8" F4 Newtonian Mirror Cell Mods

[laser_jock99]

Following some earlier work on my 12" scope to make it hold collimation better I have applied similar mods to the smaller scope.

To increase the Z axis (up & down the tube) stabillty and to move the mirror up the tube 10mm I used some longer, stronger springs.

New long spring beside the old spring.

Instead of just three springs this mod calls for six of them. The useless 'mirror locking' bolts are thrown away so now there are three active & three passive springs.

To stop the mirror cell shifting in the X-Y axis on the loose fitting collimation screws I used a cone engagement method again.

I fashioned some longer M6 adjuster screws.

M6 tamperproof Sheer Nut

New adjuster screw with conical part of M6 sheer nut beside the old adjuster screw.

The mirror cell holes need a light counter sink so the cone sits nicely.

The finished mirror cell- there is now very little mirror shift once collimated!

[laser_jock99]

Parts list & sources:

3 x M6 female knob - can be brought from e-bay

http://www.ebay.co.uk/itm/M6-THREAD-40MM-KNOB-FEMALE-05468-/321130668333?pt=LH_DefaultDomain_3&hash=item4ac4e1212d

Or much cheaper direct from Boneham & Turner

http://www.boneham.co.uk/shop/product/lobe_knobs_brass_boss

Some M6 studding/threaded bar to cut to length for the adjuster knobs- ScrewFix or E-bay etc

6 x compression springs (Springmasters p/n C5514610) - wire diameter 1.6mm, overall diameter 13.72, free length 25.4mm, no. of coils 7, rate 7.37 N/mm

http://www.springmasters.com/compression-springs/147-165.html?zoom_highlight=c5514610

3 x M6 shear/security nuts - again obtainable from e-bay

http://www.ebay.co.uk/itm/M6-SECURITY-SHEAR-NUTS-A2-STAINLESS-QTY-10-/380791230072?pt=UK_BOI_Industrial_Supply_Security_Equipment_ET&hash=item58a8ed0e78

3 x M6 x 16mm hex socket head bolts/screws to retain the passive springs.

http://www.ebay.co.uk/itm/Pack-of-10-M6x16mm-Stainless-Steel-Hex-Socket-Head-Bolts-Screws-/390726149845?pt=UK_DIY_Material_Nails_Fixing_MJ&hash=item5af917e6d5

[JRM]

    Did basically the same with my 12" Antares dob, little different hookup on mine though, I took the locks off and after replacing with the stronger

springs there is no need for the locks,holds collimation very well.  Nice job.

Rick

[Chris]

Good work Laser_jock. I see this thread being very helpful to others with budget fast Newts

[Macavity]

The pictures / explanation are excellent, but I'm still a tad confused (Bear with me? <gasp>)

The role of "Active" screws / springs is the same, yes? But longer screws, stronger springs etc.

Screw tips engage with the mirror cell, the "telescope ring" has improved (countersunk!) holes.

The Shear Nuts (As I can imagine!) significantly lessen the screw "wandering about" problem? 

But I am less sure about the role of the "Passive" screws / springs? Captive threaded into the

scope ring, these were just tightened against (rather naff! lol) lumps of metal on the mirror cell?

Now these are improved by adding springs? But is that the only change? The Bolts now just 

stop the springs flying out? Are they still tightened against the mirror cell "blocks" as before?

I'm sure a VAST improvement - Can't wait to try it out on mine! Just trying to visualise it...

Almost goes without saying - But again, Great Work on ALL this stuff!

Edited December 19, 2013 by Macavity

[laser_jock99]

Yes - you pretty much got it. The stainless steel hex head bolts you can see are only there to stop the 'passive' spring from slipping. They are very short and not intended to touch the mirror cell.

I found the 'mirror locking' screws on all my F4 (sometimes F2.9!!) scopes particularly useless. As soon as they touched the mirror cell collimation was minutely shifted. Worse still there's a danger of interfereing with the collimation process if they end up touching the mirror cell at any point. At F2.9 the effect is worse since collimation accuracy is more critical in that configuration.

I found the addition of the three extra springs added enough extra stability/firmness that 'locking' wasn't needed anyway. All the adjustment is still be done on the three 'active' springs (in practice you should only need to adjust on two of them - leave the third one be!).

Edited December 19, 2013 by laser_jock99

[Macavity]

I found the 'mirror locking' screws on all my F4 (sometimes F2.9!!) scopes particularly useless. 

Excellent! Pretty much my own thinking - Adjustment of the (default) *collimation* knobs

was almost overridden by any change to the "locking" ones - And not in a good way!

I have the secondary adjustment sorted / sussed. The primary was a stumbling block...

I have great faith this can be a good VIDEO light bucket. As noted, even focal reduced!  

[Macavity]

Shameless <BUMP> for this revelation - And Eternal Thanks, Laser Jock! 

Finally got around to this mod... For the FIRST time (my GSO 8" F4) primary

adjustment actually WORKS? lol. But seriously - Wahey!

Another source of "random" SPRINGS of similar ilk: http://entexstocksprings.co.uk/

Personally, working with springs: #65, #81 and #2021 <G> Some (awaiting delivery)

M6 x 30 x 40mm "Five Lobe Knobs" from Bonehan & Turner: www.boneham.co.uk

Aside: For £400, I never asked for a "£3000 Professional Astrograph" (Video Astronomer Me!)

But I do sense even "Standard Newt" owners might benefit from the *stronger spring mod* too? 

Edited April 1, 2014 by Macavity

[laser_jock99]

Shameless <BUMP> for this revelation - And Eternal Thanks, Laser Jock! 

Finally got around to this mod... For the FIRST time (my GSO 8" F4) primary

adjustment actually WORKS? lol. But seriously - Wahey!

Another source of "random" SPRINGS of similar ilk: http://entexstocksprings.co.uk/

Personally, working with springs: #65, #81 and #2021 <G> Some (awaiting delivery)

M6 x 30 x 40mm "Five Lobe Knobs" from Bonehan & Turner: www.boneham.co.uk

Aside: For £400, I never asked for a "£3000 Professional Astrograph" (Video Astronomer Me!)

But I do sense even "Standard Newt" owners might benefit from the *stronger spring mod* too? 

It's definitely a mod worth doing on these F4 scopes. The collimation stability gain is well worth the low cost of the components. Why on earth the manufacturers don't ship the scopes like this baffles me? It doesn't cost any more than standard the springs would......  

[Stu Wilson]

I'm getting all the bits ordered up for when new scope arrives hopefully this week.

Question. How much thread have you got out of knobs? 25mm?

 

Cheers

 

Stu