10" GSO Newtonian Mirror Cell Mods

[laser_jock99]

Something I've done before on other scopes- but worth repeating again.

Typically the supplied primary springs with Newtonians are at best 'adequate' or just not up to the job of firmly supporting a heavy mirror.

Stock GSO spring vs the ones I fitted


The other issue seems to be too much play in the through holes for the adjuster screws. It allows the cell shift in the x-y direction.

Holes too big!



To counteract this 'play' I made some cone engagement adjuster screws.

Old adjuster vs new adjuster with cone.

The mirror cell is slightly countersunk to accept the cone.

How they will fit together to stop lateral play.

My modification uses six springs instead of three. The three 'locking screws' are thrown away and replaced with extra 'passive' springs.

The re-assembled cell. Short bolts are used to retain the passive springs.

End result- more stable & easier to adjust.

[happy-kat]

Any chance please to a few suggesting links to bits that could be bought?

Do you power the fan with one of those small 9v rectangular batteries?

[Macavity]

A great idea indeed - Which I subsequently and happily emulated.

Since I have the (where I got mine) information to hand:

My "Lobe knobs" from: http://www.boneham.co.uk/

Compression Springs: http://www.entexstocksprings.co.uk/

The latter search proved a bit more of a challenge, but following the general scheme,

(steel) springs of strength of 7-8 N/mm (wire diameter ~1.5mm) restricts candidates...  

(Just select the inside diameter and [max / min] length appropriate to your setup)

[fwm891]

The length of your new springs is greater than the GSO' springs - that will shift the primary up the tube slightly does that cause any returning cone cut-off at the secondary as the cone will be slightly wider from being shifted forwards?

[laser_jock99]

The length of your new springs is greater than the GSO' springs - that will shift the primary up the tube slightly does that cause any returning cone cut-off at the secondary as the cone will be slightly wider from being shifted forwards?

It will almost certainly have some effect on the returning light cone - some of the full main mirror's light gathering aperture will be lost around the edges of the secondary. However the secondary will be 'better filled' and possibly more evenly illuminated?

The main reason for wanting to shift the the primary mirror up the tube though is the extra back focus distance gained.

[Macavity]

<thinking> I reckon all the above is a very reasonable assumption! <grin>

In my case, stronger springs, moved the main mirror approx. 1cm. The

light cone width of an F/4 system increases approx. [mirror shift] / 4 ???

(Gasp, wheeze: "similar triangles"?) But frankly, I suspect "not much"!

But, either way, with (as mentioned above) advantage of increased in-focus

and not too much vignetting? I did once bung the dimensions of my GSO into:

http://stellafane.org/tm/newt-web/newt-web.html (Surely an entertainment in itself).

Interestingly, it suggested vignetting might arise because the main OTA is a

bit too small in diameter! (Another good reason to get the optical axes aligned)

But again, one senses, well within remit of a *good value for money* scope?

As Winter draws on (fnar?), my only remaining concern is secondary dewing!

Behind-the-mirror heaters do exist! But it is a brave man who ventures to get

the secondary *unstuck* from it's holder. It can be done <nudge>... <nudge> 

[stoffe]

Great mod! I hope it works as expected? Did you change anything with the mirror holder too when you was in the workshop?

[laser_jock99]

Great mod! I hope it works as expected? Did you change anything with the mirror holder too when you was in the workshop?

I have been using this scope since May and it's much better at holding collimation better between slewing the scope from one side of the sky to the other. In this respect it works- but I still find it worthwhile doing a quick collimation check mid session just in case something has shifted (it is frustrating to waste half a nights work for the sake of a 2 minute check). For the same reason I re-check the camera focus with a Bahtinov mask after a couple of hours as these budget steel tube Newts tend to 'shrink' during the night!

No mods were made to the mirror cell itself although I did check the centering of the collimation spot as these can sometimes be off from the factory.

[stoffe]

Thanks for your answer, I think I will do the same modification on my TS Optics 8" f/4. It is a GSO telescope.