Pixies

Yep - noticed that just now. Just need a clear night!

Have posted something in the DIY forum about repairing the DMD-3 controller:

This is about a repair rather than pure DIY. So if this is the wrong forum, my apologies. I recently purchased a pair of MT1 motors, gears, etc. plus a malfunctioning DMD-3 controller. For the Vixen Super Polaris mount I'm trying to restore. I thought I'd see how these things worked and whether the mount was worth trying to set up with auto-tracking eventually. But first, I just want to see if I can fix the controller and use it for simple tracking and control. I stuck a long pointer on the shaft of a motor, connected the controller and tried to work out what was going wrong. It turns out the basic sidereal tracking on the RA channel is flakey. I timed full rotations of the shaft and they weren't consistent. 11-13 minutes, roughly. With the pointer, I can see that the shaft rotation occasionally sticks or shudders, too. Also, if I try and slew (x2 or x16) and change direction (hemisphere) it doesn't always change immediately. The Dec motion appears to be fine. I swapped motors over too, to confirm that this isn't motor-related - and it isn't. I also stuck my multi-meter intro the stepper motor sockets and can confirm that all 4 lines of both RA and DEC are working. So it's not a cable/connector problem, Now, I'm no electronics engineer, but I used to be a bit handy with a soldering iron in my youth, so took the controller apart. I had a shot at working out the rather simple circuit board and here is a quick sketch of what I think is a basic schematic, NOT a circuit diagram: So, this is how I see it working - bottom up. The crystal and the mc14060bcp (binary counter) supply a regulated clock signal to the RA and DEC channels. These take input from the x2 and x16 slew speed switch - adjusting the output clock frequency I assume. The first stop are the Toshiba tc40103bp ICs (down counters), which use frequency division to produce the clock signals for the stepping controllers. On the RA channel, it also takes input from the x2 and x16 slew speed switch. Then on to the stepping controllers: The DEC channel has a single stepping motor driver IC - ucn4202a. This includes output transistors internally with an output sink current of 500mA . The output from this goes straight to the DEC stepper motor. However, the RA channel uses a different stepping motor driver IC - pmm8713. This has only an output sink current of 35mA, so its output goes on to a Darlington Sink Driver - TD62103P , with output sink current of 500mA, then on to the RA stepper motor. (Anyone suggest why there is this difference? I assume it's related to the fact that the RA channel has a constant output for sidereal tracking, but not sure why the 2 channels don't just use the same component models?) Anyway. Judging that the fault affects 2 areas: the sidereal tracking signal and hemisphere (direction) changes looking at the diagram, that seems to indicate a possible single component failure - the RA stepping motor driver IC - pmm8713. I assume that a failure with the Darlington Sink Driver would mean one of the 4 RA stepper output lines would be failing, which doesn't appear to be the case. So my first attempt will be to replace the pmm8713 chip with a new one. If that doesn't fix it, then it's back to the drawing board. Anyone have experience with a similar fault, or properly understand these types of controllers able to comment? If I'm barking up the wrong tree, it will be good to know before I get the soldering iron out.

Re the exit pupil. What's the focal ratio of your scope? You calculate the exit pupil as eyepiece focal length / scope focal ratio. For example, in my F6 dob, a 12mm eyepiece gives a (12/6) 2mm exit pupil. Some say that perceived contrast is best with an exit pupil around 1mm-2mm. I tend to agree. 8mm - 12mm eyepieces seem best for a lot of faint DSOs in this scope for me. Here's a good thread if you have a couple of spare hours:

If you are in the UK, the bright summer nights just now, mean that a lot of DSOs are not easily seen. Once proper astronomical darkness returns, try some of the brighter objects: Globular cluster: M13 Planetary nebula: M57 Galaxies: M81 & M82 Do you know what your 'Bortle' rating is?

I have stumbled across it - and it's very useful. Thank you for taking the time to update a very old post!

New/old stuff has arrived: The RA output from the controller is dodgy, but I might replace with Arduino goodies. Also - old bent rusty on left, old straight clean on right:

Some goodies for the Vixen SP mount (fixer-upper):

Can you confirm that the secondary holder is able to be pulled right back on to the 'vane assembly' (not sure what this is actually called). If it really is as far as it can go, something is definitely wrong. Any pictures?

Try and find a low southerly view, it'll be the darkest part of the sky and you'll have the galactic-core part of the Milky Way, plus Sagittarius, Scorpio, etc. You might be able to bag some of the brighter Messier/DSOs especially with a wide-field view or binoculars. Mind you, I can't say how light the skies will be at that lattitude.

Nope- sorry To bring the secondary away from the primary, loosen the 3 adjusters and tighten the central screw. This will pull the secondary back towards the vane assembly. When you hit the adjusters, back them off a bit more and tighten the central screw again. Keep going until the secondary is under the focuser. However, from my understanding of @godra's first post, this is what he is doing, but the secondary hits a limit before it is correctly under the focuser. The spring is just to stop the secondary from flopping around when not being pulled against the adjusters. I would be suspicious that the adjusters aren't fully backed-out or the spring is misplaced and stopping the secondary from being fully pulled back.

I have a few - not exactly as wide a range - a 7mm (University Optics) and a 6mm (sorry Michael) and 4mm Circle-T orthos. When all else fails trying to split a tight double, these are the ones to do it.

OMG! That's horrendous! Who on earth wants to do something like that. If it is deliberate, I hope someone is caught.

Looking at an image of a Tal-1 secondary (from Astro-baby's strip-down guide): Assuming the Tal2 is similar, it would be interesting to see whether the secondary holder is a far up against the vane assembly as it can go, or whether something is stopping it. You have the 3 adjusters backed out so that they don't protrude out at all? The adjuster bolt hasn't been replaced in the past, with something longer, which is fouling inside the holder when you tighten it up? The spring isn't incorrectly installed, stiffened-up, etc.? I've never used a Tal in the flesh, so just guesses. I would suggest removing the whole vane/secondary assembly to investigate, if you are happy doing something like that.

I wouldn't worry about that too much. As it's only the secondary that can be adjusted - it's limited to what can be done. Just look to get the dot (eye hole) inside the doughnut marker on the primary.

Are you able to take a photo of the view? A smartphone camera is usually small enough to get a reasonable picture through the small Cheshire eye-hole.

Hi there. You are correct. The benefit, I guess, would be for someone using a wide-angle 2" EP to start, then swapping to this zoom without having to use another fitting. Originally I thought it might help with stability, but certainly in my equipment, the 1.25" fitting is fine. Also, if using the specifically-made barlow, then you have to use the 1.25" fitting.

I was using a Baader zoom to help locate and find a good magnification. Of course, all these need quite high magnification, so it was mainly a 8mm Vixen LVW and a 7mm University Optics ortho. I tried with 6mm and 4mm orthos, but the seeing wasn't good enough and contrast was poor too.

My Vixen 80M Custom cost me £40, and that included the mount. Split Epsilon Lyrae at around x80 the other day. I was using 1.25" eyepieces though and haven't tried with the original 0.96" fittings and eyepieces.

You need to see whether it affects your ability to adjust the secondary. I found that the secondary kept slipping back in to the same position whenever I tightened the bolt and I couldn't get it into a position I wanted Easy to fix though. Like John said, you need to insert a large steel washer. I'll find a link to my old post from last year.

Try Go Stargazing: https://gostargazing.co.uk/stargazing-event-organisers/

Yep - that's how I fixed it

Unless you over-tighten the secondary bolts and gouge grooves into the soft metal of the secondary holder that then prevent you from making any fine adjustments again 🙄

Had a try at some tonight in my Vixen 80 f11.3 STF 2741 (HR 8040) - split at x113 with a cool blue/grey secondary to North-East of primary STF 2751 (HR 8065) - notched at x113, secondary to North-West. Uncomfortable viewing position! STF 2583 (Pi Aquilae) - even at x130, elongated and notched occasionally. Secondary to West, primary trailing. Seeing was pretty average and the skies light. This long (for me) refractor is a new acquisition and I am still to work out how best to mount/position it. At times it was very uncomfortable looking towards the Zenith. I still have to work out a good height and seating arrangement, my drum stool doesn't go low enough and my Manfrotto is not tall enough without becoming too wobbly.

Used the dob tonight and found it! Blimey - that took a lot of peering at charts to compare the view and find it. It's pretty faint and hard to find with the light skies here. Waited to 1am to get the best view. I reckon it's the same brightness as HD 230191 - Mag 10.0