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

[SnakeyJ]

Took a long time for the kids to settle and for me to get out, though finally managed to put this together.

Internal - showing the fans fitted and cable routed away from vent holes and collimation bolts.

External - showing the fan filter covers and the mini xlr connector for power (in line with focuser and dovetail when mounted).

External - when mounted back on the telescope, showing the knobs and teflon washers.

The fans will be powered by a standard 12V PMW controller (LED dimmer from ebay), which is already mounted on the Pier power/control board.

I made a couple of other minor mods, wrapping the stainless mirror clips in black flocking (DC-Fix off-cut) and replacing the secondary bolts with M4 x 40mm stainless capheads.   Shimming the OC1 will have to wait until tomorrow!

[SnakeyJ]

The mods to the mirror cell certainly helped and I've managed to get some decent images of Jupiter over the last few months, despite the decidely average conditions and moving mid March when there were a couple of decent nights.

However, I have still been getting something of a clunk and some shift as the scope flipped over the meridian forcing me to either force flip and collimate at the start of the session or collimate both sides.

Tonight I have finally worked out the source of the problem.    The aluminium back plate is secured with three self tapping screws in to little panel clips that are pushed over the three holes at the rear end of the OTA.    The clunk was actually one of these clips sliding forward or back in the oversized hole.

Back at the end of January a s/h OO 250 9 point cell came up on SGL classifieds at a very sensible price (thanks Starflyer).   I purchased the cell on spec on a suck it and see, or to save for my 10" dob project.     As I had cross words with the long suffering misses last night, I retreated to my man cave and decided to flock the OO and see if the cell fitted.    Pleased to say it fitted like a dream, so after burning some midnight oil, I now have a fully flocked OO Europa 250 with the new 9 point mirror cell fitted.    The cell is a very nice bit of engineering indeed and it is secured with nice solid M5 bolts, that perfectly and securely fit the three drilled holes in the OTA.   My only niggle would be the three small nylon collimating nuts were too small and didn't give enough purchase - these have been replaced with the nice oversized stainless steel ones I sourced for this mod.  I will just need to find some gasket strip or trim to dress the bare end of the aluminium OTA and get the scope collimated.

I will now re-use the original cell for my dob project, which is next up should I need to retreat to the man cave again

[NGC 1502]

Hi Jake, my old style OO mirror cell is attached to the tube as you describe, 3 self tap screws into panel clips, not the best idea as you've found out.........   

Those old cells were very poorly ventilated, just a single small hole in the middle......which is why we both got our drills out.......

Your new cell is much nicer  good job.

Regards, Ed.

[SnakeyJ]

I ordered some car panel edging strip from ebay to dress the bare aluminium ota - http://www.ebay.co.uk/itm/111643644090 and this arrived today and is now fitted, completing the mod.    The strip pushes on and gives a very nice secure fit, though I had to fit it in three lengths to allow for the cell mounting fittings.   Other than a little bumb where the tube is folded over, it gives a very neat finish and is heavy duty enough to take the wear of standing the tube on its end.

I will buy another 2m of this to dress both ends of the ota for my dob project - a steal at £ 2.25/m including first class postage!

[Ajohn]

Has anyone tried adding "push screws" to these. Some don't use springs. One screw adjustment pulls and the other pushes so there is no play - other than bolts moving around if they can. The springs on the pulls should help setting up as the mirrors normally shift slightly when the push screws are used to lock the setting up - so the pull screws have to be backed off a mite and then the push ones tightened. If nothing can move around this set up should hold collimation. There is a need to make sure nothing is being bent when the screws are tightened as well.

John

-

[SnakeyJ]

Has anyone tried adding "push screws" to these. Some don't use springs. One screw adjustment pulls and the other pushes so there is no play - other than bolts moving around if they can. The springs on the pulls should help setting up as the mirrors normally shift slightly when the push screws are used to lock the setting up - so the pull screws have to be backed off a mite and then the push ones tightened. If nothing can move around this set up should hold collimation. There is a need to make sure nothing is being bent when the screws are tightened as well.

John

-

John, I have a push pull system on my old Bresser Orion (rebadged Vixen R200S) - whilst it does work, it's quite a fiddle to do in the dark of a field and last time I tried to tweak the collimation I put it badly out and gave up for the night - I haven't touched it since.     In this case the adjustment bolts have cross point screw heads, which is definitely not ideal - big knurled knobs would offer much more control and save trying to look down the eyepiece and feel with the screwdriver.   This aside I'm sure it could be made to work.

My old Explorer 200P had screw/spring collimation adjustment on the mirror cell, with the addition of knurled lock bolts - though I removed these almost immediately as they got in the way and pushed the collimation out.

[Starflyer]

I ordered some car panel edging strip from ebay to dress the bare aluminium ota - http://www.ebay.co.uk/itm/111643644090 and this arrived today and is now fitted, completing the mod.    The strip pushes on and gives a very nice secure fit, though I had to fit it in three lengths to allow for the cell mounting fittings.   Other than a little bumb where the tube is folded over, it gives a very neat finish and is heavy duty enough to take the wear of standing the tube on its end.

DSC_0695.JPG

I will buy another 2m of this to dress both ends of the ota for my dob project - a steal at £ 2.25/m including first class postage!

Glad to see you got it fitted Jake, they really are great mirror cells.

[Ajohn]

It's an ancient method of keeping optical bits in place.

Instrumentation people are fond of using socket screws - the key handles are long so minute turns are easier to do. It's also still possible to buy precision screws - I asked for unbrako equivalents from a local screw supplier but they are more expensive than the usual type. It might even be possible to get metric fine even though ISO seem to have decided they don't exist any more.

I bought some 40 TPI model engineer taps and dies for this sort of thing some time ago from the London Tap and Die Co.   Not used for this yet but I know I am likely to need them. I've bought a mirror blank now so if that goes ok they will get used. I did a couple of years of tool making training before going into design. Every die stock that came in was opened out by something like 0.010in so that split dies could be opened out allowing rather precise threads to be cut. Open them too much and they crack. Might help people get threads that mate well to prevent rock but I'm not sure anyone makes taps and dies that accurately any more. ISO again. The other way to help with rock is to make the female part longish - say 2 to 3 diameters.

There are also some 0.5mm pitch taps about at various sizes thanks to the thread being used one things like board lenses. The male parts might have to be screw cut but metric has the advantage that a 3 square needle file can be used to finally fit the thread short lengths at a time. Do use one with a handle and the lathe running at low speed - I had one more or less go through the palm of my hand once. Fortunately it didn't do any serious damage - just hurt in an odd sort of way, more like a severe ache.

On the other hand when both the push and pull screws are tight the threads will be resting on their flanks so wont be going anywhere. Maybe the best option on the parts shown would be long push nuts as they could stick out of the back of the  cell. The pulls could also be nuts on all thread / a stud. This sort of thing would minimise the space needed in the scope's mirror cell.

John

-