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Replacement Mirror Cell

Stephen

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Stephen

Stephen

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I'm really starting to loath the cell in my 200P - just finished setting up for tonight and it's rediculously out of collimation again.

It's been sat static and not bashed since the last time I used it!

Mirror clips are at the suggested (piece of paper thickness) distance from the mirror, and I left it collimated and the locking screws (Bobs Knobs) just in contact with the cell.

If I wanted to go down the road of replacing it, what are the suggested options? Ideally not looking to spend an absolute fortune but as with anything astro related, I'm sure it's not going to be £20 and packet of ginger nuts :D

Cheers!

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gbealnz

gbealnz

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The above CNC ones look the part, and I am sure they WILL do what you are wanting, but also venture to suggest they will cost you about as much, maybe more, than the scope. Silly as it sounds, I spent a similar amount on a focuser, so I have just shot myself in the foot, LOL.

Have you tried stiffer springs? You say "locking screws", so you mean a "push-pull" system. It should be exactly the same if it is a push pull system. Not the secondary moving as well is it?

Gary

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Moonshane

Moonshane

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collimation is part of owning a fast newt. I collimate even my 6" f11 newt before every session (usually only the primary needs a tweak). it took me less than 30 seconds to do it tonight and I cannot see what the issue would be with doing this? especially as you have to wait for the mirror to cool anyway?

I don't see a need for locking screws personally, and on previous scopes I have removed them (OOUK not Skywatcher). tighten the adjustment bolts as far as you can and then back off one turn. this keeps the springs compressed and therefore reduces any movement but even during an evening's session there will be some movement in a newt mirror.

The cell would be of good quality etc but it's debatable whether a 9 point cell is beneficial below 12" of aperture (certainly 10"). Given the choice I'd sooner have another eyepiece.

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Stephen

Stephen

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One of the big benefits of a good CNC mirror cell is the ease of collimation. OO does them of course, but FLO has just started stocking the American made Optical Supports CNC mirror cells.

Thanks for the link, not prepared to spend quite that much at the moment :D Looks like a quality piece of kit though.

Have you tried stiffer springs? You say "locking screws", so you mean a "push-pull" system. It should be exactly the same if it is a push pull system. Not the secondary moving as well is it?

Gary

Yes push/pull. Secondary only needs minor adjustment, the main culprit is the primary. It's not got springs (see below) so this may be something to investigate ...

collimation is part of owning a fast newt. I collimate even my 6" f11 newt before every session (usually only the primary needs a tweak). it took me less than 30 seconds to do it tonight and I cannot see what the issue would be with doing this?

It isn't an issue doing it, more the amount it's shifting. The secondary seems reasonably fine, the primary centre circle has shifted hugely out.

I don't see a need for locking screws personally, and on previous scopes I have removed them (OOUK not Skywatcher). tighten the adjustment bolts as far as you can and then back off one turn. this keeps the springs compressed and therefore reduces any movement

The SW one I have doesn't have springs, just three rubber washers. I wonder if it would be feasible to add springs to the system ...

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Stephen

Stephen

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Springs arrived and installed : all I can say is "tight!"

It doesn't require excessive force to adjust, but it's nowhere near as light as the original rubber o-rings.

Pics attached, can't test collimation yet as I'm modifying the secondary as well (the reason being is shown in the last photo ....)

Testing that at max compression, the bolt won't hit the primary. I've used Bobs Knobs locking bolt in place of the adjustment bolt. You can see the distance I've moved the mirror up the tube (it's about 12mm) and the rubbish o-ring that was there before.

post-23057-133877633463_thumb.jpg

Reassembled, new adjusters backed off 1.5 turns and making sure everything is still good.

post-23057-133877633471_thumb.jpg

Now I ordered four of these because I'm thinking about replacing the spring between the spider and the secondary holder as well - it's got a lot of flop to it! Here's a comparison shot.

post-23057-133877633479_thumb.jpg

And finally, here's the rather nasty looking secondary holder. This has had some rough treatment by a previous owner! Two dimpled aluminium washers (32mm with 5mm hole) should sort this out.

post-23057-133877633487_thumb.jpg

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Stephen

Stephen

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Slight progress update, the springs I used on the primary were too long for the secondary - as such I've ordered some smaller ones but temporarily used a metal washer (32mm, 6mm - 10 for £1.18 from B&Q) to assist.

My suspicions over the cell seem to be warranted as the collimation had slipped by ..... 1/8 turn on one adjustment bolt over three days.

Quite happy really :)

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Stephen

Stephen

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Oh I probably should add that the primary mod has moved the focus point back away from the secondary slightly as expected.

I previously used a 35mm extension to focus my 1.25" EPs, I now need to use the 50mm. (As I have a low profile dual speed Revelation focuser)

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Damo636

Damo636

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Really surprised you had so much trouble with the 200p holding collimation. I haven't had to touch mine in weeks. Out of curiosity, are you using a laser or a cheshire? I sold a laser collimator recently as i found every time i inserted it in the focuser i got a different reading. The laser was perfectly collimated itself and i use a self centering adapter in the focuser. I trust the cheshire solely now for the primary. The springs look a worthwhile upgrade though. I installed the washer at the back of my secondary holder some time ago when i fitted the Bob's Knobs and it makes collimating the secondary a breeze.

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Stephen

Stephen

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Really surprised you had so much trouble with the 200p holding collimation. I haven't had to touch mine in weeks. Out of curiosity, are you using a laser or a cheshire? I sold a laser collimator recently as i found every time i inserted it in the focuser i got a different reading. The laser was perfectly collimated itself and i use a self centering adapter in the focuser. I trust the cheshire solely now for the primary. The springs look a worthwhile upgrade though. I installed the washer at the back of my secondary holder some time ago when i fitted the Bob's Knobs and it makes collimating the secondary a breeze.

I can only suggest mine has been abused, it was purchased S/H from a couple who didn't really know anything about it. It has the war wounds to prove :)

I use a cheshire - didn't like what I'd read about lasers so decided the old ways were best :)

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