View Full Version here: : Reducing the length of my 18inch dobsonian?

06-11-2017, 01:06 PM
Howdy everyone!

I am looking for advice or ideas on modifying my 18" dob. I am interested in reducing it's 2140 (solid) tube length (stands at approximately 2200 on base) to something a bit shorter. The scope is wonderfully portable, and the tube comes apart and sets up in under a minute..no problems about that aspect. I love the scope but being able to take 500mm off it would make it easier to use and keep my feet on the ground! . Is this possible and does it have great impact on visual use?

I guess I am trying to find a way to enjoy my 18" scope without constant ladder climbing!! I love the views, and deepsky ability but just thinking of the height! So instead of long and fat, I am thinking shorter and fat!! :rofl:

Thanks kindly!

06-11-2017, 01:44 PM
Here is an idea.
Dig a hole and do it nice, so that some of the scope is under ground level.
I could imagine this would work for a permanent set up.

As to doing anything along the lines you mention I dont think it is possible.


06-11-2017, 01:54 PM
Fair enough! I had also not taken into consideration, as another member pointed out that mirrors are optimized for certain focal lengths! Well a great idea that kind popped quickly! I will just have to be creative and make a sort of round doughnut walking platform that allows me to stand on 'my feet'!!
Thanks kindly!

The Mekon
06-11-2017, 06:25 PM
Two possibilities - both expensive.

1. Have the mirror re-figured and coated to around F4
2. Fold the telescope by introducing a flat secondary.

Otherwise - buy a good three step ladder- that should get you to the eyepiece at the zenith and just enjoy.

06-11-2017, 08:54 PM
Thanks kindly!
I think the step ladder will do fine! I may just sell it off and just get a nice 12 or 14" (possibly with GOTO) or learn to live with it and enjoy the nice sized aperture! :D

06-11-2017, 11:28 PM

Try contacting these guys and see if this periscope with a relay lens will work on a Newt. Sales blurb says its available for most telescopes.



07-11-2017, 01:26 PM
Hi Bill
I saw one of these in a movie the other night I thought it was something the prop dept thought up but it seems they are functional.


07-11-2017, 01:30 PM
A deck would do it. By my eye it would only be about 2 feet above the ground. Have the hole where the scope meets and pivots on the mount and it would not have to be all that large a hole I expect.

I dont like the idea of a ladder in the dark one mistake and you could break something.


07-11-2017, 01:39 PM
Yes they look really interesting and I will check that out! If worse comes to worse I just have to love what I have or move it on! Anyway I will see what DFM have to say!

Andrew H

10-11-2017, 04:29 AM


Seriously, i don't know what else can you do. Try to find a catadioptric scope with a big mirror and a not-so-large focal length, if that's even possible? Not good for a dobson mount, however.

10-11-2017, 05:11 AM
Lots of extra glass = light loss

10-11-2017, 07:38 AM
Hello 4-Os,

Well anything's possible. If you want to use it without using a ladder you could consider (somewhat crazily):

Using a much larger secondary lower down the tube, with the attendant greater constriction, larger mounting boss diameter & length
Using corrective (converging) optics before the secondary. These would also be large, given the focal reduction you're after. Your own Hyperstar, so to speak.
Adapt the telescope for video astronomy and add a finder scope further down the tube

With the exception of option 3, they're all very bespoke, definitely not off-the-shelf.


10-11-2017, 09:28 AM
You think you have problems. Imaging observing from the upper walkway of this monster. :scared: I'd never get a steady image due to the knocking of my knees.

10-11-2017, 12:02 PM
The periscope advert states that it works with optics of f/8 or longer.

10-11-2017, 12:13 PM
Your right Raymo, I reread the blurb at the bottom , I guess for a DOB, the periscope could only be used with a Barlow to increase the focal length.

19-11-2017, 01:38 AM
By radically changing your mount design you could lower the scope to the point where the the mirror section will just skim above the ground.
By the looks of your pictures when at zenith, the bottom of the toa is about 150 -200 mm above the ground. Lowering the ota by that much probably is not enough though.
The design Iíve used allows the ota to swing through the circular base and lowers the height of the whole ota. Not an insignificant amount of re-work for you.

22-11-2017, 09:47 PM
I can add something here as I know the scope!
Two more ideas at least.

1/ If you want to use your wheeled platform idea change from the wheels below to a design where the castors are attached to a frame at a point above the platform. Find the height of the attachment point from the height of the castors that provides clearances so the wheels turn okay.
Remake it in steel.
Not much gained?

2/ Remove the feet from the lower mount ring - that saves 1.5inch. Make up steel brackets that attach to the lower ring and castors.
Not much gained either.

3/ If you add some kgs of weight to the bottom end of the telescope you shift the COG down the tube. Then you can move the altitude-bearing-steel-ring-attachment part down the telescope. Then you can replace the 3 azimuth bearing frame sections. Risks overloading the tube frame. You couldnt simply add weights to the mirror cell. The tube frame could be reinforced simply enough with added struts and thick rings made to match from 5mm(?) aluminium sheeting. An engineering workshop would do a good welding job. Theres an equation to work out the distances or just trial and error it on the ground with the steel ring off. Mock-up a frame in timber that has a design that is similar to the original and see how it swings.
If its good and the weight / cost is ok then proceed.
Gain could be substantial. Still gotta be 1800 to the eyepiece...