Planning a 20" (500mm) Scope (Nasmyth??)

[swamp thing]

Hi I've changed the topic title for you Jim. Good luck with the build, look forward to seeing the beast take shape.

20" is a big lump of scope.

[J_M_Franklin]

Thanks Clive, good couple of links there, and the calculations page is far superior to what I had found...

[J_M_Franklin]

Hi I've changed the topic title for you Jim. Good luck with the build, look forward to seeing the beast take shape.

20" is a big lump of scope.

Thanks Steve, appreciated...

[J_M_Franklin]

I'm planning my Obsy at the same time, already designed my house which will be built in about 3-4 years time on a nice parcel of land, originally considered building the Obsy on the house, but decided against that due to heat transference from the living section of the structure. I have decided to keep it a simple affair, but I may make it twin domed for the 500mm and for the other scopes I have and will likely buy in the meantime..

[Mav359]

Hope the door from the garage is big enough to get the scope out when you have finished

Top project, Well done

[J_M_Franklin]

Hope the door from the garage is big enough to get the scope out when you have finished

Top project, Well done

Garage is full of tools and equipment, no chance of building it in there. I recently demolished the garden shed as the floor was rotten and the current house, which has to last till we build ours, has a postage stamp garden so I need the space for the ankle biters (kids). I will design it all, build it in modular style, ensuring all sections go together, then when I actually buy the optics and want to complete the project I will either have the workshop I have designed on the house, or I will rent a small industrial unit for a short period to put it all together, check it over, get all the kinks worked out and then flat pack it ready for observatory installation...

The better half through something into the mix today, she asked if I was going to make it a robot scope so I could control it even when we are away, and that got me thinking...already planned on 4 PC's for the control room, Imaging, Control, Internet and secondary control/imaging computer for the smaller scopes and as a backup in case of an issue with the primary...but sat upstairs, doing nothing at the moment, I have an 8 dual Core Xeon machine with 16GB of memory and 4GB of storage that was built as a family server and web host machine, but this plan was shelved when BT informed me they would not give me a static IP on my fibre line unless I shelled out about £260 month for a business account...I told them to put in that place where the Sun only shines if buried upside down...I think I now have a use for this because I could use this as an internet server in the Observatory so I could make the telescope not only robotic but remotely controlled for anyone with the correct login information....so there is a possibility of "renting it out" to other amateurs on nights when I am not using it...but the fee would be one that would simply cover the cost of electricity and maintenance at the scope/observatory...something nominal like £10 per night...but that last part will have to wait until it's all done and dusted and working..just a thought process at the moment.

[Peter Drew]

As you would have to use a Cassegrain design to avoid the large obstruction presented by a flat secondary mirror, can I suggest you consider a Dall-Kirkham configuration. This employs a 70% corrected paraboloid primary and a spherical secondary, somewhat easier to figure than the classical Cassegrain. If the spider vanes of the secondary and tertiary mirrors are lined up with each other you should still get only four diffraction spikes.

[J_M_Franklin]

As you would have to use a Cassegrain design to avoid the large obstruction presented by a flat secondary mirror, can I suggest you consider a Dall-Kirkham configuration. This employs a 70% corrected paraboloid primary and a spherical secondary, somewhat easier to figure than the classical Cassegrain. If the spider vanes of the secondary and tertiary mirrors are lined up with each other you should still get only four diffraction spikes.

Hi Peter, I have considered that and I have been having some discussions with actual scope makers, not the big boys, but some of the professional amateur makers (if you know what I mean) and most, not all, have encouraged the Nasmyth design as they are very rare beasts in the amateur world. I have asked about the various designs, they're generally negative about Dall-Kirkham design due to various aberration effects unless you use the corrected version.

I am looking at a long focus design, in excess of f/10, and that rules out the D-K design, but the design I want would allow me to use the scope as either a classical cassegrain or at the Naysmith focus, so i would get two telescopes in one, depending on what I want to do.

[Stu_2011]

Thats a damn fine train of thought there James, the obstruction would be less that a standard, but it would have to be firm to avoid flexing as the scope moved...Titanium?? (Joking on the titanium)

Titanium isn't actually that stiff as metals go, it's strong but not stiff. Titanium has an elastic modulus (i.e. stiffness) of 110 GPa, steel is typically 200 GPa and aluminium is 70 GPa. Carbon fibre is the material if you want light weight and stiff, and a good design is just as important to produce a rigid structure.

Sent from my iPad using Tapatalk

[Moonshane]

changed the typo. I am just shy of 6'4" and was about to post the same thing John did but if you are going all EQ on us then I agree the design you mention might be the best. personally though for this sort of thing (especially if imaging, I think I'd go for some kind of Dall Kirkham or R-C design? might be better in the long run.

[Peter Drew]

I'm not sure what aberration effects have caused concern re Dall-Kirkham systems? I've made several from 8" to 17" aperture at F20 with excellent results, my understanding was that within reason, the longer focal lengths were superior to shorter. It's true that if used as an alternative swap over Newtonian arrangement to provide a F4 or 5 which used to be an option then the 70% correction was not ideal. I mentioned the D-K as it would be likely that a beginner would have more chance of producing a working set than a classical Cassegrain set.

[Astrobits]

Reading through this thread I think some clarification is required.

The figure on the mirrors of a cassegrain, be it D-K, Classical or R-C ( Richey-Chretien ) is independent of the focal lengths one wishes to chose. All sizes can be made in the three types. I think that it is generally accepted that the image at the edge of the field of view is worst in the D-K and best in the R-C but the difficulty of making ( and cost if purchased ) increases from D-K to R-C. This mostly affects those who do astrophotography, visually the D-K is quite acceptable. It is possible to add some corrective optics into the D-K design and there are some "corrected D-K" telescopes on the market. One of the limitations of the cass design is that the secondary amplifies the focal length so it is difficult to make a cass a wide field instrument. The faster the system the faster the primary has to be and the bigger the secondary is. To get the optimum system a cass is normally made to about f/16-20 with an f/4 primary and a secondary that amplifies 4-5 times and is around 25% the diameter of the primary. The Nasmyth variation simply adds a flat near the primary to deflect the focus out through the Dec/Alt axis and does not add anything significant to the rest of the optical design.

Peter is correct in his post and one of the designers/inventors of the D-K, Horace Dall, had a 15" D-K telescope ( made by his own hands of course ) in his garden in Luton and usually didn't bother to use it unless the atmosphere allowed him use it at extremely high magnifications.

Nigel

[rwingnut]

Jim,

I am late to the game, just wondering if you have made any progress on this project?  I have had a 14" mirror in storage for over 6 years, just getting back into things myself.

I did build a 8" Newtonian on a "stationary eyepiece" mount I designed... Built of wood over a two week period and fared pretty well on it's first light.