Spider vanes and focuser position

[bambuko]

Is there any optical reason why spider vanes layout shown (simplified) at B and C would be undesirable?

Layouts as shown at A and D seem to be more common for some reason? resulting in a tube distortion (particularly on Chinese OTA which seem to be made from aluminium foil rather than sheet) as you tighten the vane and in vanes getting in a way of the focuser (as is the case on my TAL-1).

A and D are plainly ***** from a point of view of tube strength and ease of arranging the focuser, so there must be some reason I am missing why things are done this way?

Or more general question - does it matter at all where the focuser is placed radially around the tube?

[cantab]

With arrangement B or C the spider fixings and the finder attachment might interfere.

[bambuko]

Thanks cantab,

Definitely not with C (it is totally clear of focuser base).

Possibly with B (as drawn) - but that can easily be taken care of.

It definitely interferes in A and D, plus (as I have said before) focuser hole weakens the tube right where you are attaching one of the vanes.

Just have a look at one of the Skywatcher contraptions and you will see flat area on the OTA right next to focuser where the vane pulls it

So... back to the original question - is there anything optically against B and C ??

[Peter Drew]

"C" will give 6 diffraction spikes on a bright star instead of 4 if that bothers you. 

[bambuko]

"C" will give 6 diffraction spikes on a bright star instead of 4 if that bothers you. 

Thanks Peter Drew,

Will D also give 6 spikes ? - i.e. is D any different optically from C ??

I can find a lot of information about diffraction spikes, no problem.

What I can't find is the information about how (if at all) is the spike pattern affected by a position of focuser in relation to webs?

[Peter Drew]

Yes "D" is the same in this respect. As far as I can see, the orientation of the focuser relative to the spider vanes just alters the orientation of the diffraction pattern as seen through the eyepiece/camera. 

[bambuko]

That's what I was thinking... but was hoping for someone more knowledgeable to confirm.

If this is the case, than why does everybody seem to be adopting A ?

Structurally this is the weakest arrangement, causing not only distortion of the tube but also poor securing of the vane nearest to the focuser...

[Peter Drew]

Good question, perhaps it just looks "right". 

[bambuko]

Well ... I don't blame Chinese (after all they only copy) but who was the first one to blame?

It certainly doesn't look right to me

[John]

My newt doesn't use any of those. While there is some diffraction with this design, you don't get the spikes:

[bambuko]

Thanks John, for that design - I like it

So... if you were to rotate the focuser so that it is not equally and symmetrically disposed about the two vanes - would you expect any additional distortion?

(or to put it another way - do you agree with the suggestion that it doesn't matter where you place the focuser?)
Having said so, it is purely academic question, because your design addresses bot concerns I had about "vanes ve focuser" and therefore does not need any adjustments to the focuser location.

The only thing (compared with the usual straight vanes spider) that would concern me with your two circles design, is the ability to adjust vanes to get secondary central (I guess you just twist and pull?).

[hunterknox]

"C" will give 6 diffraction spikes on a bright star instead of 4 if that bothers you.

I'm sure it's true but I can't fathom why. What am I missing?

[John]

Thanks John, for that design - I like it

The only thing (compared with the usual straight vanes spider) that would concern me with your two circles design, is the ability to adjust vanes to get secondary central (I guess you just twist and pull?).

I guess so but mine were commercially made and fitted (Orion Optics) so I've not had to adjust the vanes themselves. 

[John]

I'm sure it's true but I can't fathom why. What am I missing?

I believe you actually get 8 spikes with a 4 equally spaced vane design but one set overlaps the other so you get 4 slightly brighter spikes. With the 3 vane design you get 6 separate but slightly dimmer spikes. I think this is because the diffraction occurs at the edge of an object in the light path so from either side of the vane, even though it's thin. A single vane design creates 2 spikes, one from either edge of the vane.

I'm happy for those with more expertise to tell me I've got the wrong end of the stick (or vane) on this 

[Stu]

Some info on different effects of secondary design at these links.

http://www.astronomyhints.com/spider.html

http://www.fpi-protostar.com/cm-faq.htm

http://www.rfroyce.com/spidrunq.htm

Stu

[Stu]

....and for those wanting some maths.....

http://www.telescope-optics.net/spider.htm

Stu