2nd Light on 12.5" Cassegrain

[skybadger]

This is an update on the build of the 12.5" cassegrain which amounts to a 2nd light on the built OTA after a slew of changes.

Here's what it looks like :

Here's a foucaultgram of mirror of the primary I managed to capture .

If anyone saw the Pathe news clip released recently of the astronomer putting the mirror on the back of their Newtonian in 1950s, that was the maker of this mirror set!

I have re-assembled this telescope having made significant changes to the entire ota.

During the summer I have:

re-bored the holes in the skeleton frames to ensure alignment of the frame tubes by stacking the frames and re-drilling together.

Re-made the mirror support plate with the mirror clips rotated by 30 degrees so they don't interfere with the hexagonal-layout of the truss tube structure

In re-making the mirror support plate i have also paid particular attention to keeping the central bored hole concentric with the edges.

I have remade the mirror mount skeleton frame. This carries the focuser and the mirror mount plate and they were not sufficiently concentric on the first attempt. The focuser plate now takes 60mm Vixen threaded attachments and specifically the Borg low profile helical focuser.

I have finished all the aluminium tube clamps and fitted them to the skeleton frames. They are necessary now as the frames are now loose on the tubes.

Re-assembly:

I re-assembled the scope frame and mirrors and did first trials in the garage: I used the laser to align the focuser on its independent mount to the centre of the secondary holder. I fitted the secondary mirror, ensuring that the focus point for near objects was a good few inches outside the focuser, having learnt from the first assembly.

I collimated using a cheshire eyepiece to ensure that the secondary reflections were concentric before adjusting the primary to re-centre.

Basic adjustments completed, I mounted the scope on the observatory mount and re-collimated using the 2" auto-collimator which I find quite sensitive at indicating good secondary concentricity. The reflections of the vanes for the secondary were aligned in one axis but not the other - odd.

I did a first view on Vega. It was high at the zenith and bright in the dusk sky. The out of focus rings at 40mm were asymmetric so I adjusted the primary to even them up. It required a lot more adjustment than expected to do.

I changed to the 25mm Meade 4000 plossl eyepiece ( mag: 3200/25= x128 ) and nipped up the star image to be concentric either side of focus again using the primary collimation bolts. I find that I can either have the star image condense into the centre of the rings as you approach from either side of focus or you can have good concentric out of focus rings but the star centre moves that tiny but important bit off to the edge of the shrinking ring just as you come to fine focus, but not both. (This I think means something is not quite aligned but I can't see what).

I moved to Deneb, for mount alignment purposes. Smack in the middle and no real change in the image due to movement of the OTA.

I moved to the central star (Gamma?) and then to beta to view the double. The double was wide open with lovely bright yellow and blue colour but the image was soft. A bit squashed spider even. The legs of the spider wafted as I watched and no amount of focusing would get crystal round stars.

I then moved to the double double Eps Lyra. There it was, again, E1 and E2 wide open with clearly discernable doubles but the close doubles were not clean; they were fuzzy, even using the 9mm eyepiece (3200/9 ~ x345)

Finally, making use of what dark sky was left in the face of the rising full moon I moved over to M57, M27, M29 which were all relatively large and bright and then closed up, leaving the scope pointed at 10 deg alt and 90 deg az just to catch the dust..

Situation:

The focuser works fine. Visual focus needs to be moved to the full extended length of the focuser to enable mounting a DSLR at focus when drawn fully in. So the secondary needs to come towards the primary by about 20/sqrt(M-1).

The primary baffle is too long. I need to determine the required length compared to actual. Currently it's 37 cm from the front of the mirror. I cannot see all the secondary from the inside edges of the focuser, or even a little light around the edges.

Holding of collimation between slews looks good but the initial collimation change i had to make might be due to the swing from horizontal to vertical - I shall have to look into that and mount the mirror with the tube vertical.

Underlying collimation looks flawed. I need to resolve why the vanes aren't symmetric in the reflections ( maybe they aren't mounted symmetrically - is this really something to worry about ?) and also why a auto-collimator collimation had to be adjusted so far on a star.

Finally, why does the star collimation do that trick of disppearing into the edge of the rings as I come to fine focus and how do I go about tracking down the cause ?

Will that solve my soft and squidgy stars issues ?

I should note that the background stars are tight and round more or less to the edge of the field in the 25mm eyepiece and its the brighter stars that are soft and troubling me. Having seen a foucaultgram of the primary which to me looked fine even for such a fast primary (f/2.7), I wonder if its roughness I am seeing in the mirrors. I have no way of testing the secondary other than by making a matching nulling mirror. TBH I can't see that happening.

I shall try to collect some video evidence next time, now I have a first view.

I shall also try to use the gauge on the helical focuser to judge the symmetry of the out of focus rings on either side of focus for the purpose of spherical aberration measurement and validating that the secondary is at the designed separation. Its about the only means I have for doing this test.

Any thoughts and hints on progress gratefully received.

Mike

[dweller25]

This is a great project, thanks for sharing

[laudropb]

Very interesting report. A lot of work but I am sure you will reap the benefits when completed. Thanks for posting.

[Peter Drew]

Mike. I have a 12.5" F2.7-F13.5 Dall-Kirkham Cassegrain. I built it some 30 years ago from supplied optics so have no accurate record of its optical vital statistics. It has an optical window supporting the secondary so I can make only approximate measurements from an external position. The secondary is 75mm diameter and the intermirror separation is around 75 cms. The baffle tube is 40mm diameter and extends 30cms beyond the primary, I can see an annulus of clear sky round the secondary with my eye central to the focuser.

F2.7 is quite fast for a standard Cassegrain primary, mine was originally intended to be part of a  Maksutov system hence the Fspeed. You seem to have gone about things in the right way and I'm sure that the residual niggles will be overcome. Correct intermirror separation is important to avoid induced spherical ab, your star image problem could be down to the optical axes of the mirrors not being in line accurately, if your optics are of the classical Cassegrain design this may well be critical, the D-K system is more forgiving.  

I am currently building a 12" F5-F20 D-K using a set by David Hinds, like you I have no measurements to work with so I appreciate the difficulties.  

[Gazabone]

Wow, that's fantastic. Hope you get many hours of enjoyment from it, well deserved.

[skybadger]

Ta all,

It's a challenge, which is why the entire tube is adjustable!

The secondary is 92mm in diameter and the primary baffle is 62mm I.D.and 37 cm long. The intefmirror distance is 56cm to come to focus 100mm behind the mirror surface. I put the low profile helical on to give more lattitude in distances while I measured the intranet and extra focal SA while searching for the sweet spot!

might try a conical baffle instead but by definition would not expect to see any sky from the centre of focused field.

Still waiting for a clear night to get some pics.I'll post them here when I get them.

Regs

Mike

[mapstar]

Interesting project and a good read. I'm sure you will get those niggles sorted quickly

Thanks for posting 

Damian

[skybadger]

Well I followed up with some ronchi grams at the focal plane

This one is outside the focal plane by 3mm

This one is at the focal plane

Finally several mm inside the focal plane.

And a picture of Gamma Cygni (Albireo) double pair obtained at the focal plane

Which leads me to worry about the quality of the mirror surface. It looks like high frequency ripple to me and some dodgy blobs. So I've taken the scope off the mount to take a proper foucaultgram like I should have very early on.

The ronchi grams are taken using a Gerd Neuman ronchi screen fitted to an afocal DSLR with a 80mm lens, taking 6 seconds at iso 1600 for each image.

Mike