200P Collimation - Mirror flop

[SnakeyJ]

Just spent the best part of a wet evening re-collimating my Explorer 200P from scratch and was getting to a point where I was reasonably happy - though have noticed that there is a slight eccentricity to my laser collimator (giving about 7-8mm travel/orbit on the primary when I rotate the laser at the focuser) - I will try and adjust this out tomorrow evening.

However, I have noticed that there is a fair bit of play on one of the primary mirror clips - if I gently push the mirror from the rear of the cell, it's slack enough to give a definite click and perhaps 1/32" travel (enough to displace my collimator alignment from centre to almost of the edge of the alignment window. This definitely need to be adjusted, though I'm slightly wary of previous posts warning against pinching the mirror and of corrosion leading to sheered bolts - so I will be approaching this with caution and will grease any bolts that are removed. Question is how tight can I tighten these to remove the flop whilst avoiding any pinching? Also would it be worth putting a bit of thin rubber mat or similar material between the clip and mirror to provide a better grip/more even pressure and to allow for some thermal movement?

Please excuse mixed metric and imperial units!

[Steve Ward]

The clamps don't want to be actually gripping the mirror as this will give more problems than not.

I adjust the clips using a strip of paper , carefully , between the clamp and mirror until the paper is held and then back off enough to remove the paper.

Mirror is secure but not pinched or in danger of being so.

[SnakeyJ]

Thanks Steve - I'll give this a go tonight.

[SnakeyJ]

Just thought on the clips I might use some spare dc-fix flocking for the pads - should help spread the load and allow for some thermal movement. Will give them a good clean after cutting to remove any loose fibres.

[umadog]

Leave them as they are. They're just there to stop the mirror falling out during scope transport when the tube is horizontal. The rest of the time it's sitting back in the cell and the clips are irrelevant. On one of my scopes I have about a cm between the clip and mirror. The usual rule of thumb is a that business card should be able to slide between the clip and mirror, but there's no particular reason why that shouldn't be two or three business cards. Anyway, so long as it's not touching you're good. You don't need to worry about the mirror expanding when it warms and then squeezing itself: it's only ever going to increase in size by a few microns at most.

I'd be more concerned about your laser collimator. For an 8" f/6 under high power viewing, the secondary tilt needs to be correctly aligned so that the laser is within 1 mm of the centre spot. The same for the return beam or the barlowed return beam. So if rotating it is causing it to move 7 mm, then the error of the tool is 7 times larger than the needed accuracy for high power viewing.

[Daniel-K]

just a thought ditch the laser and go old school and get a cheshire sight tube, ive had lasers auto collimators and found the best way is with a cheshire sight tube. i use to have my clips just a tiny weeney bit tight done the job

[bingevader]

I can't say I've felt the need to buy a laser yet, Cheshire does the job for me too!

I've heard of stopping-down, but with a business card under each clip there won't be much mirror left!

Cheers

[NGC 1502]

As long as there is a 'smidge' of clearance between clip and mirror surface, you will be fine.

You very certainly don't want to clamp the mirror in place, but neither is it necessary to have it flopping around, especially if the scope is transported in a vehicle.

Some folk do get on well with lasers, but many do not, simple tools work at least as well, if not better.

Regards, Ed.

[SnakeyJ]

Thanks everyone I'll give a gentle tighten and leave it at that rather than try to pad!

I need to get a decent cheshire, as I only have the short tube el cheapo. But I will have a go at centering the laser as if I've left the scope out/covered for a few days and often end up checking it in the dark where the laser is quite useful.

[umadog]

The laser gets an overly bad rep IMO. If you centre your laser and then check it's centered by giving it a 360 turn before each use then you have no reason to doubt its accuracy. Furthermore, if you barlow to adjust the primary then the laser doesn't have to be aligned because you're just using it as an illuminator. The primary adjustment is the most critical, anyway.

As for the Cheshire/sight-tube: you can use it in the dark. The trick is to clip a bright LED to the spider and illuminate the centre spot. Also helpful is to illuminate the 45 degree Cheshire surface, if your Cheshire is configured that way (they aren't all). I collimate in the dark and I do it with a Cheshire and an autocollimator. My main reasons for not using a laser is that my current tools work for me and I don't the extra expense or have to think about packing yet more stuff. But if you already have a laser, then it should be possible to get it working as required.

[SnakeyJ]

The laser gets an overly bad rep IMO. If you centre your laser and then check it's centered by giving it a 360 turn before each use then you have no reason to doubt its accuracy. Furthermore, if you barlow to adjust the primary then the laser doesn't have to be aligned because you're just using it as an illuminator. The primary adjustment is the most critical, anyway.

As for the Cheshire/sight-tube: you can use it in the dark. The trick is to clip a bright LED to the spider and illuminate the centre spot. Also helpful is to illuminate the 45 degree Cheshire surface, if your Cheshire is configured that way (they aren't all). I collimate in the dark and I do it with a Cheshire and an autocollimator. My main reasons for not using a laser is that my current tools work for me and I don't the extra expense or have to think about packing yet more stuff. But if you already have a laser, then it should be possible to get it working as required.

Thanks Umadog for some extremely useful info in both posts, I've spent 3hrs centering the laser and got it to 5-6mm accuracy over 4m. I can probably squeeze a little more, but calling it a night. Will have to leave the mirror until tomorrow, but getting there slowly!

[Moonshane]

it's more important that the primary does not move laterally. the clips can help with this but sometimes a packer, a blob of silicone (between mirror side and upright of clip) or some sort of threaded solution to allow adjustment are more ideal than tight clips.

if you have a cheap short cheshire like this

then I also have one and find it the best for fine tuning the final primary adjustment as there are no cross hairs.

[SnakeyJ]

it's more important that the primary does not move laterally. the clips can help with this but sometimes a packer, a blob of silicone (between mirror side and upright of clip) or some sort of threaded solution to allow adjustment are more ideal than tight clips.

if you have a cheap short cheshire like this

then I also have one and find it the best for fine tuning the final primary adjustment as there are no cross hairs.

The very one Shane - complete without cross hairs. An art I haven't mastered with this, but will give it another go.

[SnakeyJ]

Out of interest I removed the mirror cell this evening. One of the retaining clips bolts was almost seized solid and two others were well on their way - some careeful persuasion got the bolts out without sheering. Here's a pic on the clips and bolts prior to cleaning, showing the corossion:

A gentle clean up with a fine wire brush, followed by a little red lithium grease should prevent a repeat. Though the quality of the bolts is very poor (particularly weak/badly cut heads) and I will look to replace these in the longer term.

I hadn't noticed before, but the clips pull the mirror back on to cork pads around the mirror cell and seem pretty solid once adjusted with a shim of light card to prevent over tightening. Once done the rattle was cured and there was no obvious lateral movement of the mirror.

I'm not so sure on the positioning of the centre spot on the mirror though and will make up a pattern to check and replace if necessary. That said with the mirror mounted back in the scope it only took a few minor tweaks on the primary and the collimation looked pretty reasonable - the seeing isn't great tonight, though a quick star test on Vega looked pretty good. All completed in time for the clouds to roll in and obliterate 90% of the sky, but Sat24 looking good for 1-2am so will probably hold out for a little longer.

Cheers to everyone for good advice on this - looks like a decent cheshire is on the to buy list now.

[umadog]

Looks like some sort of loctite-esque material. I had the same on my Orion scope.

The position of the centre spot is worth checking. My Orion was off by about 1 mm. I checked it with a Catseye template and replaced with a Catseye triangle. It's also possible to check the position photographically. I did this after replacing and found, IIRC, that it was right to within about 200 microns.

[triton1]

I had some play in one of my clips on the 200p.Tightened the screws up with a lightweight cigarette inbetween mirror and clip.Stopped tightening when paper started to bind.The paper thickness measured .0015 of an inch so should be accurate enough

regards Jonn

[SnakeyJ]

I brought myself a long tube cheshire from MA, printed out Mels excellent guide and spent 3hrs last night getting frustrated as I couldn't get the text book view of the primary with all three clips ) even switching back to the short tube cheshire. However tonight it all worked a treat and I had the scope collimated nicely within an hour, including refitting the secondary with the nylon milk bottle washer mod ( which really does help with mirror rotation ). The main stumbling block, aligning the secondary with primary mirrors was very straightforward once I dispensed with the cheshires and used the Mk1 eyeball through a 2-1.25" adaptor. Once this was achieved I got a very quick and good alignment with the long tube cheshire, confirmed with laser - repeated the secondary and primary alignment with cheshire and laser. Finally I used the barlowed laser technique once the scope had cooled down outside and a couple of star tests (up to 200x)

Visual results were excellent with my best view of Saturn at 200x with four pin sharp moons on display.

Flipping over the meridian my star tests on Vega and Arcturus were very clear and concentric.

Thanks to one and all for the help and advice on this - very much appreciated.

Final note - my cheap nasty short tube cheshire is definitely cheap and well out of alignment.

[Moonshane]

hi Jake

to see the classic view of the collimated optics through a cheshire/sight tube combo on a scope faster than about f8, you need to use an extension tune. otherwise you won't see the whole of the secondary down the cheshire/sight tube.

also, when you say you plastic cheshire is out of alignment, what do you mean? it's only the central hole that matters really when using it to align the primary (that's all I use mine for). is the hole not central?

that said, if you are getting good sharp images at 200x then your collimation is not that far out if at all.

[SnakeyJ]

Thanks Shane - I tried racking both in and out with no joy, but will try with some extra extension.

Re the short tube, from the top the hole appears well centered, but the light path is not square with the body of the cheshire! Rotating gives a pronounced cone effect which I don't get with the laser or other cheshire.

Think this one is destined for the recycling

[CedricTheBrave]

Just out of curiosity how big is the gap between the primary and the base of the tube?

on mine it looks to be about 15mm with the locking screws nearly all the way in! I struggle to get pin sharp images and could not see the moons of Saturn even with a 2 x Barlow and 10mm ep night before last!

Think I will run through the hole set-up again.

[Moonshane]

no idea about the gap dimension but you should see Titan at least in your finder scope. use lower power and you'll see them. then higher power if required.

[CedricTheBrave]

have to say I do find the finder scope to be very good,

Need my eyes fixed as well though roll on the 15th