New SDM compact truss dobs

[Starkler]

I had a look at the SDM site (http://www.sdmtelescopes.com.au) today and noticed pictures of their new compact truss dobs.

I must admit a touch of envy seeing a 15" f4.8 (same as mine) but in compact form. It would be so nice to be able to fit a scope of this size in the car boot instead of messing with a trailer.

[anj026]

They are very tempting Geoff, even for a refractor nut like me!

[tnott]

Nice. Interesting is the shorter altitude bearing (instead of the foldable bearing of the Obsession UC) made possible by the continual strip of teflon. The 8 poles simplifies the addition of a shroud for those who want one.

[CoombellKid]

If you think about it you dont need full half circle Alt bearing. If the scope
has Alt rocker teflon bearings spaced (at their furtherest ends) 70 degrees
apart and the scope altitude travel is limited to 90 degrees you have 20 degrees
of Alt bearing never doing any work. Couple that with the first 15 degrees
off the horizon bearing mostly unfavorable to observing you could cut the
Alt beaing down to 145 degrees. If you employed full length teflon strips
for the rocker box side you will extend that leeway.

I like the full length shroud look, plus the full SC on a compact style.

regards,CS

[tnott]

Yes. Unless you want to add the Servocat, in which case you need a bit of overhang on the end the altitude motor is placed.

On the Tridob I originally did away with the extra 20 degrees but then needed to add a piece to the rear fin when fitting the Servocat.

[Rodstar]

I had the same feeling, Geoff. My mind has been wandering down interesting paths of late....the idea of having a second "small scope", like an 18" compact scope, is rather tempting (although divorce lawyers may inadvertently also become part of the equation).:P

[CoombellKid]

Yeah but!!!!!!! and I know you have one on your 16" and I know they are
a GREAT piece of kit. And I am in no way knocking your choice. I really
fail to see the benifit on a scope small enough you can peer through the EP
from the ground. Or at least maybe need a step stool. So IMO (but it's only
mine) a servo cat on this size scope or even an f/4.5 18" is overkill. A well
made truss dob of any flavour the action should be so smooth you dont
need tracking.

just my 2c

btw, I love your scope and what you have done with it. It must be a real
treat to use. I may build a compact design on the side myself.

regards,CS

[CoombellKid]

Geoff & Rod,

They look amazing would love to see and try one in the flesh. As you know
first hand he is an awesome craftsman. He is also a top bloke and gave me
a lot help.

regards,CS

[tnott]

I thought so once too.....

http://www.cloudynights.com/documents/Stellarcat.pdf

Read the summary at the end.


:P


Of course tracking is not essential, esp. if a dob is designed right. But it has a few advantages - I find viewing much more relaxing without the nudge ,nudge; it is easier to share the views with others; you can see more detail; the object can be kept in the diffraction limited center of the FOV, you don't need to rebalance for heavy eyepieces and .....



it is really COOL!:D

[CoombellKid]

Like I said, I'm not knocking your choice... it's purely up to you. And as
I already said it is a great piece of kit. But it has not won me over on the
dob smaller than 18". Sorry but I've tried with and I've tried without. :P



And this is the biggest falacy of them all and often repeated. It may work
out that way on paper. But at the EP I dont find myself going sheeezz I
loose so much detail unless the object is centered in the EP :lol::lol::lol: I
think you just need toget better EP's son. And if you find you get up tight
and cant relax due to the fact "I've Got To Nudge Again" son you got to
learn how to multi-task or just not think about it so much :lol:

Like I said earlier I'm not knocking your choice. And I never read reviews
especially off CN. But that's just me :)

regards,CS

[ausastronomer]

Hi,

Andrew Murrell, Gary Kopff and I actually used the 18" ultracompact SDM prototype at Coonabarabran in February of this year. This was an F4.5 scope. Peter has based his design on Dave Kriege's with some of his own modifications. Peter's design isn't quite as light and portable as Dave Kriege's but it is stronger and more stable. Peter's design uses a short upper cage assembly and 8x32mm poles, which is very stable. The Obsession design by comparison uses 6 poles which are about 25mm diameter or a little less and a ring as the virtual UCA. The Obsession design is lighter and more compact, but not as stable.

I liked Peter's 18" Ultracompact a lot. In use it feels very stable and just like using a classic style Obsession scope. It depends what is important to you in this regard, stability or weight and transportability.

Cheers,
John B

[ausastronomer]

Hi Rob,

I thought that once too.

After having Servocat and AN on the 18"/F4.5 for a few years I have changed my opinion to the extent that I will never own another dob without Servocat or AN fitted. I am in the process of building a 10"/F5.3 truss dob, as a travel scope. I will be fitting Servocat jnr and AN to this scope and there is no thinking about it required, other than, "can I afford it". When you have it you can always release the clutches and move the scope manually, in the traditional manner, which I do quite often when going through the LMC, SMC and rich galaxy clusters. You can't reverse the process and "turn it on", when you don't have it fitted. Unfortunately it isn't cheap, but very nice to have if you can afford it. I don't really see scope size as a determining factor. It is just that as the scope size gets larger the lower the percentage of the total scope cost, is represented by the cost of the servocat system itself. In the case of my 10" scope the servocat represents almost 50% of the cost. With a 30" scope the servocat represents about 10% of the cost.

Cheers,
John B

[Satchmo]

Hi John

Did you get your Servo Cat before the dollar crashed ?

I'm convinced about Altaz drives. I had a Dob Driver which was a great intro but was a pain to use, but I will fit a Sidereal Technology drive to my next scope because it is sitting on the shelf . Being able to completely relax to take in the detail while you are looking through your dob is like receiveing a shoulder massage at the eyepiece :)

Mark

[spacezebra]

Yes, the SDM UC is a brilliant design - 20" not that far away!

Cheers Petra d.

[Starkler]

Does anyone know if the alt bearings collapse into two pieces like the obsession or is removable for transport?
I could ask Peter directly but I'm sure other's would be interested to know also.

P.S. The gold anodised tube used on the uta is a nice touch.

[erick]

Doesn't look like it in the photos and I think I've seen mention that they don't?

[Phoenix]

Hi Folks

No the alt bearings do not collapse on the 15". Originally I was keen on this and Peter was happy to pursue it. A few design concepts were drawn up for collapsible bearings but the connection points for the 8 truss pole design proved problematic (easier with Obsession's 6 truss pole design). But the 8 truss pole design with SDM's classic stability/rigidity was what I was after in a compact design, so a compromise had to be made. Peter did have a close look at collapsing the bearings and I think if this was a requirement for your scope he would take another look at it. After all he is a genuine problem solver and superb craftsman.

The servocat was a no-brainer for me (other than cost). While there is a lot of 'fun' pushing around a scope, not to mention a great way to learn the night sky, the nature of my work means that I 'never' have enough time to observe so I am keen to get to what I want to look at quicker - that's my excuse and I am sticking to it. And as John B mentions, I can always release the clutch and I'm back to learing the night sky. Just personal preference.

Cheers
Nix

[Phoenix]

Can't wait to see some photo's of Petra's 20" SDM UC. I think Peter and Petra started on the 20" before the 15" got going. Let us know when some photos become available.

Thanks also to Mark, who posted in this thread, for a wonderful Mirror (so Peter Read tells me, as I have not as yet picked up the scope - frustrating!).

Cheers

[CoombellKid]

Ummmm errrrr perhaps someone didn't tell you servocat wont increase the
number of objects you see... it's the Argo Navis that will do that. The
servocat only does the push to bit. And if you wanted to have a race against
someone that pushs with an Argo Navis you would Whipped!!! everytime. :P

True :thumbsup:

IMO for what servocat does. I think it is well worth the money, like I have
mentioned _several_ times it's a great piece of kit. But and having used it
(you know we do get to see and use these kits up here, so we dont need
to be sold on them). I still have no valid reason for it use on a scope like
a short FL 18" or anything below. Money and cost bare no importance to
me if I see a valid reason, which I haven't yet. And no offence John but
I would feel like a right "W" putting one on a lil 10" but that's just me, and
again it ain't the cost. I mean why would you do that if not to make yourself
a big headed pullhard.... I mean really get a gripp. But then again that's just
me, but I know I'm not alone in thinking that.

The next scope I build, looking to start towards the end of next year will
have servocat, but that's because I'm going to need a ladder to get to the
EP. Not because of some ego I need to let loose.

regards,CS

[astroron]

I would like a Servocat on the 16" the main reason being at high magnification an object zooms through the field at a rapid rate of knots.
Trying to observe faint stars in faint galaxies would be a great help with a drive mechanism.
Ron

[CoombellKid]

arrrh! Ha!

Ron has a reason that actually seems valid to me.

regards,CS

[ausastronomer]

Rob,

It has nothing to do with ego.

Having servocat on a 10" scope effectively turns it into a 12" scope by allowing you to resolve much fainter detail in dim targets, as a result of the accurate tracking and increased comfort.

The motorised tracking is the big benefit, goto is neither here nor there.

Why do you think double star and planetary observers put their 4" refractors on a tracking eq mount, which generally costs more than servocat (for a good one) for a telescope less than 1/2 the size of my 10" ? Ego ?

Cheers,
John B

[CoombellKid]

Unfortunately John and I hate to burst your bubble but it's still just a 10"
scope, it's a simple thing called physics ;). And as for comfortability and
increased detail, where is the science that points to this. I think it's a
more subjective thing than actual reality.



Well I really dont think your comparison is a good one, kinda like comparing
apples and bannanas. Besides most of the setups you are comparing to
usually have dirty great big cameras attached to them. Have a close look
when your at your next star party. And probably why they dont use 10"
dobs with servocat installed :) I thought this is about servocat on
small dobs :shrug:

In the end it's all about what floats your boat, ego or scope :thumbsup:

regards,CS

[tnott]

The Obsession UC design uses a single long strip of teflon (and rounds out the front edge) to help eradicate the "bump" in the join in the break apart altitude fins.

Having a single piece of Teflon also means that you can design the front of the altitude fins so that they are shorter than if you had the separate pads at 70 degrees. So, this means that the SDM UC altitude fins do not go any higher than the UTA when it is storage mode anyway, negating the need to have them fold over, unless you want to store it under your bed. As long as they are above the centre of gravity it won't tip over.

If you look at the photos you will see what I mean. I would have gone this way with the 22" if I hadn't already got the parts cut first - would simplify construction.

[Satchmo]

Who needs `science' to back up or legitemise whatever people find gives them any kind of edge to their observational abilities?



Your posts are a living example..thanks !

[tnott]

The diffraction-limited field in a newtonian is actually quite small so objects are sharper in the center of the FOV. This has nothing to do with eyepieces, though good ones like Naglers will have less astigmatism (seagulls at the edges) which is another problem.

For visual observing, ultimately the discernment of detail is made by the human brain, so it is not unreasonable to suggest that it will notice greater detail if it it is more relaxed and not using some thinking processes for nudging the scope.

These problems are more of an issue at higher powers when objects whizz across the field quickly. One thing a larger scope allows you to do because of it's greater light grasp is to increase the magnification on small, faint objects like planetaries and galaxies without them dimming too much. It also has greater resolution and thus potential, when the atmosphere allows, to increase the magnification more than a smaller scope. So the bigger the scope, the more advantageous tracking becomes.

Many experienced observers have stated there is an increase in the discernment of detail by adding tracking to a large newtonian, not unlike an increase in aperture. There is, of course, no more detail collected by the telescope but the person viewing does indeed see a bit more IMO.


Anyway, it's nice to have but not essential for enjoyable viewing (a relief given the plunging Aussie dollar) and probably goes against the low-tech, low-cost, no-fuss, yogurt-powered Dobsonian ethos somewhat.:P

[CoombellKid]

When someone touts an idea like it's fact. :)



Gee thanks Mark I'm really glad you appreciate them :rolleyes:

regards,CS

[Satchmo]

What is not potenetially 'factual' about someone seeing more detail with a smaller scope with a drive than a larger one without ?

I'm not an avid observer like JohnB but I remember two oppositions of Mars ago I saw fine granualation in the markings on Mars in my 340mm F5 with newly fitted Dobdriver : finer detail than anything I have ever seen with a larger non-driven telescope. This detail was invisible when I tried hand tracking. I don't feel I need to conduct a formal scientific study to satisfy anyone else of my observations..:)