markse68

Thanks for all the kind words 😊 Yes Alfian making something bought your own gives you an attachment to it and satisfaction in using it I think, and for me having something that’s not too precious- that I can fix if needed is really liberating too. Of course the ultimate would be making from scratch, but that takes more time 😉

I managed to get a couple of hours with it last night- seeing wasn’t very good and it was a bit gusty so good testing conditions for the az bearing/brake. I think it’s about perfect now. There was no tendency to swing in the wind, just eyepiece instability, and I was able to track targets using only eye socket pressure on eyepiece 😂

However to my dismay on arrival at site collimation was off again so my spring upgrade didn’t fix that and I still need to figure out what’s shifting. On the plus side the retro reflective primary spot and collimating focuser cap meant I could quickly get it true again using just the light from my phone startup screen- clear bright circles to concentricise if that’s a word 

Actually I’ve been wondering if using the finder scope might be a more accurate way of getting collimation corrected in cases like this? I had my finder and red dot pretty accurately aligned in the workshop but after recollimating in the field they were both slightly off. If I tweaked the primary to get the eyepiece view aligned to them again I think that’d probably be more accurate than getting the circles concentric by eye over a much shorter distance? I guess provided it’s only the primary that shifted and collimation was perfect when the finders were aligned

So all done for now. Collimation was a joy and everything lines up properly now. I seem to have developed a bit of a thing for Russian scope stuff...Do skorogo comrades ✊

Couple of small bits to finish- I fitted a magnetic tool rack to the back of the tube for a counterweight- the tube can slide in the mount but it’s stiff and the weight should be quicker and easier. It’s a really strong magnet- lucky my car’s aluminium!

And I made a collimating cap that doubles as a dust cover for the focuser in delrin with a retro-reflective ring which, with the retro-reflective centre spot I put on the primary, aids collimation greatly

Next up the secondary mirror mount. I wanted to re-stick the mirror as I had seen some double imaging at high power and figured the way it was stuck might be leading to it being slightly bent when the mount was nipped up tight. It was stuck with a layer of double sided sponge tape across its whole back surface and the aluminium support plate is quite thin so must flex some.

The actual mount looks home made but I like it a lot- it’s quite ingenious separating tilt and rotate/height making collimation a doddle. 2 screws are captive in the mirror support plate but able to rotate. They are threaded through the main block with knobs in the outside to adjust. Counterintuitively you must “undo” them counter clockwise to lock position as they pull on the plate. This takes a bit of getting used to. The mirror on its support tilts over a steel pivot bar. Clever I think 👍

To relieve the bending pressure somewhat,  after cleaning off the old tape I just used 2 narrow strips of UHB tape- a super sticky dense semi-sponge tape that should hold it- I hope 😉

Day 2- first up sorting out the azimuth bearing on the ancient dob mount. Originally it was 3 blocks of nylon (It looks like ptfe but it seems a bit hard so I’m going with nylon) running on Formica. It worked...kinda. A lot of stiction made it a bit of a pain - movements were hit and miss and jerky resulting in lots of eyepiece shaking.

So I bought and fitted a lazy Susan bearing- made in the USA no less!  A layer of thin rubber couples it to the top part of the mount. I left the 3 original feet as they add stability gliding just over the surface lifted by the bearing.

Much smoother but now the opposite problem 🤦‍♂️. Slightest gust of wind and the tube spun off! 

So today I fitted a brake- a simple affair consisting of a block of nylatron on the end of a piece of studding screwing through a steel thread insert inserted in the moving part of the mount, bearing down on the Formica inside the lazy Susan. Works a treat 😊

I need to be able to split the base to fit it in my car so I’ve fitted a hefty 16mm central shaft and a quick release collar that locks the 2 parts together when in use

Thanks all😊 It’s just some M6 brass studding  I bought on eBay slit with a junior hacksaw Stu. Fresh blade gave a kerf of about 0.7mm. The vanes  are 0.8mm so the slits needed opening up with some coarse wet and dry.

Yes Steve the Tal finders are very nice- very light too despite solid build. I too really like the old focuser- there’s no slop in it whatsoever. I just got a good deal on a Moonlite and was planning to replace it but am having second thoughts 🤔 I have a nice wide angle TV Widefield 2” which I’ve been wanting to use but the blue anodised Moonlite will look very out of place on this old banger 😂

I almost went out as the skies seemed darker than usual but tired and it’s a bit windy. Moon was looking beautiful though- low and large when I arrived home

Next the finder- I quite liked the Fullerscope one but it’s huge and has a very narrow afov and very crude wobbly wire crosshairs. The one on my Tal-1 has a much wider afov, nice crisp crosshairs and a brighter clearer image. I picked up another on eBay last week so on it went. Being much shorter it also means less head movement from eyepiece to eyepiece when aiming 👍

Then the primary mirror cell- hopefully this will fix the problem I had of losing collimation when taking the scope in the car to a viewing site. The original springs the wing nuts compressed just felt a bit too soft and it was too easy to knock the wing nuts loading the scope into the car. I found some stiffer longer springs which needed cutting down to size, and I fitted thumb nuts in place of the rusty wing nuts- M6 tapped to 1/4 UNC to match the studs. It certainly feels more solid now

I’ve spent the day “upgrading” my new old scope- it’s so liberating to own something like this that’s not too precious! A bit like an old car where you don’t cry about every dent or scratch, I can drill new holes to mount things without a care 😊 (it’s already full of old holes 😉)

First up- modifying the secondary spider. This needed to be done as after collimating, the secondary wasn’t anywhere near concentric with the reflected image of the primary centre spot and the eyepiece barrel and there was no way to adjust it. The spider had its vanes bent at the ends then screwed crudely to the main tube. I hacked the ends off then extended them with studding and thumb nuts so the secondary can be adjusted laterally. After the last pic I sprayed it all Matt black. It’s surprisingly solid when the thumb nuts are nipped up.

Had a very enjoyable evening chatting with some members of Flamsteed Society on Blackheath and enjoying the various phases of the eclipse- wonderful 😊 Apparently the ancient Greeks worked out the Earth is 4x the diameter of Moon by watching its shadow! (phone pic to prove I was there)

Waiting for the clouds to clear- not long  now🤞

Hi Stu, that looks really great- are those dob mounts aluminium plate then? I’ve been working in Munich all week- on my way back home now and it looks like it might be clear in London so if not too late and not too knackered I might get proper first light on my new old slow 8.5”- exciting! 😃

15 hours ago, Paul M said:

The whole tube assembly looks like a Fullerscope to me. The focuser, access door, primary cell and tube end trim are surely Fullerscope components. The secondary mount/spider aren't like Fullerscope components that I've seen.

My own 6.25 Fullerscope of early 80's vintage came with a David Hinds mirror which was standard for the "Export" models I think.

I can't say I've heard of Jim Hysom (sorry Jim!) but along with that Dob mount you have a fine classic looking scope there! Happy stargazing

Thanks Paul, judging by the number of extra holes in the tube I think this one has had a few lives and this may not have been the original mirror or secondary mount. I’ve not read anything but good things about Jim Hysom and his mirrors- I think he was a very respected maker http://www.europa.com/~telscope/tsenglnd20C.txt

I did get a very brief first light on Sunday but only a glimpse of the moon before it disappeared behind clouds, and the gas giants never showed, but the moon was wonderfully contrasty and richly coloured- a definite step up from the 4” Tal I’ve been using, so I have high hopes for this one 😊

Would be interesting to find out how the likes of his and David Hinds etc mirrors stack up against modern high quality mirrors 🤔

Mark

Hi Stu, yes it was the one on eBay. Advertised as 9” f8 but it’s actually 8.5” (I centre spotted it so measured it) and by my measurements actually f7.5 but that’s fine- it should be excellent on Jupiter and Saturn if only it weren’t raining grrr! Off for a week- so will have to delay first light- can’t wait 😊 

This beauty 😍

Well ok it wasn’t the postman- had to drive to Cambridge to pick it up. And yea ok it’s not really beautiful- pretty damned ugly really 😉 But beneath its ugly exterior lies a Jim Hysom 8.5” f8 mirror in immaculate condition- it’s a beast! And this surprised me in a really nice way- a really nice clever easy to adjust secondary mount and the crusty old focuser has a fine focus (geared) knob! And is really nice to use with no sloppiness. Anyone know if the focuser and secondary mount are Fullerscope parts? The finder is. Of course it had to be cloudy today...

oh and it’s got a little door so you can get in there with a feather duster to remove cobwebs!

Thanks Ricochet. It wasn’t as much a bright cross like the diffraction spikes from bright stars but more like a hazy smear but yes maybe that’s what it was. Tbh I was on the way home and pretty tired and it was late so I didn’t hang around long- the scope surely hadn’t temp equalised. But it was worth it for Saturn 😊

Last night I took my trusty Tal-1 to a new viewing site- the common right near my home in Plumstead with a much better view of the horizon and a bit further away from direct street lighting- and what a difference it made! Saturn which up till now had been the boring cousin of magnificent Jupiter suddenly revealed its beauty- Cassini division, shadow of rings and even hints of banding! Up till now it had been just a homogenous blob with ears 😉 BUT Jupiter really disappointed. I tried a few eyepieces but I was getting terrible flare in a broad hazy cross centred on its disk. It wasn’t enjoyable to view at all. I worried my scope had gone out of collimation but then why Saturn so clear? I think I may have reached the limits of my primary in its original poor condition. Could this (bacterial/mould?) damage be causing the flare I’m seeing and is it time for resilvering?

And yes- one mullered bearing. Found an SKF on eBay - it’s size 62 x 40 x 12mm or type 61908 or 6908. The lower bearing seems fine so can stay

Now the upper bearing housing unscrewed with ease

Then it clicked- the threads weren’t galling up at all. In fact I probably didn’t need the special spanner at all though nice to have. You have to remove the lower threaded cap as it locks the shaft and the upper bearing into the housing 🤦‍♂️ A little heat and it came off easily

I know this isn’t very interesting to most but I couldn’t find this info anywhere so maybe it’ll be of use to someone down the line.

Made progress today- I made a spanner for the upper bearing housing using the heads of 2 m4 Allen screws to engage the notches in the housing.

Alas I still couldn’t unscrew it...

I like the spring balance assist- haven’t seen that before and was thinking of using springs on my 12” design to enable use of smaller rockers and have some adjustability for heavy eps etc. Is that something unique to that dob? I haven’t come across it before. Didn’t see you mention it (sorry if you did) but do you know who made it?

At the other end- the counterbalance weight end, there are more threads locked with grub screws 🤦‍♂️ It looks like the part the weight bar screws into is in turn only a short cap threaded onto the main shaft and locking the shaft to the inner race of the lower bearing and preventing its removal. I failed to loosen this part but in the process loosened the OTA mounting plate 👍😉

I think I’ll tackle this part next time with a bit of heat from a blow torch, gripping the other end in the vice. I don’t want to damage the threads though...

I made a bit of progress- the OTA mounting plate does screw onto the shaft after all and came loose while I was attacking the other end. Under it is a threaded collar that I assume holds the top bearing in place. Whilst the machining and design of this mount seem very robust they had an unfortunate habit of using pointed grub screws to lock threads- distorting the threads in the process and making dismantling for service a bit of a pain! I gave up unscrewing that collar after only managing about one turn before it felt like it was galling up. I had to grip it in a vice which is making a bit of a mess and will require paint touch up and re-forming of the steel notches. I tried drilling out the messed up threads where the grub screws bit and WD40, but no luck- I’ve given up for now. I think I’ll have to fabricate a special tool capable of a lot of torque to get this free.