Questions about using an SW Mak

[Avocette]

I just bought a Skymax 150 Pro which I am learning focuses by moving the internal rear mirror. It is fitted additionally with an external dual-speed Revelation focuser which I understand helps avoid mirror shift in imaging. I'm used to the SW Newtonian focuser mechanics but this Matsukov/Cassegrain is completely new to me. A few questions arise:

• Should I leave the star diagonal in a fixed position as the scope moves on the EQ mount or should I rotate it to compensate?
• Included with the scope were the original SW rear tube (is this a so-called 'visual back'?) and a 2" to 1.25" adapter, but is there an easy/secure way to attach a DSLR as there would be on an SW Newt focuser? I thought of using my Barlow with its T threaded top with lens removed in the 1.25" adapter in the focuser tube but I am reluctant for weight/leverage reasons, and I thought there might be a more elegant and secure way.
• Presumably I may need to make adjustments to both the standard Mak focuser and the external focuser to find the prime focus point with the DSLR. Are there any recommendations you could offer?

Thanks for your help and ideas.

[nightfisher]

Feel free to move the diagonal as needed when using the EQ mount, or you can rotate the crayford focuser, either will work fine.

I connect my DSLR with a T thread and 2" nose piece, i find this secure and works well. I used to have a 180 mak with crayford, i would set this at mid focus then get rough focus using the moving mirror, then fine focus using the crayford, hope this helps

[Avocette]

Great - thanks for pointing me in the right direction. I'm a bit nervous of unscrewing something vital and doing some damage!

If I understand it correctly, there is a lot of focus range in the primary mirror adjuster, although moving the primary mirror towards and away from the secondary mirror has some affect on the actual focal length of the scope. To compensate for the extra length of the external focuser, and in my case the adapter ring to make the MAK fit the SCT focuser thread, I guess the primary mirror must be quite a way up the tube. Well hope to get a chance for some experiments later this evening if the clear skies predicted by the new FLO-sponsored clearoutside.com website is correct!

[nightfisher]

How did you get on, did the skies stay clear for you

[Avocette]

Thanks for the question - I enjoyed a couple of clear hours after a delayed start (clouds left about 11:15 so the clearoutside website was a bit more than an hour optimistic). However I was mostly in 'learn' mode both with the mount (EQ5 Syynscan) and the scope. It was the first time ever I had used a polarscope so some of the time was spent messing about with alignment.

My initial conclusions are, inevitably perhaps, a bit mixed. I'm in one of those mindsets where I'm not sure what I should have expected to see (compared to one of my Newts for instance) and whether the Skymax lived up to expectations. In the early part of the session during 1 star alignment I was particularly struck by diffraction patterns on the star Vega - I don't think I've had such clear bright and dark rings with any Newt. But I wasn't really doing a 'star test' at that time. But later, just looking at various stars (including Alberio and partner) I kept seeing the diffraction rings. Now I am pretty sure that I must have been operating with the Revelation focuser and its motor drive at one end of its travel (I know - you suggested setting it half way.....) and so I think (now) that I could only approach close to one side of focus on the star and hence the diffraction rings. I used a Bahtinov Mask at a later time and I managed to get the correct X and symmetrical horizontal bar pattern, but regretfully I didn't think to repeat my viewing of stars. It was only later by chance when I had disconnected the focus control handset that I remembered I could use the Mak focuser, but this was exclusively with Saturn and I didn't go back and look again at bright stars.

I'm thinking that I would like to get back to basics (and learn more about the scope) by removing the Revelation focuser and using the original SW screw-in adapter directly with the 2" star diagonal and eyepieces (no DSLR for the moment) and getting the feel for the normal Mak focuser.

By the way does anyone ever collimate a Mak? I am left with the lingering feeling that there was some asymmetry in the diffraction rings in the accidental star tests I carried out. That's why I plan to go back to the basic Mak focuser to see if the mirror shift it causes in focus adjustment may have been responsible for the possible asymmetry.

[nightfisher]

I would advise to leave collimation until such time as you KNOW it really needs it, and by all means try it minus the crayford, my 150 mak has very little mirror shift

[Avocette]

We had some reasonable skies late last night and I was keen to test out my 150 Mak. However the viewing experience was pretty disappointing. Focusing in on a bright star (Vega), at the point of focus I could see a set of tiny black rings asymmetrically distributed about the centre with a 'hairy' comet tail in one direction. That aspect looks a bit like the astigmatism I've seen with my 10" f4.7 Dob but the black circles were new to my eye. Looking at the twin coloured stars at Alberio each had the same comet tail and black rings. Similarly with the 'double double'. No sign at all of the third and fourth stars amongst the smudges.

 

However I've just read a web paper on Collimation by Thierry Legault http://legault.perso....fr/collim.html and in the process realised that I probably haven't seen an Airy disc before! One of his computer generated charts shows exactly what I saw last night (Fig D) if you add in a bit of turbulence from the seeing.

 

Image © Thierry Legault