GSO 8" Newtonian mods

[multiweb]

There was a thread here (http://www.iceinspace.com.au/forum/showpost.php?p=1021686&postcount=1)recently asking about a GSO 12" newt. I think this post will be relevant to the 12". I have a GSO 8" and had issues with the scope not holding collimation. The problem is in the primary cell design. What works for smaller scopes doesn't necessarily scale up very well as far as weight is concerned. My 5" Celestron had the same cell design and never had an issue. It worked fine for a small lighter mirror. The mirror in this 8" newt pretty much goes where it decides to go when the scope is pointing in various part of the sky and the so called locking knobs are useless.

The first thing I did was to replace the collimation knobs and locking knobs with stainless steel Hex screws. I also drilled and taped the cell to move the locking screws right next to the collimation ones. The original locking knobs were half way in between each pair of collimation knobs and didn't do much. I also counter-sunk the tips of the locking screws inside the mirror cell so they also stop the cell from rotating of moving sideways when it's locked.

Next thing was to modify the retaining clips. I bought the scope second hand and the mirror was siliconed to the cell with clips removed. The problem with the clips is that they are rubber and they pinch the mirror down against the cell. When they're tight enough to hold the mirror they pinch the optics. When they're just right as not to pinch the optics then the mirror is too loose for its own weight and shifts sideways and even tilts.

So I changed the mounting based on what a few others do on the web. I liked the protostar cells system and that's what I tried to replicate here. The finished product looks very simple but it was a fair bit of stuffing around and trial and error to make it all fit in the annulus between the mirror glass and the tube. There is really not much room to spare in there. The concept is a no brainer. The two original screws were cut down and small alu brackets (light color) hold the mirror centered. They hold the weight. The black brackets are the retaining clip. So they just stop the mirror from sliding up. There's a bit of rubber to make contact with the edge of the glass but that's it. There isn't any pressure. Everything is adjustable.

Now I can move the whole tube in different positions and the collimation holds. I also have no pinched optics, so best of both worlds. I assume that could be easily replicated on a 12" providing they use the same cell design or similar to the 8". Probably easier as there will be more room to play with. :thumbsup:

[mental4astro]

Nice changes there, Marc.

Have you ever considered using spring washers instead of wire compression springs? I've moved away from wire springs as they are directionally unstable, and introduce unwanted lateral forces to the cell that can shift the cell laterally. In the picture you've uploaded, I can see the spring is terribly distorted as its been loaded, and highly crushed. Spring washers are unidirectional, meaning that you not only don't get lateral stresses, the entire strain on the spring is being made use of, and spring washers are shorter vertically so the mirror ultimately sits deeper inside the mirror cell.

Depending on the way you arrange the washers, you can maintain the same load capacity, double it or treble it just with their combination. Yet their vertical dimension is bugger all. To give you an idea, the 25mm spring washers I'm using on a 12" scope I'm building are 0.5mm thick, and a total height of less than 1.5mm. Yet they are rated at 25kg for a 75% dimension compression. Sit another spring washer inside it, and this becomes 50kg. Have two washers touching lip to lip or hub to hub, and it is still only 25kg.

Something to consider.

Alex.

[multiweb]

As discussed it's a great idea. There are so many flavours of spring washers which ones do you recommend? Do you have a pic or link? I'll certainly swap the existing springs around. :thumbsup:

[Shiraz]

good info there Marc - thanks for posting.

Really good idea to countersink the locking screw contact points and have now modified mine.

Interested that you are getting good results with 3 side contact points. I found that I got minor astigmatism when I used a fairly similar setup - worst when scope was pointing such that a contact point was directly under the mirror. went over to 6 silicone dabs around the edge, but there was still a tiny bit of annoying residual astigmatism (the silicone apparently expanded/contracted when setting and put the glass under uneven tension/compression). Have just removed the silicone and am in the process of modifying the mirror holder to put in 6 adjustable nylon screws on extenders to get them up to the mirror CofG - but as you say, there is not much room in there.

I guess we could just buy fully sorted scopes, but where is the fun in that? Regards ray

[multiweb]

Hi Ray, check your secondary as well. Astigmatism is almost always caused by the secondary offset or the surface not being flat so it may be stressed by the rubber gel that's bonding it to the plastic container. I cut mine lose, then glued some cork under the retaining clip and packed the sides with paper shims. Then I put only one dab of gel grip on the spider side of the plastic. So it's just 'hanging there' by that and the front clip.

The primary is too heavy to be siliconed I reckon. I'm not sure it would deform the figure as to create aberrations. It's still a thick chunk of glass. Although rubber gel as an incredible pull when it cures. I managed to deform the primary on my C11 by putting too much gel at some stage and that was much thicker. Since then I don't take chances with glass and tend to fit it tight but not bond it or restrict it in any way. The 3 unpainted bracket are what's holding the mirror weight. It makes contact with the glass sides. You can just rotate the mirror but it will not move sideways and it will be very hard to remove it up unless you're dead square. Like pushing a bearing on a shaft or inside a bush.

A good trick if you still have astigmatism is to rotate the primary 120 degrees, then check again. You'll find a position where it's gone. Mark it, then always remount your mirror in that position. Also mark the cell so you always match its orientation to the tube.

[Shiraz]

thanks for the advice Marc. The first thing I checked was the secondary - without any edge support on the main mirror, there was no astigmatism, so that ruled that out. Seems that even that much glass is pretty bendy when talking in microns :). I was still surprised though that cured silicon could distort the mirror so much, although I recall a post by Mark Suchting on a distortion problem he had with silicone attachment of larger mirrors. Regards ray

[multiweb]

When I got the primary and secondary back from coating on my 5" I freaked out. So bad I couldn't even focus. Turned out to be the gel. I had just glued the whole secondary to the alu holder. Mark S. told me only 3 dabs 1 to 2mm in diameter and use lollypop sticks as spacers then let cure. That did the trick. But I was packing sh|t for a little while before though. :lol:

[mental4astro]

The place I've sourced my spring washers is ASI Springs in Victoria (http://www.asisprings.com.au/default.aspx). The fellows there have been so amazingly helpful. Called them out of the blue and they nearly fell over backwards when I told them where I was to use them. But, they were so patient and helpful. Bloody awesome stuff, :thumbsup:.

What you need to do is determine the outside diameter of the main collimation bolt. You need to use an inside washer diameter that is snug, but not tight. Too loose and the springs won't stack neatly. Too tight and they'll bind on the bolt. How strong depends on your use and preference. I don't have anything more to help strength wise as I'm experimenting too.

[multiweb]

Thanks Alex. Will contatc them. I've been thinking putting some at the back of my C11 as well but there's not much room.

[Steffen]

Very interesting, Alex. I had a look at the site, those disc springs seem ideal. I thought by spring washers you meant those washers that are cut open and twisted…

How did you go about picking the right strength for them? What is a good Newton value to shoot for if they are to carry a mirror cell? I suppose it should be at least an order of magnitude larger than the weight of the cell (at least if one wants to do away with locking bolts)?

Cheers
Steffen.

[multiweb]

I think the Belleville washers are the best for this. I'd say you want the minimal amount of axial shift at the time you're collimating and enough sprint to push back but once the lock screws are in the springs become irrelevent because all the slack is taken off.

[Steffen]

I would prefer not to use locking screws at all, hence I'd rather have strong springs.

Cheers
Steffen.

[multiweb]

Is this for a large Dobs?

[Steffen]

No, an 8" GSO :)

Cheers
Steffen.

[multiweb]

Ha ok. Same scope then. Let us know how you go with it.

[multiweb]

Last night was ok in Sydney as most of the smoke was out west, thanks to the easterly breeze we had all day. Not so today... it's all back on top of us.

Out of the box the scope was collimated pretty close. Everything is centered, flats show that the illumination is centered and uniform so I'm pretty happy with that.
I thought I had the spacing correct for the MPCC but it doesn't look like it at all so I'll have another go and tweak that to get rid of the field curvature.

But everything else looks good so far. One more tick in the win column. :)

[multiweb]

Last night did a little more testing and used another coma corrector with a different spacing. Less reflections. Field curvature is improved. Still need to increase spacing a tad. I as also tested the automated focuser which helped a lot. Seeing was horrible but still pretty happy with the field improvement. Getting closer now. :) Most importantly the scope survived a trip indoors and back out without losing collimation. So the primary cell works.

[multiweb]

Moved the MPCC out by another 1mm and I'm now pretty close. Field curvature has dropped a lot. I reckon another half a mill and I'll be spot-on. I'm about 1.5mm past the recommended spacing right now. Pretty chuffed with the primary cell. Collimation hasn't moved in a week now and I have been handling the scope back and forth a couple of times. Larger version here (http://www.astropic.net/astro/GSO8_ir_m8_600mm_PM_B_HD.jpg).

[RobF]

Great work Marc. Sounds like you're stretching every bit of performance out of her!

[Tandum]

Marc, there have been many threads on these so called imaging scopes. Every person I contacted was having issues with the primary cell. My issues where at the front end instead. In my case the primary worked fine. I did have it out of the scope while fault finding and maybe that reseated everything and got it working right?

In my instance the spider moved in/out of the scope as the scope moved.

All fixed and I achieved some of the tightest images I've ever had from that newt. Very sharp indead. 5hitbox of a focuser, but who else makes a 3" focuser for a newt? Baader is 3.25", FT don't do newts, ummmmm.

I figured I'd just get out of it completely.

[multiweb]

Yeah I think it'll do nicely and give me some very nice pictures. It required some stiffening mechanically but optically it's fine. I've just sorted the OAG last night so the next test should be on the money :)

[Lee]

Looks like a good idea... will have to try this....
This week after my abortion of an attempt with CCD Inspector, I collimated with the laser, then tightened up the locking screws, only to get this rubbish....

Everything looked better after loosening off the locking screws....

Are RC's this much trouble??

[multiweb]

Is that a corner at 1:1? It doesn't look that bad. Are you sure it's not field rotation? What was the sub length. GSO RCs use a corrector as well I believe so maybe spacing needs tweaking?

[Lee]

1:1 near centre frame.. sub length 10s... I think the tightening pinched the optics somehow??

[multiweb]

ah... yeah if it's center frame then you have an issue. What does the out of focus pattern look like, both in and out of focus? Is it oblong?

[Lee]

I haven't checked into out of focus patterns.... Although during focusing at the start of the night, the donuts are round (fairly)...

[Lee]

Well... thanks to this helpful post I've installed some stiffer springs, some C-700's from CSC spring company (Bunnings have the range). They really hold the mirror cell a lot stronger than the standard ones, it really takes an effort to compress them, the standard ones compress under the mirrors weight....
I've left out the locking bolts, and am getting round stars across fields from one side of the sky to the other so far....
Thanks for the post.... :)
I still think I need a better spider....

[cometcatcher]

Interesting thread.

Way back when I was making newtonians and mirror mounts, I would use valve springs from the car wreckers. This worked for all scopes from 6" to 16".

I am thinking of getting a GSO 8" F5 for imaging. They make these things much cheaper than I could build one.

[Wavytone]

My first 8" Newtonian used valve springs in the primary mirror cell, but like most things with springs it never remained collimated for long as the wing nuts on the back would shift. Not a great idea IMHO.

In the end I removed the springs and used lock nuts on the inside of each threaded rod, opposing the wing nuts. A bit more fiddly to adjust during collimation, but once done it DID remain collimated - well enough that I could remove the entire cell from the tube to clean the mirror, put it back and no adjustment was needed.

[Shiraz]

just to say thanks. following earlier discussion on this thread, decided to re-check my secondary. The darn thing has developed astigmatism and there is now nothing I can do to get it to be flat - guess it must have been made out of poorly annealed material and recently decided that it would de-stress itself. Anyway, your thread has been very helpful :thumbsup: regards ray

[multiweb]

That's really unusual Ray. Never heard of plastic deformation with glass. I could be wrong but I've had stressed glass always going back to its original shape in the past. Have you tried to stick it in the freezer for a couple of days?

[Shiraz]

Beats me?? I used this secondary in a planetary scope for a couple of years and would undoubtedly have noticed if it had any problems - it certainly does now though.

I am not intending to stick it in the freezer - will use the flip top receptacle next to it :hi: - only $50 for a new one

EDIT: Found this on a mirror blank site http://castle-emerald.com/ This is important because when glass is manufactured, it invariably includes internal stresses (which are impossible to avoid) and as these stressed regions exert internal pressures within the glass, they act to deform the mirror over time. During the re-annealing cycle that our blanks are subjected to, these strains are allowed to ease leaving a blank that will not deform over time.

Maybe it can happen - hope the primary is made of sterner stuff