Well, I thought I'd start a new thread as suggested by a couple of forum members where I'll post information and pictures on the latest project.
I will be constructing an 18" f4.5 truss Dobsonian made in the traditional way using Kriege's The Dobsonian Telescope. I'm not going to deviate too much from the plans in this book, so it may look "boring" or just like all the other 18" Dobs, but, I guess, It'll be my Dob. My aim is to have it complete for Summer 2008. Not an unreasonable deadline considering we are also shortly commencing building a new home. It will be the first scope I have owned in nearly 20 years!!
After a lot of ideas and advice from forum members in other threads on IIS, I have started this week by ordering/purchasing:
I hope to start the secondary cage in the next couple of weeks (work permitting), once I master the art of making the rings on a router. I have a few questions and some issues with some of the calculations concerning the secondary mirror and eyepieces, but I'll post these questions under different topics as I go.
There's not much more to tell at this stage, but as time progresses, I hope to add to this thread with updates and pictures.
The Kreige design may be 'boring' now and a little heavy, but I have yet to see an alt-az design that works better in the field.
Two snippets of advice from me - Oversize the secondary because you WILL do some imaging sooner or later, and pre-drill the rocker box for ServoCat as it is easy to do on the build, and a complete pig to do as a retrofit (and yes, you will get that too, eventually).
All the best with it and I'm sure it will turn out great.
Your not making cell yourself Steve?
Thanks Orion, I just sat down and looked at every component of the scope which I have to construct, and I made a decision to have some parts ready-made, and the mirror cell was one of them. I could make it, no problem, but I want to concentrate on the timber work. And I'm confident the mirror cell will be first class.
Quote:
Originally Posted by AstroJunk
Oversize the secondary because you WILL do some imaging sooner or later
Hi AstroJunk, I pretty scratchy when it comes to calculating this sort of thing, but I'm working from Kriege's book, page 95. Here is what I've calculated so far (all in inches) using a 20mm eyepiece:
Secondary cage I.D. = 19", or 9.5" radius from centre of cage to inside surface of cage.
Focuser board thickness = 0.75"
Focuser = 2.5" (haven't decided on either Feather Touch or JMI)
= 12.75 distance from seconday mirror to focal plane.
using the equation, the result is 3.49" secondary mirror minor axis! Or 3.5".
Now that's fine for a 20mm eyepiece, but what if I use a different eyepiece? all these parameters change. Did Kriege use 20mm because it is usually in the middle of the range for eyepieces?
Quote:
Originally Posted by AstroJunk
...and pre-drill the rocker box for ServoCat as it is easy to do on the build, and a complete pig to do as a retrofit (and yes, you will get that too, eventually).
Ohh yeah, no problems here. I will start researching ServoCat, Argo Navis and encoders when I even start on the mirror box. I'm going to take up Gary (Wildcard Innovations) offer for some advise on this when the time comes.
Now that's fine for a 20mm eyepiece, but what if I use a different eyepiece? all these parameters change. Did Kriege use 20mm because it is usually in the middle of the range for eyepieces?
I ran your config through Scope Designer and a 3.1" diagonal will give a 0.25" diameter fully lit field and the 3.5" an 0.75" fully lit . At F4.5 the light drop off would be very gradual so you don't need to worry about eyepieces of long focal length and wider field stop. You might want to go with the 3.5" diagonal and allow yourself a little more freedom in dimensions. It certainly won't make any perceivable difference at the eyepiece. Its always sobering to remeber that a typical SCT has a 40 percent obstruction across the secondary baffle.
That being said I like a clearance of at least 3/4" -1" around the primary to avoid any heat currents that might be hugging the inside of the cage , but thats just a hunch.
...You might want to go with the 3.5" diagonal and allow yourself a little more freedom in dimensions. It certainly won't make any perceivable difference at the eyepiece...
Thanks Mark, I remember you said in passing comment that a 3.1" should be okay. I was going to get back to you on that. Good to see both my poor maths skills and poor knowledge of optics have not let me down this time around!!
I have had lengthy discussions with Peter Read from SDM telescopes on this very thing, as it relates to 18"/F4.5 scopes. I have an 18"/F4.5 Obsession which uses a 3.1"secondary, Peter Read uses a 3.5" secondary on his 18" scopes which still only creates a 19% central obstruction.
If I was to build an 18"/F4.5 Obsession clone I would probably go with the 3.5"secondary but I certainly don't notice any light dropoff with my scope having the smaller secondary.
I have an "oversize" 3.1" secondary on my 16" F4.5. Avoids any edge problems with the secondary.
I have a minimal size 1.3" secondary on my 8" F6 travel scope. I can notice some minor difraction effects from the secondary edge when using ep's with 24-27mm field stop size.
Both mirrors are from a reputable supplier - Protostar.
We were talking about thread inserts this morning and where you can buy them in Melbourne. I could not remember the name but said that there is company over at West Heidelberg that sells them. Well I have finally located the address it is:
I completed the secondary cage struts today. I have shown three struts in the first image but I have actually completed four of them. After much agonising over the final diameter, I chose 1 1/4". The struts are exactly 12" long (Kriege suggests 11" but I like even numbers). I polished them with some 1500 wet and dry paper and they look good, but I'll probably end up covering them with foam tube from Clark Rubber.
The threaded inserts I eventually bought are an interesting variation from the type shown in Kriege's book. They have one set of "teeth" and a flanged end, both of which are pressed to each end of the thread. The flanged end fits neatly into the end of 32mm aluminium tube with a 1.6mm wall thickness.
Phew!! At last some progress!! Next week, the secondary rings. Meanwhile, I've ordered a 3.5" secondary mirror, spider, mirror holder and anti-dew heater from ProtoStar tonight!
Drill a 8mm hole in a spare bit of ply, and bolt one of those struts hard onto it then give the top of the strut a jiggle. Does it move?
I have a sneaking suspicion that the flange may prevent the insert from getting enough 'bite' to make it really solid, and that tightening it will make no difference as the flange will just sit more firmly on the ply rather than stiffen up the assembly. On the other hand, I could be completely wrong!
In dob building, your optics are only as good as your loosest strut ...
Hi AstroJunk, Yeah I know where what you mean. I hammered them in with a woodworking mallet. Then I screwed a hex head bolt all the way into the insert and hammered only on the bolt head. Then I screwed it to a scrap, like you've suggested. They are rock solid. I'm very happy with them.
This weekend saw some progress on the Dobsonian project. I purchased 3/4-inch (19mm) B-B Grade 9-ply Hoop Pine in a 8'x4' sheet. This cost $130. I was going to buy the A-A grade marine ply but it was over $200 for the same size sheet. I visited three suppliers and decided to purchase from Construction Plywoods in Airport West.
I cut the secondary cage rings with great assistance from my Father. Then the rings were placed together and the centre holes drilled for the strut bolts. I recessed the holes for the struts with a 1 1/4-inch forstner bit down to a depth of 1/8-inch, and on the outside of the rings I recessed the washers so they are flush with the timber.
I routed a recess for the focuser board which will curved to match the inside tube. I will make that next week, hopefully when the Starlight Instruments Feather Touch focuser and the ProtoStar secondary mirror and spider arrives from the US.
I will attempt to bend a length of pine into a curve to match the curve of the rings.
I am using brass bolts and washers which I feel will complement the rich baltic stain I will give the timber. I have always liked brass fittings and fasteners on timber products. Now I'm off to buy some stain...
Well the Feathertouch focuser (finally) arrived today from Starlight Instruments. Also in the post last week was the spider, 3.5" secondary mirror, secondary holder with built-in anti-dew heater, from ProtoStar.
But back to the focuser. What a beauty, hey! Really well made, solid construction. Even just handling it out of the box it feelslike it is of the highest quality. I'm glad I purchased it.
Now all I have to do is finish the focuser board. I've had mixed success with making a curved focuser board. My main issue has been creating the correct curvature.
Thanks Mark, I remember you said in passing comment that a 3.1" should be okay. I was going to get back to you on that. Good to see both my poor maths skills and poor knowledge of optics have not let me down this time around!!
Another consideration in choosing a secondary size is if you want to use a paracorr you will need to bring the focal plane out another 15-20mm, which will demand a larger secondary.
In the designing of my 15" f4.8 SDM, the calcs suggested that a 2.6" sec was just big enough, but once I discovered that I like using a paracorr, it would not be. I'm glad that I went the next size up and at barely over 20% secondary obstruction, Im not losing anything.
Here is the mirror cell. It is an 18-point mirror cell built by SDM Telescopes. This was one part which I didn't have time to build myself. Besides, the quality of Peter's work is exceptional! A very high quality mirror cell. If the workmanship of my mirror cell is any indication, anyone who owns an SDM scope, has a custom-built scope of the highest standard.
Maybe my next one!
