Features of SDM#039

erick (Eric) · #1 16-05-2010, 10:13 PM

While I am waiting for a combination of a clear night and free time to give the telescope a nice calm enjoyable workout, I thought I'd show you some of the particular features of this scope. Some of these are new developments for SDM telescopes. (some inspired by my previous setup!

)

I'll do this over a number of posts, each to a particular feature.

First, to set the scope in size perspective, here is the full setup (minus shroud) with a suitable male model. This is a small SDM telescope - not the smallest ever - but at the very low end.

erick (Eric) · #2 16-05-2010, 10:15 PM

As we worked out how I would lift the entire rocker box/mirror box assembly, Peter Read suggested some cutouts in the rocker box.

These serve to:-

1) provide easy to use "handles"
2) reduce weight
3) look really cool!

erick (Eric) · #3 16-05-2010, 10:19 PM

First use of a non-laminate material for the azimuth bearing. It is a textured metal finish which contacts the teflon pads. Similar for the altitude bearings.

Looks beautiful. Works well in daytime testing - butterly smooth but good "stiction" properties. Will have to see how it performs overnight as temperature drops but I'm anticipating it will be close to perfection!

erick (Eric) · #4 16-05-2010, 10:21 PM

Not sure if it is the first time Peter has used a curved spider, but I have one. Peter spent a while to get the balance between curve and rigidity to his satisfaction. Early testing shows no strong diffraction spikes.

erick (Eric) · #5 16-05-2010, 10:23 PM

Peter recently went to a synthetic material for the split blocks. Easy to use. Should not alter their mechanical characteristics or dimensions over temperature ranges or age.

erick (Eric) · #6 16-05-2010, 10:26 PM

Peter fitted a universal joint on the top of the Argo Navis stalk/platform. I have found this useful as I go up and down on my gas-lift chair. I've lightly greased the top ball and can adjust this for easy, but confident adjustment in all directions.

erick (Eric) · #7 16-05-2010, 10:31 PM

As supplied by Moonlite and fitted by Peter with a bit of ingenuity since I had the focusser rotated 90 deg (the mounting holes which are supposed to also mount the slide are rectangular, not square.)

I've added a little knob from Bunings so I can better move it and have also replaced the heater (third photo). The supplied heater had six resistors in series with a resistance of 217 Ohms. Never even warmed to touch. I have 12 resistors with a total resistance of 47 Ohms. gets nice and warm now.

(Edit: and before you ask, yes within the power specs of the resistors - 3.9 Ohms, 0.25 watt)

erick (Eric) · #8 16-05-2010, 10:34 PM

Peter reversed the direction of the altitude encoder and after the shaft is lightly screwed to the connector to the mirror box, the encoder is held motionless by gravity (the metal plate rests against the bolt). In this way, any residual axial shift is not resisted and the cables never move, reducing wear and breakage of cables.

barx1963 (Malcolm) · #9 16-05-2010, 10:35 PM

THis is a thing of beauty Erick. STOP NOW before I do something foolish like ring Peter and order one for me!

mbaddah (Mo) · #10 16-05-2010, 10:40 PM

Erick what an absolute gem of a scope! Interested to know how movement of the scope is with the new changes as the night progresses?

I'm so envious can't wait for my SDM to get built

erick (Eric) · #11 16-05-2010, 10:41 PM

Peter designed and built a replacement for my rough heated eyepiece box, and it is a beauty. Matches the telescope in every way - varnished wood finish; the hinges. I love it. It sits on two stainless steel risers, one of which carries the power to the box. Peter installed a heater controller I built. The heating is provided by a 10" dewnot strap which lies on the floor of the box. I provided some additional outlets - one of full 12V and one controlled. I have built a little cluster of red LEDs on an RCA plug which can run off either (full 12 Volt or controlled) to provide some low intensity lighting inside the box. You can see it in one photo. Haven't yet tried at night to see if it is dull enough. Pictures show a full load on board.

Edit - forgot to mention that Peter came up with a very clever mount/dismount arrangement that is so simple and effective to use.

erick (Eric) · #12 16-05-2010, 10:47 PM

One unexpected addition had to be some counterweights to balance the loaded eyepiece box. But Peter came up with an excellent design - looks good in addition to being very functional.

The whole assembly can be removed entirely from the rocker box, and put back together for easy transport and storage. The black knobs come off, the two weight bars are removed and the bolts are then unscrewed from captive nuts in the rocker box. Then they can be put back together.

I have almost tried to lift the rocker box/mirror box with the counterweights still attached, but noticed in time. I'd sure notice if I did try - the counterweights are heavy.

erick (Eric) · #13 16-05-2010, 10:53 PM

There are so many great features, but two I am just going to love are:-

1) Quick setup and break down. Anyone who watched me setup previously knows how long it took to put together, install and wire up the encoders and Argo. Install onboard battery. Install all the heaters etc. (Dobs are supposed to be quick!

) I reckon 5 minutes tops now!

2) Powered Ground Board!

Hurrray! No more tangled leads. Once I managed to get my power lead caught up in the azimuth bearing on my previous base - no fun in the dark with a fully loaded dob!

AstroJunk (Jonathan) · #14 16-05-2010, 11:18 PM

That is an amazingly well thought-out scope Eric!

I recall Peter being very enthusiastic about the painted metal Alt bearing surface, so it makes great sense to use it on the az as well.

I'm looking forward to a review after a dewy all-nighter - I bet it all works a charm

pgc hunter · #15 18-05-2010, 09:04 PM

Fantastic Eric! Lots of excellent features which really make observing a joy. We've had a few clear nights now, what's your excuse for not having first light yet????

I've had my sdm out for 2 long sessions now and can report back on some of the features.

- The dew heaters work very well. I've had some of the worst dew I can ever remember last Saturday, to the point where the scope was dripping..with puddles forming on the ground under it... literally.. and the heaters kept the Telrad, finder and secondary bone dry,

-The shroud holds even severe dew at bay, at no point did water drip on to the primary. You will see hundreds of water droplets hanging off the inside of the shourd (and outside), yet they don't actually fall from it. It's quite a bizarre sight!

-The textured metal bearings are smooth without any of that annoying sticktion, even in the 4C temps I was out in. Balance holds perfectly, even when viewing near the horizon with a relatively heavy EP such as the Vixen LVW series

-THe powered ground board is da bomb!

-and make sure you don't forget to close the mirrorbox lid before removing your dew laden shroud