice On Dob Mirror Cells Please...

[Stargazer McCabe]

I'm seeking advice on a point brought up by Didier Vincent at Sud Dobsons in a recent email exchange. The DIY section seemed the best place for it, but if it isn't I have no objections to it being "re-homed"...

In a nutshell I have been considering having a Truss Tube Dob built for us. I've been evaluating various options and companies / individuals that could assist us with optics and construction.

We'd rather work with someone who can help us build a "custom" scope, rather than having the optics and accessories dictated to us and one individual who has been quite thought provoking is Didier at www.sud-dobson.fr

In a recent exchange, I happened to mention to him that we currently own a 14" Flextube. In his reply he posed the question "why don't you convert that into a Truss if you are happy with the mirrors and size..?"

Undoubtedly sourcing Optics from John Nichol or OO (if I can stand the wait !!) would yield a really good mirror. But while I've never had our 14" mirror tested, it seems a pretty good example and we've had no complaints to date. So I haven't dismissed this option out of hand. The secondary is of known 1/10th wave quality from OO.

However, the construction of the SW Mirror and its cell is slightly unusual as the mirror is "ribbed" as shown below...

  

Didier has commented that there are two ways he could incorporate the mirror. One way to do so would be to use the mirror and the associated supports supplied by SW. Along the lines of the "kits" he provides. http://www.sud-dobson.fr/kits.html

The other comment he made referenced using only the mirror and adapting the design and construction of one of the mirror cells of his "bespoke" Compact Truss Dobs. http://www.sud-dobson.fr/compact.html 

While incorporating our mirror would save on overall project cost, my gut reaction to this idea has been to question the idea. But I have no real reason for doing so... 

Setting aside the fact that an OO or Nichol mirror might be of guaranteed, tested and higher quality, could those with experience and knowledge of mirror cells (such as @swamp thing, @mapstar,  @Astrobits, @saac, @Chriske, @Peter Drew ) let me know whether there's any significant "engineering" or "design related" flaws to Didier's suggestion please ? 

Thanks in advance

Simon

[Ben the Ignorant]

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

http://www.jpastrocraft.com/cells.htm

http://www.webstertelescopes.com/mirror_cell.htm

I never needed a custom cell but I modded my GSO lightly, these are the texts I studied.

[Astrobits]

I am not familiar with the SW mirrors/cells but it appears from your pics that SW mounted this mirror from a central boss only. Certainly the molded ribs would not permit the conventional triangular mounting systems as there are eight ribs and it is not going to sit neatly on 3 x 3 supports. I would suggest that you keep the SW cell mounting system as is and arrange it in the mirror box of your choice.

Nigel

[Stargazer McCabe]

@Astrobits Thank you. I appreciate the guidance Nigel.

I'll continue my research. It may be a better end result to get a known, conventional, mirror from someone like John Nichol and mount it in the conventional triangular mounting system... But I'll bear in mind your thoughts if discussions about using our existing mirror is to be pursued  

[Stu]

I didn't know that the Flextube had a ribbed mirror. It looks like it is supported by the three pads attached to the three rings, an unusual but presumably effective solution. I guess it has benefits in terms of cooling and weight and perhaps other things.

Regardless of the quality of the SW mirror, I would think that if going to the expense of building a custom dob it would make sense to maximise the performance by fitting a premium mirror.

Without a side by side comparison you can't say for sure, but I would expect a Nichol mirror to out perform the SW in areas such as contrast and light scatter. The other benefit  would be that you could optimise the focal length for your particular observing tastes and perhaps eyepiece height preference. You would want to understand mirror thickness and associated cooling time too before going down this route.

Sounds like a fun project to be starting

 

[swamp thing]

1 hour ago, Astrobits said:

I am not familiar with the SW mirrors/cells but it appears from your pics that SW mounted this mirror from a central boss only. Certainly the molded ribs would not permit the conventional triangular mounting systems as there are eight ribs and it is not going to sit neatly on 3 x 3 supports. I would suggest that you keep the SW cell mounting system as is and arrange it in the mirror box of your choice.

Nigel

I would agree with Nigel. Use the SW cell system and arrange it to fit in a mirror box of your own design. 

[Stargazer McCabe]

@Stu & @swamp thing Thank you for the swift and helpful replies. It certainly reassures that nobody thus far is saying Didier’s suggestion has a significant flaw. 

As you say Stu, the route of a new mirror from OO or John does allow a greater number of options to be considered. Focal length and diameter being the most obvious ones, allied to optical quality if we felt we could endure the sort of wait @faulksy documented so comprehensively ?

To be fair, while I acknowledge a new mirror is bound to be better than the existing SW one (which individuals say are very roughly 1/6 wave on average), folks more experienced than me that have looked through it have said it’s one of their better ones. Therefore, if we were to commission a 14” there’s a strong case for Didier’s suggestion. 

Would we really see the difference that another few thousand pounds on the mirror would yield under the sort of skies and conditions we encounter..? Some might, but I genuinely wonder whether we would and whether that extra expenditure would “pass us by” being brutally honest ?

An interesting conundrum to ponder. It’s increasing our understanding and knowledge so whichever way we go, we’re grateful for the input and thoughts. Thank you again...

[Stu]

41 minutes ago, Stargazer McCabe said:

if we felt we could endure the sort of wait @faulksy documented so comprehensively ?

I think for a number of reasons I would opt for a Nichol mirror if you do choose to get a new one. I guess one consideration which is both practical and financial to a degree, is what do you do with the Flextube without its mirror? Bin it? It would obviously have a significant resale value with the mirror which could offset the new mirror cost.

I’m actually not arguing one way or another, just giving a few thoughts.

[catburglar]

Unless you plan to go up in aperture, I’d be tempted to keep the SW mirror and cell. Any improvement in quality from a custom mirror at the same aperture  is likely to be marginal. 

You don’t say why you want to change, but  presumably you want something that is lighter/more portable. If this is the case I’d put the money that you save because you’re not buying a mirror into optimising the mechanical elements that impact on setup, usability, stability, thermal issues and light baffling.  E.g thicker/carbon fibre truss poles , integrated dew control for primary and secondary with internal cabling etc.

[Stargazer McCabe]

@Stu John certainly has been exceptionally helpful and “responsive” to date. Whereas others...

The Flextube could possibly “donate” more than just the primary if we went that route. The secondary could be used as it’s an OO 1/10 wave with heater and the focuser, although only a Moonlight CR2, could perform an initial task...

However, to be fair, I could see the existing scope serving someone else for many years to come with its Push To capability with Serge’s Encoders. So I’d feel it a tad draconian to rob it of bits to the point it ends up being tossed...

[Stargazer McCabe]

16 minutes ago, catburglar said:

Unless you plan to go up in aperture, I’d be tempted to keep the SW mirror and cell. Any improvement in quality from a custom mirror at the same aperture  is likely to be marginal. 

You don’t say why you want to change, but  presumably you want something that is lighter/more portable. If this is the case I’d put the money that you save because you’re not buying a mirror into optimising the mechanical elements that impact on setup, usability, stability, thermal issues and light baffling.  E.g thicker/carbon fibre truss poles , integrated dew control for primary and secondary with internal cabling etc.

That all makes a lot of sense and echoes some of the conversations my wife and I have been having.

Our reasons for considering the Truss design are exactly as you suggested. The Flextube, although great, is bulky and heavy. Both in terms of its base and Tube.

As I have said in other replies there are pros to each route and it’s an interesting and good conundrum to have... 

Thanks for helping the deliberations ??

[Stub Mandrel]

If you expect to be shelling out $$$$, why not get the SW mirror tested by Orion Optics or Vacuum Coated Scientific Mirrors?

If it comes out very well, use it in the new scope, if merely good, sell the flextube complete and put it towards the new scope.

[mapstar]

14 hours ago, swamp thing said:

I would agree with Nigel. Use the SW cell system and arrange it to fit in a mirror box of your own design. 

Agree with Steve on this one, as it will involve significant cost and may not prove any better.

[mapstar]

13 hours ago, Stu said:

 It would obviously have a significant resale value with the mirror which could offset the new mirror cost.

Selling the 400p flex tube would be the way I would personally go. 

Just looked at the prices on those sud dobs 

[Stu]

1 hour ago, mapstar said:

Selling the 400p flex tube would be the way I would personally go. 

Just looked at the prices on those sud dobs 

A bit confused now Damian, are you for keeping the SW mirror or selling? Not that it takes much to confuse me

 

[mapstar]

8 minutes ago, Stu said:

A bit confused now Damian, are you for keeping the SW mirror or selling? Not that it takes much to confuse me

 

Selling the lot and starting afresh Stu. It would then be a Nichol mirror in there 

[Stu]

10 minutes ago, mapstar said:

Selling the lot and starting afresh Stu. It would then be a Nichol mirror in there 

Ah good, that's what I thought. That's what I would do ??

[Stargazer McCabe]

This might interest those following this... 

http://astro.dialou.fr/materiels/optiques/orion-skyquest-xx14i/transformation-de-lorion-skyquest-xx14i-par-sud-dobson-partie-1/

[Chriske]

Hi, missed this on, sorry for the late reply.
A conical mirror normally does not have these ribs. In fact the mirror will perform sooner without these ribs.
I had a similar experience in the past. Using high magnifications these ribs will be visible until the mirror is at ambient temp. Depending on your mirror-cooling-system it will be visible for a long time or not. The deformation in the mirror looks like spikes around a star. Looking at the diffraction pattern of a star you'll be surprised.Very slowly it will disappear. But again it all depends on what type of mirror-cooling you'll be using.

 

[Chriske]

Looking at these pictures again, I would add more cooling fans. This is to speed up things but most of all to spread  cooling more evenly over the back of that mirror.

[Stargazer McCabe]

Thanks to everyone. I had no reason for my initial reaction to Didier’s suggestion other than just “gut feeling” I suppose...

I now feel I can formulate a more informed reaction which still follows my initial one to be fair. 

It also allows me to make a more reasoned and informed “capital expenditure” application to the Memsahib ?

(The joys of being married to a Global Commercial Director ?)

I’m minded to apply for a 20” and if I get away with it ?? But I’d settle for an 18” or, if I pick my moment badly, a 16”?

Thanks again to everyone who chipped in. 

[Dave_D]

I'd love to get my grubby little paws on a conical primary...