Today I decided to take my Mak-newt fully apart and see what on earth is going on in there.
Found and fixed quite a few strange things.
One thing I observed was that the mirror spot seems to be way off center. Would there be a good reason for this? What's the best way to fix it? i.e. how to get off the old spot, what to use for a new spot and how to get it dead center.
Culprit and self portrait attached. You be the judge.
Hi James, the best way is with a sheet of paper......... http://codeidol.com/other/astronomy/...t-Your-Mirror/
I doubt you could collimate your scope with that spot off center (unless you do it purely by star testing).
I would love to know what other strange things you found with your Mac Newt.
I would love to know what other strange things you found with your Mac Newt.
The 'fix up the mak-newt' project just started today, but I suspect the mirror spot issue might be the root cause for the secondary adjustments being maxed out.
There was a loose screw in the focuser, and it was a fair way out of alignment too - although it might have been in an attempt to compensate for the mirror spot somehow.
The tube is carbon fibre and the focuser is Aluminium - so no problems there, but many of the hex screws and washers have rusted and they don't look too good. I'd love to replace them with some shiny new ones if i can find any. At least I know what all the nuts/bolts/screws do - it should be reasonably easy to get it working at its full potential (finally). A cooling fan might be in order too.
I was really surprised how heavy and steeply curved the corrector plate is. It's more like a corrector brick. Now I know why they don't make 16" mak-newts.
Yes, definitely re-apply the ring binder center spot.
Go to a fastener specialist shop and replace all your rusty cap screws with stainless steel ones .It does beggar belief that premium scopes can be built without stainless steel hardware.
To mark a new spot dead center in a Newt.mirror, cut a square piece of paper the same size as the mirror. Fold the paper twice and cut out a small piece, perhaps 5mm, in the corner that forms the centre of the paper. Place the paper over the mirror and, with a dark coloured marking pen, make a dot through the hole that you cut out. Perhaps 10 mm in size.
Ken
Last edited by solissydney; 31-08-2009 at 06:22 PM.
Look what I found on the reverse side of the ring holding the corrector / meniscus. (the other side doesn't have any printing) I always suspected it was a prototype, now I guess it's confirmed.
I tried to clean it up but it still doesn't look very good. It needs to be re-anodised or maybe just spray painted black.
Have you got and LED torch ? Reflect an image of the torch back on to a piece of white card placed next to the torch in the same plane as the LEDS. Measure the distance from the mirror to the LEDS and halve it for the FL. 750mm thats marked would be the nominal focal length for the Newtonian the mirror was originally destined for , so your actual FL could easily be 730mm.
I got the new stainless steel screws today. I dropped them off to be black powder coated, along with the aluminium ring that holds in the corrector / meniscus. Since I got the scope for free, I figure I might as well fix it up properly.
Anyhow the lady in the powder coating shop recognised what it was straight way and showed me her nice 110mm (I think) William Optics scope she just happened to have with her. That was a bit of a pleasant surprise. My parting instructions were to make my parts look as nice as her scope. Let's see how they go.
With a bit of luck I hope to have it all finished by the new moon.
Time for an update!
The powder coating is all done and it looks great.
But then I hit a bit of a road block - I couldn't manage to get the collimation right with the laser collimator. I splashed out and got a quality 2 inch sight tube/Cheshire combo, auto collimator and new 'hotspot' center spot.
I was also suspecting astigmatism and mirror movement as well so I also removed the mirror clips and mounted the primary on 3 small pad of silicone goop.
Bingo!
I did a quick test the other night with almost full moon - stars finally look good near the corners. Corners of this test image are magnified 2.5x. Taken unguided with a 500D prime focus, no flattener or coma corrector etc.
I'm not sure what is causing the vertical spike on Alpha Centauri - maybe the focuser is reaching down inside the OTA too much? I'll have to look into it.
don't know whats happening with the diffraction spike, though your top two corners seem flared quite substantially compared to bottom left, so your collimation is still out to some degree.
Im not sure about Mak newts, as they have the corrector. if the corrector isn't lined up properly your going to have problems.
Using the cats eye gear to get good collimation for the mirrors is easy business, but i think what you might have to have a look at is doing a star test to align the corrector plate. I am only guessing as i have never owned one of these.
Your guess is right - according to Telescope-optics site, the tilt of the corrector is critical (last sentence on that page). However there isn't any way to adjust it (other than shims) I and can't think of any other way to test it other than a star test like you suggested. Luckily the mechanics of the corrector mount looks good at this stage, so I'll leave it be for now. I'm actually more worried about 'pinching' the corrector - it's fully set in a chunk of aluminium and it would be very easy to over tighten.
I wish I could say the same about the tube! Note to all ATM'ers out there - 2mm carbon fibre tube is **not** thick enough for the base of a focuser for imaging. When I have the catseye installed, I can flex the tube near the focuser with a reasonably small force (say equivalent to a few KG camera hanging off) I can see the hotspot move out of collimation. I'm going to have to reinforce the tube under the focuser. This is a real shame because I'm going to have to do it with Aluminium.
ahhh now i know where your coming from with your flexure. Cats eye collimation gear is very sensitive to movement of ANYTHING which is why i went to town on my parts for that very (and legitimate) reason! (I too have a auto collimator + calibrated chesire with hot spot)
To easy your worried soul i would say that a 6 x 25 mm flat bar of ally rolled to a slightly larger diameter of the tube so that when you bring it together the ends just meet and it is applying force out would work a treat, then drill and tap say 6 pan head screws M4 x 6mm and that should sure up any flexure without weighing the whole thing down like a brick!
The carbon fiber should really be a composite design with CF and a Layer of FG and then CF again. with the twill offset so its not falling the same directions every time.
That is just some thoughts from my end of the stick hopefully you get some decent results.
(the other side doesn't have any printing) I always suspected it was a prototype, now I guess it's confirmed.
the dude who got it got a fantastic score!...
now i know where your coming from with your flexure. 
