Hi I’m ~6ft tall and would like to know what the biggest dob I would get so that I don’t have to use a lager. A step stool would be ok but preferable that I could just stand and view objects right above. Is there a 18” that matches this ‘requirement”? Thanks.
Biggest Dob without a ladder
Started by power_overwhelming, 09/19/2003 09:39PM
Posted 09/19/2003 09:39PM
Opening Post
Posted 09/19/2003 09:57PM
#1
I believe it is Ed at Tscopes.com that builds a "folded" 20" dob (at least I see a 18"... he may make a custom 20"). The secondary is angled so that the eyepiece is lower.
http://www.tscopes.com/FST/index.html
Other than that, I'd say a 15" scope would be about the max. They get pretty tall after that!
http://www.tscopes.com/FST/index.html
Other than that, I'd say a 15" scope would be about the max. They get pretty tall after that!
Posted 09/20/2003 04:13PM
#2
Mark Cowan does custom f/ mirrors.
For an 18" here's a Dobson range of f/'s and heights:
Height = FL - diagonal distance(~11") + thickness (~1.5")+ base-and-cell(~3.5") clearance:
f/4 72" FL >> 66" Height
f/4.2 75.6 >> 69.6"
f/4.4 79.2 >> 73.2"
f/4.6 82.8 >> 76.8"
f/4.8 86.4 >> 80.4"
f/5 90 >> 84"
f/6 108 >> 102"
One item: NEWT OK's a 2.6" diagonal for the f/5 18" for natural EP's up to 50mm. That's using a 2" EP focuser with a 1" height and typical construction numbers.
I was doing numbers on a 16" this morning. Your 18" came to mind. The prototype for my 10" f/5 went well, so a prototype for the 16" is in the works. Sidewalk Astronomers plywood ideas with a truss-and-strings OTA and various design follies. Easy enough to do one for your 18".
A PLOP 18" cell is surprisingly easy. Couple of evenings. Hardest problem is getting the holes drilled dead-on to mm accuracy.
Your low-enough 18" looks doable.
For an 18" here's a Dobson range of f/'s and heights:
Height = FL - diagonal distance(~11") + thickness (~1.5")+ base-and-cell(~3.5") clearance:
f/4 72" FL >> 66" Height
f/4.2 75.6 >> 69.6"
f/4.4 79.2 >> 73.2"
f/4.6 82.8 >> 76.8"
f/4.8 86.4 >> 80.4"
f/5 90 >> 84"
f/6 108 >> 102"
One item: NEWT OK's a 2.6" diagonal for the f/5 18" for natural EP's up to 50mm. That's using a 2" EP focuser with a 1" height and typical construction numbers.
I was doing numbers on a 16" this morning. Your 18" came to mind. The prototype for my 10" f/5 went well, so a prototype for the 16" is in the works. Sidewalk Astronomers plywood ideas with a truss-and-strings OTA and various design follies. Easy enough to do one for your 18".
A PLOP 18" cell is surprisingly easy. Couple of evenings. Hardest problem is getting the holes drilled dead-on to mm accuracy.
Your low-enough 18" looks doable.
Posted 09/21/2003 02:15AM
#3
I have looked through a 30" f4 that didn't need a ladder. However, there was a rather large secondary and then a tertiary mirror that was at eye height. I thought the sharpness was OK, but the thing that struck me was seeing so many colors on Jupiter.
Charlie
Charlie
Fort Lewis Observatory: (37.238, -108.052) ~2360m (7744 ft.) elevation.
Darkness - typically 6.5+
Scope - Meade 16" LX200; f6.3 focal reducer
Focus - JMI Smart focus
Camera - SBIG ST-10XE; (~.5"/pixel)
Guiding - AO-8 and/or Meade 5"/DSI/PHD
http://www.fortlewis.edu/observatory
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