Stub Mandrel

I bought a coated 60mm achro doublet from Astromedia and made a fine guidescope with it, it cost under a tenner. Sadly they are sold out. I've been looking for something similar and found this, which I'm not entirely convinced is much better and doesn't even appear to be coated: https://www.laser2000.co.uk/product/dlb-60-250pm/achromatic-doublet-60mm-diameter-250mm-focal-length And you think apo scopes are expensive?

I don't like working with the brown stuff if I can avoid it. I think some other people's builds will be much prettier. Hopefully, it won't involve more than cutting out some very simple shapes and drilling some holes. The 3D printed sectors have worked, I printed clips to hold strips of 15mmx2mm aluminium in place and they a seem to have worked in combination with clear gorilla glue and some strategically placed clamps. That means I don't have to machine any fancy curves in wood. I've ordered a NEMA 17 0.4nm stepper, hope it's up to the job, should be OK if geared down, or I may go for belt drive.

I believe! I've looked at lots of Ascom focusers. Some of them are ridiculously complicated. I just want something that will show up in Sharpcap, connects to 12V (I know how to set the current limit on the stepper board) and a USB port and focus in and out. This sounds like it will do the job. Am I right that all I need is a Nano (and put @tekkydave's sketch on it), a stepper driver (like a pololu) and a stepper than work out my own connections. I assume exact ratios are unimportant as long as the stepper can handle the weight of the camera. Do I need to add a special driver to my computer or will Ascom do this?

Black paint over the chip should restore contrast.

You have made me paranoid enough to do a more accurate calculation! The tube is balanced so the CofG is effectively at the pivot (I have a magnetic rail for balance weights). It's actually 690mm to the pivot. The tube weighs about 15.5kg* and the base bass weighs about 14kg. The baseboard means the CofG of the base part is rather below half way up it (this will remain the case as the baseboard of the platform will have similar mass, being rather bigger, if slightly thinner, and made of ply rather than lighter MDF. My estimate CofG of the base is at 300mm or lower, done by suspending it from a handle. Distance between the two CofG's is 290mm. Position of the combined CofG is 152mm from the base's or 452mm up. Add 80mm for the pivot height = 532mm. At the mid point of the platform the polar axis will be 52 * TAN 300 = 383mm up So, the CofG will be 149mm above the polar axis. That's much less than my original guess of 400mm. With the bearings 180mm either side of the polar axis, the triangle of stability will be 90mm wide at the mid point. Working backwards from a tipping point of 90mm lean gives an angle of 37 degrees. 37 degrees is way outside the expected envelope for using the scope, even if the base is placed on a slope (I will be fitting adjustable feet for polar alignment). *So really I should just put it on my HEQ5 which is rated 18kg for visual (I wasn't planning on guiding and long exposures...)

OK, let's assume (wrongly) that the CofG is at the pivot. This is worst case as actually it is much lower due to the weight of the base. It's under 700mm to teh pivot. At the mid point of the platform the polar axis will be 52 * TAN 300 = 383mm up, lets say 300mm to allow for the bearing being below the platform (actually by far less than 83mm). So the CofG will be less than 400mm above the polar axis. Lets lean the platform to its extreme of 7.5 degrees. Actually no, let's go to 10 degrees for a margin of safety. The CofG will be 70mm to one side of the polar axis, which means we need the triangle of stability to be at least 150mm wide at this point. The north bearings will be 350mm apart, so at the half-way point the triangle of stability will be 175mm wide. So bearing in mind that (1) the CofG will be lower than this, probably by 100-200mm (2) I will probably mount the scope closer to the north bearings to put more load on them (3) the platform would have to run past its end stops or be leaning over significantly to reach such an angle I don't expect any stability issues.

Plus if there is a tip hazard I can just pop a couple of counterweights (or lifting weights) in the base. Also, rather than fool around with clamps, locating the feet etc. I'm just going to put three friction pads and a pivot on the platform so it will be spin a nut and swap over the upper part of the base.

So say many websites. Why? Physics say that the COG must remain within the footprint defined by the three bearings for stability. I'm not aware of any unique physics that applies to dobsonian scopes.

Same tube and base, I'll look for a photo... Utterly strange, I don't seem to have a photo of the whole thing! I need some pics for my book so I'll drag it out when the sun shines...

Sorry, only just seen this. They are very big as the photo above shows. Just fits in my Xtype with the rear seat down, I don't think it fits cross-wise.

Going from scratch to suit what I have handy.

10" Dob (restored since this pic):

A new project! I'm making an equatorial platform for 52 degrees, so OK both for here and South Wales with a few mm of tilt and most of the UK with minor adjustment. I want to combine 3D printing with other techniques. Here's the South bearing, the pin is a push fit in two ball races and will be glued into a 3D printed bracket on the base. I'm printing the second of two sectors now, in two minds about facing them with aluminium strip, if I do I will need to pre-bend the strip as it's very springy! I found it a devil of a job to find simple instructions for the size of the sectors. If you use inclined sectors, imagine a horizontal line from the lowest point of your north bearing arc to the south bearing. The length of this will set the proportions for other dimensions, I decided on 600mm. The diameter of the bearing sectors = bearing separation * SIN (polar angle) The sectors need to be angled perpendicular to the polar axis

Not sure about this, it's far shorter than the tube. A lot of people shorten the focuser tube though.

Plenty of folks. Consensus seems to be that the Baader has better control of reflections of very bright stars, but is fussy about spacing - the Skywatcher is fit and forget.

Lovely pics Kevin. Unfortunately it seems almost no-one is aware there are three grub screws around the base of the Skywatcher focuser, you can loosen the fixing screws and adjust the grub screws to change the tilt.

I used to use a single sheet of A4, but discovered that flats with an uncovered board work just as well because it is so far out of focus. But I've noticed my habit of rotating the panel has created circular scratches that have created a bright ring so I'm going back to a single sheet of A4 chiefly as a protector.

I field mine with sand and fitted braces to it that improved it quite a bit, but getting a HEQ5 tripod was a worth it.

It surprising how many people will say that without ever having tried. Now I have an HEQ5 I would say the biggest benefit is that computer control is simpler than with my homebrew system! In practical terms the potential performance (payload) is similar enough, it's just a that the bigger mount is more tolerant of being out of adjustment. Look what people achieve with a Star Adventurer and that's even more modest in spec than an EQ3.

I would have hoped Intes could do a bit better than that?

Sweet!

If I was buying now I'd probably go for a Creality Ender which would work out cheaper than the kit I bought and much less than £400.

Interesting... my 10" is glued and gives a pretty good star test, although collimation was slightly off. Another astrophotographer's reaction on seeing my vanes was to ask 'do you get trinity diffraction patterns' because he had them in the past. It does seem triangular apertures may produce the sort of shapes I saw. I need to repeat the exercise on a brighter star to get a clearer image.

I use one that looks identical. I think I got it of eBay. I use it at lowest setting, tell Sharpcap to make a flat from 64 frames but set the maximum capture rate to 2 frames per second. That gives me half a minute so I slowly rotate the panel through a full circle during the capture to ensure any gradient is cancelled out.

It can't be mirror pinch as it's glued at three points, the 'clips' don't actually grip it, just locate it centrally. I wonder if it's a result of reflections inside the white tube combined with the shapes I've used. ... time passes ... I've done some exploring online and the nearest image I could find was caused by secondary mirror clips... now I've glued the secoondary but when I removed it from the old holder under each of the clips was noticeable patch of grey blooming. Before assuming the primary was distorted by its original mounting (unlikely with a mirror 6" diameter and 1" thick) I think I'll get the secondary recoated as well, it can wait a while as the effect is only going to be an issue if I was trying to split tight doubles.