Mirror Cell Mods for OO Europa 250 - Curing the Flop!

[SnakeyJ]

Thanks to my new Hotech laser and some helpful advice from Neil and Lee in one of my Jupiter imaging threads , I've found a significant amount of mirror flop in my OO Europa 250 mirror cell.   I was initially tempted to get a replacement 9 point cell from OO, though £ 240 + delivery seemed a little excessive even though this has excellent reviews.   Alternatively, there was a s/h OO cell for sale on ABS and I came within an inch of purchasing this before Neil said that he had already tried this mod and found that the new VX cell does not fit the Europa tube.

Saved from parting with my hard earned on a fool's errand, it seems that some DIY is called for to make the best of the standard Europa cell.

Initial investigation reveals that the mirror is securely held and centred within it's aluminium cell.   The issue appears to be that the mirror and cell are flopping around on the three extended collimation bolts.     Whilst I could tighten these back, pulling the mirror cell down the OTA, this would move the focal point back such that I could not achieve focus with my SX filter wheel in the image train.

I will get some stronger springs and replace the collimation bolts, but have some reservations about the strength of the thin steel back plate, especially as I plan to cut three holes out for fans.

Looks like I've another project on the go

[sbooder]

I just swapped the mirror cell springs for car valve springs.  I have not had flop or needed to adjust collimation for a while now.

[SnakeyJ]

I just swapped the mirror cell springs for car valve springs. I have not had flop or needed to adjust collimation for a while now.

That sounds pretty heavy duty - I will try and look some up. Did you replace the knobs at the same time?

[sbooder]

No the knobs work fine with the new springs.  I will try and find out from which car they were from.  My uncle got them for me, and as a mechanic, as well as an experienced AA, he uses the same ones every time.

[SnakeyJ]

Here's the picture of the cell with the mirror removed.

Disassembled, showing springs and fastenings:

Back Plate from the cell, showing oversized holes:

The cell is constructed of cast aluminium 9mm thick, with 3 x M6 40mm stainless steel countersunk bolts.     The bolts are all tight/fast in the aluminium, but the play seems to come from the rear end, where they pass through 7.5-8mm holes in the 2.5mm powder coated steel end plate.    The collimation knobs are a motley collection of M6 knurled nuts and stainless steel washers.    The springs are 9 x 31mm, made from 1.25mm wire.

The play at the back plate allows significant lateral and rotational displacement of the mirror, especially with an F4.8 telescope (somewhere around 2.2mm sweet spot).    I'll rebuild and attempt to measure this.

[Dave Lloyd]

I think valve springs might also be good to maintain tension whilst adjusting brake disc pier heads.

[SnakeyJ]

Looks likely that someone has played with this before (other than the odd collimation nuts).   I've just reassembled and found the optimum rotation for the cell against the back plate - this has reduced the lateral play considerably.   There is still some rotational play, but again this feels a lot better than before.

Now to source some nice oversized M6 knurled knobs - I will take a quick look on Astroboot and Fleabay

[SnakeyJ]

A little expensive, but I quite like the look of these M6 knurled nuts, which are oversized 25mm x 18mm (dia x h) - http://www.ebay.co.uk/itm/Thumb-Nut-M6-Thread-Knurled-316-Stainless-Steel-/281522738759

The extra diameter should allow for finer control.

[SnakeyJ]

Can anyone recommend a UK supplier for springs and knurled nuts?

[NGC 1502]

Hi Jake. Your Europa mirror cell is identical to the one on my early model OO Dob. I fitted shorter / fatter springs from a small local ironmongers. The mirror had a bit of radial play in the cell, I adjusted the nylon screws, without causing pinching.

The inadequate single central ventilation hole was fixed by drilling a series of 12mm holes as per the dodgy attached pic.

Collimation just needs the occasional tweak, and cooldown is much better. The scope is stored in an unheated room.

The cell is well strong enough with the metal removed, but your call how much you remove !

Regards, Ed.

[SnakeyJ]

Hi Jake. Your Europa mirror cell is identical to the one on my early model OO Dob. I fitted shorter / fatter springs from a small local ironmongers. The mirror had a bit of radial play in the cell, I adjusted the nylon screws, without causing pinching.

The inadequate single central ventilation hole was fixed by drilling a series of 12mm holes as per the dodgy attached pic.

Collimation just needs the occasional tweak, and cooldown is much better. The scope is stored in an unheated room.

The cell is well strong enough with the metal removed, but your call how much you remove !

Regards, Ed.

Thanks Ed - you've spaced and drilled those holes very neatly.    The end plate is a bit thicker and stronger than I was expecting, so not too worried here.    I'm thinking of adding three small fans internally, so I will probably cut 3 x 30mm holes and have the fans on the inside (OO CT style), with a small hole for a power connector somewhere towards the edge - though I might add 8 x 12mm holes around the centre if it will mark out okay as yours looks so good.    I'll run the fans from a PWM controller for variable speed and give some vibration control.

[laser_jock99]

I get my springs from here:

http://www.springmasters.com/

and knobs, screws & handles from here:

http://www.boneham.co.uk/

[SnakeyJ]

I get my springs from here:

http://www.springmasters.com/

and knobs, screws & handles from here:

http://www.boneham.co.uk/

Thanks Jock, I will take a look over at springmasters and will save both link for future reference.

[SnakeyJ]

Knobs arrived in the post on Christmas eve - whoops, the hole and thread doesn't go all the way through. Hoping I can resolve this on the lathe.

[Gina]

If you have a lathe then you could make your own knobs I would have thought

[SnakeyJ]

Thanks Jock, I will take a look over at springmasters and will save both link for future reference.

I should do, but need a knurling tool. A ball tool I could make [emoji3]

[SnakeyJ]

Not too bad, just around 1.5mm to drill out.    With about 14.5mm of thread I could probably get away without doing this, but it will give me confidence to see some thread while I am adjusting the primary screws.

[SnakeyJ]

I've just ordered the replacement springs from www.assocsprings.co.uk in 316 grade stainless.   The wire diameter is 1.5mm vs the 1.2mm of the OO springs.    I've guessed the load rating, choosing the best compromise between strength and solid height.   This may shift the focus out a few mm, but a little more in focus adjustment would be welcomed and may even allow me to try the Revelation x2.5 barlow that I've never been able to use outside of my filter wheel!

Details are as follows:

Part Number          D0 (mm)  d (mm)  L0 (mm)   L1 (mm)  Sh (mm)  P1 (N)  R (N/mm)  Weight

C03600591250X   9.14         1.50       31.75        24.61      18.64       70.50   9.87           0.011630

Not cheap @ £ 20 including delivery, though half price for qty5+ so I ordered 6 for the spares.

Meant to add that they have quite a useful search tool for narrowing the choice down @ http://www.assocspring.co.uk/p/33/compression_springs-stainless_steel-316_type

[SnakeyJ]

Finally all the bits are in line and the rotten weather presented the opportunity to get on with this project.

Mirror cell removed and marked up, holes centre punched.   The design was rather busked.   All up weight of the back plate is 750gms pre-mod and the material is aluminium rather than the steel as I had originally thought:

Holes drilled, piloted with 4mm, 6mm and then 10mm holes:

Darn it - my 38mm holes saw wanders with the hand drill (must get the bench drill home some time).

Decided to drill around with 4mm and then puch the centres out:

Need to find a half round file to neaten up, but almost there.

I'm installing 3 x 40mm x 10mm computer fans in the large cut outs.   These will be covered externally with ABS covers and filters.   These neatly cover the errors from hole cutting.

The whole back plate will be covered by a shower cap type soft cover when not in use to prevent the spiders moving in.

[SnakeyJ]

Using the lathe and my new tail stock chuck, I drilled out the end of the Stainless steel knobs and have extended the M6 thread all the way through.

The new springs are a good fit and much stronger than the originals.   I've added some teflon washers and these now adjust very smoothly and the whole cell seems much more solid.

Pics to follow as I've just dissasembled and looking for some 4mm nuts and bolts for the fans.    

Hope to get this all reassembled this weekend, but want to try and replace the mirror centre spot with a catseye hotspot.    I tried soaking this off with acetone last time to no avail, but will try again with some IPA.

[SnakeyJ]

Quick comparison of the old and new springs (new one on bottom):

Back plate re-assembled (still to fit fans as I have no 20mm x M3 bolts!):

I've just blacked out all the cut edges with permanent marker which looks a lot neater.    Need to find a phono bulkhead connector and drill another hole for the fan power!

[NGC 1502]

Hi Jake, nice job  I too use a shower cap (as per your earlier post) on the primary cell end.

Regards, Ed.

[SnakeyJ]

Hi Jake, nice job  I too use a shower cap (as per your earlier post) on the primary cell end.

Regards, Ed.

Thanks Ed - I took a chance on Astroboot and got the SW 10" Dob covers for £ 8 - http://www.astroboot.co.uk/images/astroboot/dynamic/800w-532h/1398872478-Image058.JPG

The hard cover will need some adapting, but I am keen to replace the OO original which is very poor and flimsy.    The shower cap is perfect for the other end!

I'm a bit stuck at the moment without 20mm M3 bolts to mount the fans and covers, but I have put everything back together in the obs.    Collimation was very nice and simple, but will have a play to see how this drifts when the scope flips over the meridian.

[SnakeyJ]

That seems to have done the trick

Although I've nothing but cloud and rain, I was able to clamp the hotech in and check how far the collimation drifts as I move the OTA around in RA and DEC and both sides of the meridian.     Before the mod it was shifting completely off the viewing screen of the hotech.   Now at worst I'm seeing a drift of <0.25 of the screen from centre one side of the meridian to the other.     It returns faithfully to position when returned to the original position.     Some of this may be accounted for with a little slack in the focuser adjustment, which I will try to adjust out when I have more light to see by.     There is also a slight deviation in the orthogonality of the focuser, which may need a shim or adjustment (not so familiar with this focuser and can't seem to find a manual or fitting instruction online).

A little more work to be done, but this is definitely a huge step in the right direction

[SnakeyJ]

Finally got all the bits together, so will complete this tonight