NEW SDM build

[sasup]

I would like to get a group thought on the aperture size I should get. First let me say I do have an idea what I want and have done the research to a point. My request is purely one of finding some bit I may have over looked, plus I just love hearing you lot go on.
I have put deposit on a 16" but as the build wont start till the new year, it gives me time to fret and change. I,m thinking I'll go with the 16" but a 14.5 is $500 US cheaper in the mirror and would make a lighter overall scope. The large aperture is great of-coarse but when do we lose the portability and ease of use for size?
Also I have been told I can do any color I want so:
14.5 vs 15 vs 16 inch, red-green-blue or a mix of all lets hear it.;)

[erick]

Decisions, decisions :confused2:

I think we can safely leave the choice of colour to you :)

Aperture? There is really no question - as large an aperture as you can afford, handle, transport, use etc.

So the questions to answer are:-

1. Cost? But, averaged out over the life of the scope, the extra is not much.
2. Bulk/volume - ability to handle and transport?
3. Weight - ability to handle and transport?
4. What focal length? How high will the eyepiece be? Can you observe with feet on the ground or will need a step or two? How will you be if you need steps?

[sasup]

Thanks for that. I agree my big question is size vs portability. I am leaning to the 14" now just due to moving it around. The mirror would be 14" f4.5 so the f Height is roughly 3" higher then the LB12" f5 I own now at 1500 tall. This seems to be about right as I am 6 foot tall. Also the build time for one of marks mirrors. I know there are other mirror polishers but I really want one of Marks.

[mbaddah]

Hi Stacey,

I am also awaiting a 16" SDM build due sometime early next year. I have rocked back and forth between 14-16" apertures. Initially I had settled on a 14.5", thinking it was small and light. Only recently it was changed to a 16". They are both LARGE size dobs and you will need to wheel both scopes around. Peter Read has told me several times that it's easier to wheel a 20"+ scope than to carry a 12" dob!

Hence bulk/weight shouldn't be a factor between these sizes. If you want something smaller and fit into a sedan, then your limit is most likely a 12.5" (just). I think more importantly to consider is costs and focal length. If the costs permit, go the 16" 4.5 :) Or even better, an 18" f4 :D

[jamespierce]

You have seen my 16" F4.5 both setup and in the back of my wagon, so you have a good idea what it's like. It is possible to get it in and out of a car, or up and down a few steps solo, but it's really not ideal. It is right at the limit of what I can fit easily in my wagon though (the truss poles only JUST fit across the back seat, and don't fit in the back without folding down a seat (which takes up more space)

Last time I was up at Peter's he was building a pair of 14.5s - they are a little smaller, but I wouldn't say significantly so - think about it this way, the mirror box can be 1.5 inches smaller in both directions, but all the hardware, motors, handles, truss clamp blocks etc are the same size.

So yeah, it will be a little smaller, but not a lot smaller. Might be quite a bit lighter though.

A 12" however is MUCH smaller in the sdm style, in part because you can get away without needing the wheely bars etc.

If you're going to setup and move it around solo, consider the 14.5" - but you're tall enough than a 16" should still be at your eye height at the zenith (and in reality you never really observe right up at dobsons hole, esp not with a servocat) ... you are getting a servocat right ?

[AstroJunk]

I agree with Mohammend regarding ease of movement when these dobs are set up - They are designed to be easy to position with wheelbarrow handles etc.

The biggest factor is really the vehicle you plan to transport the scope in. If you have to physically lift any part, then that will strongly influence your choice. I use ramps to wheel my 20" into a van, so no issues for me. I know that the 14" fits into a sedan, but it's not light.

Of course the next factor is eyepiece height. I don't mind a small ladder for 20" views, but I don't think I could go much larger (it's actually quite tiring to do a long nights viewing shifting ladders between objects). Anything 16" and below should be fine.

[sasup]

thanks guys ya im leaning to the 14.5 due to car size. I use the jazz to travel as its cheap on fuel. The height is ok cuss its a f4.5 at 14.5mm. I am also looking at the compact 17.5. Hi James I am switching over my servo-cat as well as the moon lite focuser and Argo.

[jamespierce]

You're on the right path, the best telescope is the one you use - don't underestimate how much difference it makes being able to move it around and set it up easily. Also - get the moonlight filter slide to match the focuser, loving mine. Finally - don't forget a dew heater setup for the UTA.

[skies2clear]

Stacey, it's so personal in deciding, as you have stated about your preferences so far.

I can only give my point of view, but if I had a 12" scope already, as I do, and you do, I would opt for the 16". Going from 12" to 14.5" may not be a big enough step for me. But if you are looking at a practical larger scope, with premium parts, then I can understand the decision to not go larger than a 14.5" and have this as your special scope, as it will be from SDM with DSO primary.

I went for a 16" and haven't regretted it. Both the 16 and 12.5 have Marks mirrors. You will be very happy....that's all I'll say.

Enjoy your new scope when it arrives!

[gb_astro]

Hi Stacy.

John Bambury has a 14" SDM (#33).
Set up next to my 14.5" it looks more like a 13".
Peter seems to have made a special effort to make it as compact as possible.
Have a look at the photos of it on the SDM site, (noting that Peter
is not a very tall guy).

http://www.sdmtelescopes.com.au/SDM033.html

Would be worth contacting John ( IIS alias ausastronomer) to see how he handles it.

gb.

[jamespierce]

My 16" was also designed to be as compact as possible (a decision I think Peter regretted as he did all the mods to fit the servocat in later)

[norm]

Hi Stacey,

Having owned my 14.5" SDM coming up 2 years in Jan 2011, I would say that weight is one of the most important factors when getting one built.

Its not so much when its setup and up and running, but the logistics of lifting can be difficult. Where possible ask Peter to reduce the weight, either by getting cutaways in the mirror cell or rocker box, maybe using aluminium for the mirror cell, even the lid for the mirror cell could be in thinner ply or tough ABS plastic. The wheel barrow handles/tyres - see if Pete can come up with a way of a quick release system - that potentially 3 kilos less you need to lift as a whole.

I know it may not sound much, but trust me, shed that weight! I manage to lift mine into the Triton. I wouldn't say I'm built like a brick Sh!thouse but I can lift things OK. In 15 years time, it maybe a different story !

Also factor in what you'll be lugging in the car, its not just the scope, but other accessories and all in all it can take up quite some room.

Some things I think are a must (hindsight is great).
- get the hard hat secondary box, don't muck around with the soft cover.
- get a hard case for the poles and see if it can cater for the Argo stalk and Argo (assuming you'll be getting them). I've got the soft covers and they work fine. However, ended up making an accessories box for my other odds and sods, plus argo/stalk.
- get a filter slide built into the secondary. Trust me, you won't be screwing in filters in and out on your eyepieces, its just labourious.
- get the metallic scope cover - can't remember its name.
-go the whole hog with the dew heaters etc. (could be standard now).

If you haven't seen an SDM in the flesh, PM me and I'm more than happy for you to come around and check it out and have a play. I'll be away for a week or so after XMAS, but anytime after is fine.

Good luck.

Norm :)

[sasup]

Hello norm. I have opted for the whole hog but i am using my old servo with new drives, Argo and cr2 focuser. I am hoping to get one of mark's mirror or Zambuto. Thanks for the tips.
Hi gb I talked to John today, great bloke.
James: I went with the lot so full astro dew protection and covers ect.
So as of now Im going with the 18" compact low-rider in red or black!

[sasup]

OK after some communication and help from peter reed, John Bambury and Alax as well as the good forum members I have decided on a 18 inch F4.5 low-rider in compact form. It will look like SDM number 028 on the SDM scope page.
Now the low rider telescope is a interesting way to take a Tall ,fast scope and keep your feet on the ground. For the 18" I believe the secondary is a flat 3"mirror turned to 30 degrees. Here is a web site if you have not heard of a low rider before.
http://www.biophysik.uni-freiburg.de/Reiner/ATM/lowrider/lowrider_e.html

[jamespierce]

Wow, look forward to seeing that when it's done ... why stop at 18" !!! I thought Peter wasn't making the compact designs anymore, how did you get him to change his mind ?

[xstream]

Hi Stacey,

I'm glad to see you like SDM #028 ;) I also believed Peter wasn't going to make the compact design again because of the extra time and effort it takes to get everything just right.

We're more than happy with ours. It had to be compact and portable because of our life style (Grey Nomads) so the scope goes where ever we go. 18" is probably the end limit of portable I can lift it on my own but wouldn't want to do it regularly, so the norm is a two person lift.

If you want to discuss anything about it just pm me and I'll give you my number so you can give me a ring. As for the red or black talk to Peter about the green he and I have talked about a compact would look magic in it!

[gary]

Hi Stacey,

Good choice.

Best Regards

Gary

[Satchmo]

You are correct , SDM does not make the Obsession style 'compact' currently. As I understand it from Peter , Sasup's scope will be a standard SDM design with a lower profile mirror box much like SDM 035.

Putting my ATM 'er hat on for a moment :

The 'low-rider' design has the disadvantage that it is somewhat more a challenge to baffle from stray light, and having to look up into the eyepiece all the time. Depending on your age, these low riders design could literally be a `pain in the neck'. It will also require a larger flat probably 4- 4.5" diameter or so compared to 3.5" normally . I personally don't think it is worth these disadvantages just to lower the eyepiece height by 10" , but I'm happy to stand corrected if I'm wrong in the presence of the finished article.

[jamespierce]

Like Mark I'm a curious/dubious of the low rider style - it seems like it could end up with some awkward angles both up high and down low ... I reckon a good adjustable chair, or a little step might be a better solution. An 18" shouldn't need a ladder, just one step.

Check out these babies - http://www.catseyecollimation.com/cperch1.html

http://www.catseyecollimation.com/summitwscope-b.jpg

[sasup]

Ah bummer I thought mark would have loved the idea of a low-rider. I must admit I have my concerns but scope baggers cant be choosers and I have to find a way to make it work or down size back to a 14.5. Now that I have made up my mind to try the 18 its hard to go back in aperture. I do have a viewing chair and im not scared to use it; However I do need to make up my mind before peter starts to build as the pole height is shorter in the low-rider then in the classic style.

[jamespierce]

I've just loaded and unloaded my scope twice in the last two days (holiday/family trip, observing next week) - gotta say I wouldn't mind if it was a touch smaller or lighter ! How about a compact 14.5 with one of Mark's mirrors - super portable, great optics etc. Aperture isn't everything...

[gb_astro]

Stacey, Phil Townsend, who lives on the Central Coast, has SDM #34,
an 18" F4.5 with Argo/SevoCat. He used to be a regular at the Pony Club
but has not been there for awhile.
If you wanted to see a standard 18" in the flesh he might be able to help.

gb.

[jamespierce]

I think the problem with 18"s is that unless it's an F4 mirror you start getting into 'arkward to use' size ... at which point, why not a 20, or 22 etc ... You're very definitely getting into two person lift territory etc.

[sasup]

James I see your point but its still early days. I think I want the 18 and im almost sure of it. Peter seems to think it would weigh in at 90lb or 45 kgs roughly... This was off the top of his head and I can lift that. My issue is with the height. If I dont go the low rider then im at a focal length of 2080mm right?

[GrahamL]

I can't make my mind up on those lowrider designs around the net .

The lower focuser height does seem to come with a fair bit of baggage
always thought the angle could be a little tough on the neck and shoulders. I wasn't aware you also had to increase the secondary size/shape and baffle the focuser to ?

Looking at (sdm 10 ) .. and throwing a pair of decent stillettos on Peter the zenith height dosn't look to bad , trim a little more back in the build and you could probably live with it and a very small two step ladder.

But is that what you wan't ?... custom built scopes are a big investment .. there was a very nice 18 ' f4 at astrofest this year
and there are some very nice sub f4 scopes of larger apperture kicking around which seem to be ground based for visual observing .
If thats really important to you I'd try and have a chat with the owners of around the 18/20 " apperture and gauge there thoughts .

good luck.

[sasup]

Thanks nightstalker I to im thinking maybe Ill just live with the height but still the low-rider has me curious. The great thing is if it does not work, one only needs to replace the poles,secondary mirror and punch a hole in the uta. so either way i cant weight. merry Christmas.

[sasup]

OK heres the latest. 18" scope, semi compact, low rider and with diagonal.

[jamespierce]

What do you mean and with diagonal ?

Low rider, but with a diagonal so you can change the viewing angle ? How extreme can you go with the lowrider ? Could you go as far as 45 degrees, then use a 45 degree prism ?

[sasup]

I think the thing to watch for here is the amount of times the light must be reflected. The secondary is at 30 degrees and then the focuser is at a slight angle ( to be determined). A diagonal will help, possibly with ease of use at low angles. I think it has some risk in going with an unproven application like the low rider but equally if there is a way to keep my feet on the ground and still play with large aperture, thats great. here is a photo of a 22" as I remember.It would be good not to be the Gennie pig but what the hell.

Would be great to catch up soon. Have you ordered your frame?

[gb_astro]

Stacy you are going to need about 9cm (3.5") of extra focus travel to accommodate the use of a 2" diagonal as an option.
Can't really see you getting it with a newtonian focuser. ??

gb.

[sasup]

Thanks for that ,Ya I am going to have some more talks with moon-lite. peter and I have thought of that and its one of the things Im working on.