12" Cassegrain build complete

[skybadger]

Hi all

I have just completed my 12" Cassegrain ATM build so I'm fairly chuffed.

This is a diary post of photos of the build. 

I must admit that the scope is  build from existing optics from George Hole and Sons of Brighton. The primary is a 12.5" f/2.7 and the secondary is 92mm diameter and its CoC is unmeasured. I bought these second hand from a chap on Astrobuysell for £250 two years ago.

So this has been a bit of a shot in the dark since the only way to test the optics is really to make a workbench to hold the optics, which is as near as counts a telescope.

I took the idea for a truss tube scope from a chap n the internet who had built his Newtonian in a similar way.

A picture of the original telescope

i'm fairly sure this shows a secondary mirror using a slider at the upper end on a single arm. The focuser is a fixed tube and I had the mirror mount and it was a 2" thick piece of wooden board with 1/2 bolts, it was massive....

A picture of the initial frame in a trial assembly.

You can see the 1" diameter 14SWG aluminium pipe being used as the trusses in a hexagon straight configuration. The whole point is to allow the stiffener sections carrying the mirror, the saddle and the secondary respectively to move relative to each other so I can setup the system.

A picture showing first trial fit of the saddle parts

The saddle parts carry a dovetail either side across the scope. This one is fitted with inserts on the inside surface (to prevent pull-out) and drilled for a Losmandy dovetail. There is a second stiffener to go on the top edge which is all rebated using a router and glued only, no  screws. 

For a sense of scale, the overall length is 85cm, ID is 13" and OD 17".

More to come.

Mike

[dweller25]

Mega

[DirkSteele]

Fantastic job! 

[chiltonstar]

Fantastic! I can't wait to see the results.....

Chris

[WaveSoarer]

Excellent. What a great project.

[skybadger]

Cassegrain part 2

Part 2 is all about the mechanics  - The mirror mount, the secondary and the baffle.

The mirror mount repeats my strapping mount from the 12" Newt - 4 blades made from 15mm steel strapping terminated at one end in a M8 bolt, split and screwed to the strapping and bolted through an

L bracket on the secondary frame. The central boss was turned and slitted on 4 side on the lathe and then 2 holes let in to each slit from the side 10mm apart, to screw the blades to.

The boss is drilled 12mm through, reamed for a 12mm diameter stem to slide through snugly. I started with a wooden disk for the mirror mount itself with an alloy plate holding a captive ball rod

end as a central pivot. In the end I replaced the ugly square plate with a turned full diameter plate to proved the surfae required for the adjustment screws.  Since the secondary boss was

undersized compared to the mirror and slit in quarters, I found it easiest to turn another alloy circle slightly larger to hold the adjustment screws, tapped m5 at 120 degrees aparts.
This just screwed to the central boss.
On my newt I had come up with a mechanism to allow the mirror to be screwed in and out, axially along the tube, for height adjustment. For this one, I never got round to that and just fitted a

spring to the outside to pull the stalk through. The adjustment screws then determine the distance of the mirror along the tube. This does mean collimation might affect the image scale by changing

the distance. I'll probably fix that in the long term. 

For the attachment of the secondary mirror I took a strip of 1mm Alloy sheet and beat a narrow lip on it around the edge off an alloy pipe section and screwed it to the wooden disk to hold the

mirror itself. The sheet had a thick plastic coat on the inside to lay against the glass and the lip only lies over the unsilvered rim.
 I've since flocked the outside of this with flocking sheet to preent stray reflections.

The primary mirror mount is fairly standard. I used plop to calculate the dimensions for a 9-point support in three centrally-supported triangles. Each triangle rested on a round-topped coach bolt

which I centre-tapped for a screw and spring washer. The triangle was screwed to the top and thread-locked, with a small bit of play availble for tip-tilt. This resulted in low profile supports. I

even had to file the head of the screws flat to get the clearance. Each triangle point was topped with self-adhesive rubber feet whihc seem to work well.
The mount board is attached to the mount frame by a backing board. THe backing board is pierced for ventilation and provides 3-point collimation, tightening against sprung bolts through the board

using wing nuts on risers to not interfere with the adjacent set screws for locking collimation. The lock nuts, since they bear on a pywood disk, have the acorn locknuts thread locked on top.
A similar arrangemnt is in place for the focuser mount. This mounts off the forward face of the
This arrangement is also used on the focuser mounted to the backing frame using a circular alloy disk I turned from sheet. The focuser is a Revelation Schmitt/Cass focuser which comes with a

mounting consisting of a circular piece with a dovetail lip and clamping screw system to provide rotational adjustment. The curcular piece normmally screws in to the back of Schmitt/Cass. I turned

the skyward face of this to hold a 63.5 mm OD Aluminium pipe as the primary baffle as an interference fit and also tapped three holes near the edge to mountit to the pate. The plate is on a 3-

screw push-pull system to allow the focuser and primary baffle with it to be aligned tothe secondary. The primary baffle is fitted by hand and has no further adjustment so it better be right !

I did have to come up with a fairly complex way of centering the mirror and clamping it to the backing board. This was due to narrow clearance through the spacers and the need for a strong

retaining mechanism to hold an 8 Kg mirror. The clips are IKEA cupboard wall clips with felt glued to the inside lip edge. The entire L-bracket slides radially along a nylon t-bolt track, using a

screw to pull it in toward the centre. More nylon screws limit the mirrors movement while small nylonset screws adjust the proper spacing between bracket and mirror edge so the clips don't overlap

the edge of the mirror too far.   

[skybadger]

Part 3 of 5

Taking a step back now, the dimensions of the telescope were driven by the optics and the saddle plate sizes. I modelled the optical dimensions and baffle dimensions using some assumptions for the secondary based on its diameter and the optical equations for a classical cassegrain. These equations relate the image size, magnification of the secondary, back focus distance etc together to describe the entire optical system. The model that best fit was a 12" f/10. That gave a 625mm distance between primary and secondary and a back focus distance of 290mm. However the back focus distance, required to put the focal point beyond the focuser for imaging, was an input rather than an output, more of that later.

The baffles were calculated using Mike Lockwood's (see Cloudynights.com ATM Cass forum) calculator, hence the length of the primary baffle at 380 mm and a requirement for a 100mm diameter secondary baffle, the latter so far  not implemented.

Both baffle dimensions, inter-optics distances and tube sizes were calculated together in a spreadsheet to keep track of the Mass, Centre of mass and relative positions of components around the saddle plate. I used the goal-seek capability to adjust components distances from the saddle plate centre as the system fixed centre of mass to work out where everything needed to be and still function optically. The primary variable for this is the distance the secondary mirror lies forward of the saddle, move that mirror and you move the entire optical system with relation to the saddle. That might explain why everything optical is fixed onto the tube frame and the optics on their frames can move independently up and down the tubes or as a unit by pushing the tubes through the saddle to balance the overall CofM. The primary mirror for instance was almost inside the saddle due to its large mass.

[skybadger]

Part 4 of 5

The assembly of the scope took most of the time: a lot of evening and a fair few weekend sessions. To lock the frames onto the tubes I had to make 24 tube clamps that would screw to the individual frames at each tube hole and lock it tightly in position. These are made from 24 alloy offcuts that are faced on both sides, turned out to 24.5mm, slit and then mounted on a jig I made to locate and drill three mounting holes all identical. The first ones were blind tapped but then I got in a flurry and forgot to not drill through and so don't look as good as they are tapped through. I started the clamp holes on the curved surface using a milling cutter to get a flat surface and then swapped to a drill of the same dimension to drill clear to the slit depth and finished with a tapping drill to drill a further 10mm for the clamping thread. Each set of 6 is then clamped to the same spare length of 1" tube and surface turned in the lathe to give them a good finish.
The same jig as used to drill their holes is then also used in the tube holes in the frames to drill through at the correct dimensions. I particularly didn't want to use screws in the plywood so close to the edges and the screws compress the plys between them and the clamps which I thought was a good thing as it will improve the stiffness and resistance of the wood to separate as it is moved up and down the tubes. The tubes themselves are fitted with tube stoppers and given a short hand sanding to even the finishes off.

Mirror frame with cutouts.

Each plywood frame was washed in thinned varnish to penetrate the wood for preservation. The mirror carrier was also washed in black stain on top of this varnish.

The nylon mirror clamp t-tracks were screwed directly to the wood.  I coould have through-bolted and may yet change. Through bolting allows me to tighten them up after setup with the mirror in place. Screws don't.
The collimation bolts are coach bolts with low profile heads, through from teh front of the mirror carrier, through 8mm thread inserts, through 1" springs and finally through the carrier frame to wingnuts.
The collimation clamp screws are simple set screws throuh the carrier and m4 inserts on the mirror side to prevent pull-through. 

Assembly of the mirror end has been more difficult than expected.
First I realised that the mirror clamps line up perfectly with the tubes, making it very difficult to tighten the radial clamp screws and mirror clamp screws.
Next the focuser mount and collimation plate is very difficult to put in place with the mirror carrier in place, so to add and remove it means removing the mirror carrier.
Finally the mirror had too be manouevered through the tube on to the mirror carrier since I had to tap the mirror carrier frame on with a soft mallet to get it where I wanted it and didn't want to go through the pain of removing it. 

First assembly - view down the scope without mirrors onto mirror carrier and focuser plate showing tube clamps:

So, some changes to make :
1, ensure that the mirror support triangles centres are offset from the hexagonal tube pitch to ensure that all the mirror clamps are accessible - the space betwewen the triangles is where the clamps must live, hence the relationship. This means rotating the mirror backing plate by 15 degrees.
2, Make the frame fit to the tubes more accurately. I cut and drilled all frames separately using geometry for markup. It didn't work well enough to get all holes and centres perfectly aligned. The mallet provides gentle persuasion. They need drilling all together as a single unit and all the other piercings marked out before the centres became inaccessible by removal.
3, The centres of the focuser and baffle are guaranteed to be aligned, but adjustments have had to be made to ensure that the centre of the mirror can be lined up with the centre of the baffle.
The lack of clearance of the mirror and carrier clamps means the mirror needed to be threaded through the tube and lowered over the baffle ontot the carrier. Not a fun job.

However its done, the clamps are on, the mirror all works and is relatively centred, roughly collimated and now the optics can be checked.

All assembled:, without secondary

More to come but getting there..

[Gina]

Wonderful

[faulksy]

thats lovely

[skybadger]

Part 5 of 5 but still no conclusion. You know how HHGTTG was a trilogy in 4 parts ? Somehow this is a quinology in 7. I feel like Robert Jordan or Frank Herbert.

So I assembled the optics according to the spreadsheet spacings and put an eyepiece in.
Nothing but a fuzzy blur. That's not disappointing  - yet. Focus should have been about 50mm past the end of the focuser at least, but using the afternoon sun, was definitely within the focuser body. One hand used to tip the scope on the black and decker and the other to hold a piece of paper in the beam.

View down the front with optics:

So I moved the secondary mirror frame towards the primary from 625mm ( modelled spacing required) to 585mm in increments of 5 mm or so until I got the image where I wanted it. A change of about 6%. This means the focal plane is now outside the focuser and eyepieces need a 40mm extension or so to come to focus on objects at infinity.

Focuser on rear frame:

My worry here is that Cass optics are designed to work at a specific spacing. I might have been able to specify the spacing had I the optical measurements for the secondary mirror. Since I didn't the next step is to measure them. Measuring a parabola or hyperbolic with convex surface without its mating surface is hard unless you directly measure the surface using Foucault through the back of the mirror. This normally requires the secondary mirror back been polished. But its not. ( aside: how clear does it really need to be to do this ?)  The test  remaining to me then is to check the image quality with a Ronchi grating against a real or synthetic star and adjust the secondary-primary  distance until the lines are as straight as they can be. If that means that the image disappears back inside the focuser then I will have to come up with a different focuser solution.
On the other hand, the view of the distant trees so far has been great, not showing any obvious distortion. I should really get a foucault-gram of the primary. I have been trying but I need to put a webcam on it to get my eye close enough.
I will mention that the mirrors also have a star etched into their sides, so I have also made sure that these line up in case of this marking the optimal rotational alignment. You never know. Another thing to check - whether rotating the secondary will improve the image by cancelling wavefront errors or make them worse...

Scope all flocked with secondary and baffles:

[skybadger]

Part 6 of 5:

Finally I have tried this OTA on two mounts - a Vixen GP-DX rated for 7KGs load. This OTA must weight about 15! for which I cannot get the counterweights to balance it, unsurprisingly. It does look good though.

OTA on Vixen:

And from the user end:

 

I then took it into the observatory and tried it on the static mount. I found a couple of things: the Altair Losmandy dovetail bolted to the OTA saddle does not sit well on the Altair dovetail receptacle on the mount - the mounting holes in the dovetail are not counterbored deep enough for "standard" Allen cap screws. I even turned down the heads until they did lie flush and then there wasn't enough hex shape broached into the metal to take a hex key. I shall have to increase the counterbore depth on those holes. Bit of a design flaw there or I am getting something wrong ?.
It did look good though.
<no picture available>

Final design flaw.
It doesn't fit !
Even the short length of a Cassegrain is such that it interferes with the dome roof on transit and on rotation around the dome. I shall have to increase the dome height or reduce the pier height. I was so enjoying have a tall pier so I can stand observing comfortably underneath the existing Vixen 8" cass and 5" refractor combination. Its just as well I am looking for a new dome too, that way I have options to explore.

[skybadger]

Finally:

Part 7
I finally put the telescope in front of a Foucault tester last night. My tester provides moving slit and ronchi testing capability. I mounted the OTA on a table facing the tester and moved the slit of the tester to the centre of curvature of the primary ( about 1.8m away) and completely ignored the secondary. As far as I could tell under Foucault, the shadow is a straight line with no hills or valleys, as it should be for a good spherical mirror.
I need to get better at this to allow me to take measurements and confirm this quantitatively though. For that I will need a Coude mask or a pin strip and lots of practice.
Under ronchi testing, the lines were wide and contrasty owls eyes either side of the central hole with no apparent hooks or kinks near the outer edge (TDE) or inner edge. I tried to get a picture with the camera but couldn't get close enough. I'll try again with a small webcam later.

Summary and next steps
I've learnt a chunk with this project.
1, The importance of marking out absolutely everything before you start cutting/drilling since you lose the reference centres very quickly and working backwards to find alignment points is hard.
2, Using the models to fit the optics was essential. It told me what would work and what wouldn;t before I cut material
3, I need to learn to tap thread straight. On a flat surface. Doh!
4, Try the space model or early trial assembly on the mount  as early as possible. Given the optics size, the scope can only be one size, so I don't feel bad about it not fitting the dome currently. i'll fix that one way or another.
5, Clearances - a trial assembly wil pick up these issues but sometimes you build on the fly, like the mirror supports and early decisions don't allow later changes. To get clear access to the mirror supports I need to rotate the entire rear mirror mounting frame by 15 degrees.  Since I had drawn it out in cad throughout the design stage of this I should have picked that problem up earlier.

As you can see in the last photos I flocked the primary baffle inside and out and the secondary outside, as well as changing the secondary support slightly to use a spring on the outside to pull it away from the mirror onto the adjustment screws.

For flocking I used the velvet self-adhesive material from Wilkinsons. Outsides are easy and very trimmable but to get the inside layer to stick I had to roll the velvet back on itself taped to a water pipe down the center to ensure even adhesion over the 42cm length. Still it worked and now the internal reflections are gone . The final thing will be to turn some internal baffles to prevent glancing reflections and define the image aperture. Once I have adjusted the optical model to reflect reality, I should be able to calculate the size and spacing of those using a bit of geometry. I'll just turn some tight fitting disks and bore them out to a knife edge lip.

Collimation for this beast means aligning the focuser and secondary boss together along the optical axis , aligning the secondary optics to return an image centrally to the focuser and then adjusting the primary to be normal to the secondary. Even using a synthetic star, it will need to be 80 yards or so away to achieve acceptable focus distance without extraordinary extension barrels.

Next: First Star light

[skybadger]

Part 8 of 5

First light!

I have this on the mount finally in the observatory.

I had to build an observatory to fit mind. That ill be a different thread...

First light was to get the scope on the mount in a vaguely balanced way and get initial collimation and focal point measurements down.

First result: distant trees are not at the same focus as stars.

2nd result: the revelation focuser with its 50mm extension is annoying if the focuser only has 35mm of travel. ie there is 15mm for which you cant get an eyepiece in focus in either combination.

3rd result. inside and outside stars were rotten. I had done an initial alignment using a Cheshire eyepiece but there was significant alteration required to get a collimated star image.The first activity was to ensure that there was a sliver of ble sky around the secondary from all vantage points in the focuser. The next was to ensure the secondary reflection still lined up concentric, the last to adjust the primary.

Finally I got round stars inside and outside of focus using a 40mm eyepiece. I seem to get full field illumination and I don't think I can go to a longer eyepiece. The field of view appears to be about 40' of arc. Plenty big for a low power full moon. SO I tried it on M32 while it was up there. IT completely filled the field of view with a deep core and companion and wings spilling out to either side. Very satisfying !

Last activities are getting  a guide scope on the side. I need to reverse mount on the the existing Vixen dovetail there but that's not too hard.

I'll get some pictures to finish this off with and then describe the observatory build .

regs

Mike

[Peter Drew]

Very interesting read as I have an ongoing project building a 12" F5/F20 Cassegrain based on a set of optics by David Hinds. 

[skybadger]

Hi Peter.

So far all has gone reasonably well. The last fewthings to address have been :

1, dew on the primary -not while observing but by morning even with the dome shut. I intend to make a cloth blind and then possibly a mirror heater ( I never thought I would say that...).

2; getting the back focus distance right. The magnification factor must be about 10 since I moved the secondary 7mm to get a reasonable location for focus and the image plane moved out close to 70mm.

3;Collimation to get the last 10% is a heap of grief. I'm not there yet. Aligned the focuser, aligned the secondary using Cheshire and laser and did a visual on the primary. Tom it to a star and it was nowhere near. I end up pretty close but it's.not good enough. I think I need assistance interpreting those rings.

Regards

Mike

[Peter Drew]

The average back focus of a Cassegrain of this aperture is around 15cm, I would be surprised if the amplification factor is greater than 5X. The secondary in a Cassegrain is the most sensitive optical component. A quick way to check on its collimation is to look through a collicap at the secondary. The rim of the secondary cell will produce a black annulus around the secondary mirror which should be adjusted until the annulus is uniform in thickness. This is usually good enough for a follow up star test for collimation refinement.

[skybadger]

Yes, I saw that baffle annulus method and have tried it. However I can't guarantee my primary baffle is concentric and perfectly aligned over its length to use like this.

What I have right now are good out of focus bright star runs, it's as I bring it to focus the star concentrates into an airy disk battered by seeing and its really hard to get agood handle on correction direction.

The back focus distance is currently 160+54+35 where the 35 is the primary mirror thickness, the 54 is the mirror cell and 160 is the focuser and extension. Its designed for bf of about 250mm to allow cameras etc but since I haven't got the spec of the secondary other than its physical dimensions my plan is to collimate at different distances and ronchi test to find th epoint of best optical quality.

[skybadger]

Follow-up report:

I have been adjusting the secondary mirror to primary distance. I set it up to the maximum distance and have been adjusting the adjusters by 1mm each time, doing a rapid re-collimate and checking the image using a handheld ronchi screen.

At any point accessible to the installed focuser, the ronchi screen shows slightly outward curved lines outside of focus indicating that I didn't have the correct spacing. Correct spacing is indicated by having straight lines in the ronchi screen. Haven't seen that yet.

Adjusting spacing further towards the user isn't straightening the lines so I am thinking that I need to shorten the focuser. That's a fair bit harder.

Using the moon as a source, I get a 21.5mm image, which at 30" (assumed) indicates a 5000mm focal length and actual f/15 rather than my estimated f/10...

Mike

[Glasspusher]

Mike, I am sure that you are aware of this, but if the optics are not properly corrected you are never going to see straight Ronchi bands at the focus even when the theoretical separation of the primary and secondary is spot on.  It might be a case of having to accept the separation that results in the minimum curvature on the Ronchi bands see at the focus. 

John