Upgrading my SW 300p to an OO 1/10 wave => Trigger's Broom

[Captain Scarlet]

A couple of years ago, 2018, I bought a blue-tube 12” Skywatcher 300p Newtonian from @neil phillips, to use at my dark location (21.8) in SW Ireland. It has ridden my AZ-EQ6 atop a Berlebach Planet. I have thoroughly enjoyed using it and it has given me wonderful views. Weighing in at over 26kg including rings, eyepieces finders etc,the mount, in visual-only Alt-Az mode, has had no problems handling it.

I am a serial upgrader, so the first modification I made was treating it to a Lacerta dual-speed upgrade kit, which was very good but only a little later I further upgraded to a Baader Diamond Steeltrack, which really is a luxury. In November last year I ordered a Hubble Optics 1/10 wave secondary mirror, which finally was delivered in January, and in March I fitted it. It was a direct replacement for the OEM Skywatcher 70mm secondary, so no design mods were needed.
In May 2020 I really treated myself to a 1/10 wave Orion Optics 300mm mirror, and that arrived in early November. My first intention was to simply replace the existing SW mirror with the OO one. But it was not that easy. The SW mirror is 305mm as opposed to the Orion’s 300mm,but that  the difference was easily accommodated by the SW mirror cell. The bigger problem was the focal length.

The SW mirror had a focal length of 1500mm, and of course the positions of the focuser and the spider/secondary were in just the right place for that. The Orion mirror has a focal length of 1590mm, 90mm longer, and a little geometry and some scale drawings showed that simply plonking the new mirror in and leaving everything else unchanged was not going to cut it. Doing that would place the secondary too close to the primary to be able to collect-and-bounce the whole (longer) converging light-cone, losing the outer part of the cone and effectively turning my scope into a 10.5”.

Given the extent of tube I had available, I was going to have to move spider/secondary and focuser at least 70mm further up the tube to use the full aperture. Very luckily, I had exactly and only 70mm available. Phew!

Of course, I couldn’t resist nonetheless giving the mirror its First Light in a “simply plonked-in state” as a de facto 10.5”. Waiting weeks to get all that work done would be too trying for my patience. And besides, Mars wasn’t going to wait for me! I quickly collimated the new arrangement, was rewarded with a lucky clear night, and got an absolutely astounding view of Mars: from time to time as the seeing briefly settled, I felt I was looking at a picture-atlas of the planet’s features. Whether it was the seeing that night or the new mirror, who knows.
Anyway, First Light out of the way, major tube surgery beckoned. This is where it gets a bit more detailed. I needed to drill lots of new holes.

I was going to have to move the secondary’s spider-holes 7cm further up the tube, and similarly the main focuser-holes. Drilling small (up to say 5mm) holes accurately into a thin (0.9mm) steel plate is not difficult, given a set of sharp drill-bits and starting small, moving up the sizes. But drilling an 80mm hole into a thin, CURVED piece of plate was a totally different kettle of poissons. Flat plate would’ve been easy using a hole-bit and arbor, just secure the plate down on to a flat, say, wooden surface, and allow the rim-bit to wear its way through the steel. But the curve of the plate meant that as soon as the edge-bit made the first full penetration, the next tooth of the bit would just catch and everything would stop.

I thought of using a jig-saw, but trying to get that to make a circular hole in a thin plate seemed daunting. I don’t have a bench or a drill-press, or a machine-shop.

In the end I opted for the hole-bit and arbor, and angled the fast-rotating circular bit about, to gradually eat through the different parts of the circle as evenly as I could. It did catch-and-jam a few times, but I got there. However, once the inner disc finally detached, it did so at speed and across the room, bouncing off two walls before coming to rest. It was very sharp-edged. If it had hit me, there would have been bleeding! Lesson learned luckily without injury.

Some filing away of thin sharp metal edges and other de-burring, and cardboard-black painting any newly exposed shiny steel, and the focuser was re-attached, plus finder-bracket.
Next installment should show me having re-attached the spider and secondary assembly, having put the tube back onto the primary cell, having e-collimated and ready for true First Light!

Cheers, Magnus

 

[GordonD]

Wow that's brave. Look forward to hearing how it works out. Really interested to hear how much of a difference the Orion mirror makes - I have wondered about that upgrade myself before.

Gordon

[scarp15]

Very fine mirror upgrades and interesting modification to accommodate this. What will you use to fill in the original focuser hole and all of the small holes? 

[benzomobile]

Excellent work! Bravo!

How about low profile Diamond Steeltrack focuser source?

Beppe

Edited November 24, 2020 by benzomobile

[Captain Scarlet]

12 hours ago, scarp15 said:

Very fine mirror upgrades and interesting modification to accommodate this. What will you use to fill in the original focuser hole and all of the small holes?

I have a huge roll of acetate sheet, so I plan to simply cover the larger holes with pieces of that, painted blackboard-black. The smaller holes I may just cover with tape. In due course I'll be replacing the steel tube with a carbon one made to measure (saving at least 5kg) so this is an interim stage. By then there'll be only the primary cell left.

M

[scarp15]

Great yes that's a good fix idea and for as you say the interim stage. The eventual carbon tube you plan on, does sound very interesting. Paint that I use for shiny bits or touching up here and there is Krylon ultra flat.

[PeterW]

Watch yet self that disk looks bitey!

Peter

[Captain Scarlet]

Honestly I was incredibly stupid. It flew off the bit with great energy and would’ve taken my ear off if unlucky!

[faulksy]

why didnt you just extend the tube at the bottom by say 100mm then mount your primary further down ?

[niallk]

Fair play!!

[Captain Scarlet]

9 hours ago, faulksy said:

why didnt you just extend the tube at the bottom by say 100mm then mount your primary further down ?

I did consider that but I didn't as easily have the means to achieve it. I was also concerned about losing rigidity: having some flexure in an extra segment that far down might mean the whole thing became uncollimatable. Whereas I could start drilling holes straight away.

BTW I have read your own "dob-build" account: very inspiring! I plan to go "big dob" myself at some stage I hope in the not-too-distant future.

[faulksy]

5 hours ago, Captain Magenta said:

I did consider that but I didn't as easily have the means to achieve it. I was also concerned about losing rigidity: having some flexure in an extra segment that far down might mean the whole thing became uncollimatable. Whereas I could start drilling holes straight away.

BTW I have read your own "dob-build" account: very inspiring! I plan to go "big dob" myself at some stage I hope in the not-too-distant future.

good job though, and good man for doing a build. as they say go big or go home 🤣

get yourself a orion 1/10 primary ordered now

[Captain Scarlet]

Latest ...

I made the mistake of drilling the central hole for the focuser first, then realizing that I had to sacrifice a bit of the calculated distance to the spider holes because of the presence of the inner flange of the end-ring of the tube. I thought there would be plenty enough leeway in the secondary holder to accommodate the shorter separation. There wasn’t.

As the picture shows to the eagle-eyed, I’ve had to back the secondary-holder by one screw on the spider-vanes to give me an extra 10mm, allowing the secondary to catch the whole light-cone.

New longer carbon tube now ordered so hopefully won’t have to live with this bodge for too long.

Edited December 2, 2020 by Captain Magenta

[markse68]

2 minutes ago, Captain Magenta said:

Latest ...

I made the mistake of drilling the central hole for the focuser first, then realizing that I had to sacrifice a bit of the calculated distance to the spider holes because of the presence of the inner flange of the end-ring of the tube. I thought there would be plenty enough leeway in the secondary holder to accommodate the shorter separation. There wasn’t.

As the picture shows to the eagle-eyed, I’ve had to back the secondary-holder by one screw on the spider-vanes to give me an extra 10mm, now s as losing the secondary to catch the whole light-cone.

New longer carbon tube now ordered so hopefully won’t have to live with this bodge for too long.

oh- that doesn’t look like it’ll hold very steady like that Magnus- I guess if you have enough tension it should though? Where did you order the carbon tube? According to my vintage fullerscope catalogue the deluxe versions they sold had fibreglass tubes but Ive only come across the pvc drain pipe type like mine. I’d like to find out weights of composite tubes as it is quite a lump to carry any distance 

Mark

[Captain Scarlet]

16 minutes ago, markse68 said:

oh- that doesn’t look like it’ll hold very steady like that Magnus- I guess if you have enough tension it should though? Where did you order the carbon tube?

I too had reservations about how stable it would be, but after collimating it and trying to move it around in situ with laser etc still in the focuser I wasn’t shifting the spot even with a reasonable amount of “wrist-torque”. I was surprised.

The carbon tube I ordered directly from klaushelmi.de, I think he supplies astroshop.de  with their pre-specified carbon tube upgrades. Going direct to him makes a decent saving. On my 300p I’ve worked out it’ll save me 4-5 kg as well as making it very much stiffer.

Magnus

Edited December 2, 2020 by Captain Magenta

[Ships and Stars]

Wow, this is cool, nice project. I can't leave anything alone, am always modifying stuff. JB Weld is excellent if you need to epoxy anything on. You can even drill and tap it for threads. Extremely strong bond. I use it on just about everything.  

It will be the business with the carbon tube!

 

[Captain Scarlet]

On 02/12/2020 at 22:41, Ships and Stars said:

 JB Weld is excellent if you need to epoxy anything on. You can even drill and tap it for threads. Extremely strong bond. I use it on just about everything ...

Ive always plumped for Araldite, I’ll get some JB weld on your recommendation, thanks

[Captain Scarlet]

... on reflection, pun intended, I decided to reorientate the side-support grubs in the mirror cell so there is no chance of the mirror getting “jammed” between two nylon tips at the bottom. Rather have it resting not-quite vertically on a single one.

that involved rotating the whole tube in its cell by 60 degrees.

while I was at it, and this is the reflection part, I removed the spider again and the secondary to paint the spider-vanes cardboard black. They were actually very reflective, and the last thing one wants is a parallel reflective surface in the light-path. Before and after pics...

 

[markse68]

2 hours ago, Captain Magenta said:

and the last thing one wants is a parallel reflective surface in the light-path. Before and after pics...

you’d think so but when i rebuilt my spider I left it unpainted shiny stainless steel. I’d read some theorising about black body radiation leading to the vanes reaching cooler than ambient temperature and a thermal gradient resulting in effectively thicker vanes causing worse diffraction spikes. It may have been nonsense but sounded an interesting theory. I keep meaning to get some black wrap foil to put over them to see if it makes a difference but I’ve not noticed any negative consequence so far except maybe the other night with near full moon- maybe it was causing an issue with veiling glare 🤔 Most of the time any reflected light falls on the blackened inside wall of the OTA so is not really an issue I think (hope)

Mark

[Captain Scarlet]

I think thicker vanes won't affect the diffraction spikes, the spikes are all about the total length of edges in the light-path (including the edge of the aperture). Thicker vanes will just result in a little more total obstruction. But shiny surfaces parallel to the light-path, especially those actually inside the llght-path, my instinct tells me will result in disproportionate glare around bright objects.

A reflective inside of the tube itself will also be glare-inducing-contrast-reducing, but because it's outside the main light-tube-cone its effect should be reduced. But then again the sheer area of the inside of the OTA will offset that (i.e. make it worse).

I plan to test the vane-reflectivity effect by introducing a long wide shiny blade into the light-path when next observing. Actually flocking the vanes may be the next step.

Such fascinating fun, these thoughts fill my mind when going to bed and send me to sleep!

Edited December 4, 2020 by Captain Magenta

[John]

Very interesting thread

As well as their thickness, the shape of secondary supports always prompts some interesting discussions.

I have curved supports on my 12 inch dob which seem to work well but they are a bit thicker than the straight ones found on most scopes.

My vanes are dull black rather than shiny.

 

 

[Dave_D]

hehe your tube has almost as many extra holes as mine