Nine Point Mirror Cell

[Moonshane]

Hi all

I have an Orion Optics UK 12" Dobsonian and which currently has a three point cell. I have not removed the primary but my inserted finger tells me it's about 1" thick.

Looking online (why oh why do we do that!?) it appears that the maximum ratio for mirrors in a three point cell is around 6:1; mine is clearly about 12:1.

On this basis it appears to be important to improve the mirror cell and go for the nine point replacement/exchange offered by OO UK for £90.

This may also help my balancing issue as if the nine point cell is heaver than the three point cell I can achieve balance without further modification with my heavier eyepieces.

Does anyone have any comments on this please and also does anyone have any idea whether or not the nine point cell is likely to be heavier than the three point cell?

Finally, is it likely that using a three point cell over a long time could affect the figure of the mirror or even create astigmatism that would need to be corrected by a rework?

Thanks in advance

Shane

[John]

I'm in the same position but with a 10" F/4.8 OO newtonian. I've spoken to OO about the upgrade and they said the exchange upgrade price is just when you order a new scope - you and I will need to pay the full price. Also, I my case at least, I will need a tube end band as well which is another £20.

My mirror is around an inch thick as well but I've enough faith in OO that I doubt they would put a mirror in a cell that was totally unsuitable for it - mine works fine but I want i) the better rear mirror ventillation and ii) the cooling fan that the 9-point cell allows which is why I'm considering the upgrade in due course.

I don't know for sure but I'm guessing that the 9-point cell weighs a bit more than the 3-point version does.

[GazOC]

I have an upgraded mirror cell on my Dob and its a cracking upgrade for large mirror, short focal ratio scopes as far as collimation is concerned.

[davew]

Very strange. On Orion's site it says, " It is possible to upgrade your existing Orion Optics UK Mirror Cell by Part Exchange " then says, " Part echange prices in red ".

Note the word " Existing ".

For me that would make it an extra £ 25.

Optics Mirror Cells Page

Dave.

[John]

Very strange. On Orion's site it says, " It is possible to upgrade your existing Orion Optics UK Mirror Cell by Part Exchange " then says, " Part echange prices in red ".

Note the word " Existing ".

For me that would make it an extra £ 25.

Optics Mirror Cells Page

Dave.

Yes I noticed that as well Dave. I was ringing OO to place an order on that basis however I have to say that their attitude was rather unhelpful and inflexible, so I decided to hold back from ordering.

When I'm in the right mood I'll phone again and quote their wab page at them and see if I can get an exchange deal

[davew]

Hi Shane,

I've been told by someone more knowlegeable than myself that my standard 10" mirror would probably not benefit from the 9 point other than the fact that the mirror cell may hold collimation better. Your 12" on the other hand would be a prime candidate esp if it is one of the better ones.

I don't know if it's heavier. I don't think the 3 cell will damage your mirror.

John,

I am of the opinion it depends who picks up the phone !

Dave.

[Moonshane]

cheers guys. it seems on balance that it would be a worthy upgrade.

I took it that you got a discount of £25 or £30 when you trading your existing one in rather than paying the extra £25 or £20.

I agree with Dave's last comment as they have always been ok with my generally but every now and then the person on the phone is a bit more 'sharp' than usual.

For me, it does not matter too much as their gear is excellent and there's only one place to get it unless you are lucky enough to find used stuff.

[johninderby]

The 9 point cell is heavier, it's really solidly built. A 10" mirror is borderline for any real improvements from the point of better mirror support. However once you get up to 12" then it is really worth while.

The improvement it ventilation and cooling are are a nice benefit as well.

John

[Moonshane]

cheers John

definitely an improvement I'll go for in due course.

[johninderby]

Here's a photo of my 14" OO mirror with the 9 point cell to give you an idea of what they look like.

John

[Moonshane]

cheers for the John,

do you use the tape just to hold the mirror a little more securely?

also, does this cell fit with the three bolts holding my current cell?

John mentioned having the buy a tube end for his scope. there's a fair amount of space at the end of my tube so maybe I'd not need this?

best wishes

Shane

[johninderby]

That's the way the mirror and cell came from OO, the tape was already on there. The 9 point cell is held on by 6 bolts, or really three pairs of two bolts. The trim ring is needed to cover and strengthen the end of the tube.

Here's a photo showing the cell and two of the bolts for the cell and also the trim ring. Hope it makes it a bit clearer.

John

[Moonshane]

cheers John - that helps a lot.

Looks like I already have the trim ring then but would need to drill the tube for the 6 holes and cover the others. looks like a paper loop is in order around the tube. measure distances to the central points at 120 degrees and then the right distance either side. nice wheels!

BTW, what's the little black block for?

[johninderby]

BTW, what's the little black block for?

The little block is for mounting a counterweight if needed. A post screws into the block and then I have some small Astro Engineering 1/2lb weights that drop onto the post and then a wing-nut holds the weights in place.

John

[Moonshane]

great stuff. I hoped you'd say that as I might try something like this myself.

thanks for your help John et al. you've definitely helped me spend some more dosh