RE: [ATM] Hello list, some basic question for a new Dobsonian builder

["Alan Scott" <alanne1956@hotmail.com>]

Mike,


>From: Mike Shuken <mikeshuken@value.net>
>To: atm@atmlist.net
>Subject: [ATM] Hello list, some basic question for a new Dobsonian builder
>Date: Tue, 05 Apr 2005 23:11:08 -0700
>
>Hello Everyone,
>
>First I would like to say that I have no idea how this space bug suddenly 
>bit me, but it did and I'm, well, not *obsessed* but it is becoming 
>somewhat consuming!  My wife thinks I'm nuts.
>
>Nothing like jumping in with both feet, I'd like to switch from my Nikon 
>binoculars to a 10" Dobson Tube Telescope that I plan on building in the 
>next month.  I've done a ton of research on the Internet and directly 
>communicated with several amateur astronomers but I'm still stuck with a 
>few questions.  I'm hoping this highly intelligent group will indulge a 
>newcomer with what must seem like some very elementary questions!  Well, I 
>guess we all have to start somewhere...

You will be surprised how easy building a telescope is.  And - by the way - 
every has asked these questions at one time or the other.

>
>I will be using the basic plans from this web site: 
>http://members.aol.com/sfsidewalk/tenin.htm - ahh, they make it look so 
>easy!
>
>Questions:
>
>1) This primary mirror purchase is driving me crazy.  The real thrill for 
>me is deep space, but I want to be able to do some "sidewalk" observing of 
>the moon and planets to share with my friends and neighbors.  I thought 
>that a 10" f/6 would be perfect - and I had just seen a kit with a primary 
>and secondary for $399.00.  Problem is now I can't buy that kit, the 
>company only sells a 10" f/5 ($400 for the set).  I found some very cool 
>10" f/6 elsewhere but they range from $600 to $800 - and that's what I had 
>in mind for the total budget of the telescope.  Should I continue to search 
>for a less expensive 10" f/6?  Go with the 10" f/5 for $400?  Or not eat 
>for two months and go for the $600 10" f/6?  (comparing apples to apples, 
>let's say all of these mirrors are from 1/6 wave to 1/10 wave...).

Ouch.  You start with one of the hardest ones.  Here is my opinion - and 
realize that everyone has a different one.

ASSUMING THAT THE OPTICS ARE THE SAME, You will be happiest with the f/5.  
The f/5 will be shorter, and will have the eyepiece located at about the 
height of your eye when you are sitting.  The F/6 will have you standing, 
and kind of bending over.  The f/5 is easier to transport, being about 10" 
shorter.  The f/5 will also give you a wider field of view, less 
magnification, and things will look slightly brighter (assuming the same eye 
piece).

Now - optics.  The faster the scope (the smaller the f number), the harder 
it is to create a wonderful mirror.  FROM MY EXPERIENCE, FOR THE SAME MONEY, 
f/6's are generally very, very good, f/5's are very good, and f/4's are ... 
fair. So, since we want a sharp image, while having a fairly compact scope, 
where is the "sweet spot"?  I believe that it is about an f/5.  Generally, 
you get a very good mirror for a reasonable cost.

Summary - go with the f/5.

Two places to look for mirrors that I have used:
Orion.  www.telescope.com, accessories, telescope parts, primary mirrors.
Hands on Optics.  http://www.handsonoptics.com/, accessories, parts, Sky 
Instruments.


>
>2) Should my mirror be American made?  I like the idea of buying American, 
>but how about Russian and Chinese mirrors?  All things being equal, I would 
>pay a little more for a mirror if it was made in the USA, but that being 
>said, I still have a limited budget  :-(
>

Chinese is fine.  Orion and Discovery use Chinese glass. Quality is 
generally excellent.

>3) The spider in the plans is made from three cedar shingles and a piece of 
>wood dowel.  It does not appear to be adjustable *at all* except by tapping 
>it back and forth in the tube.  Is buying a commercial spider worth the $50 
>or so?  How hard will is be to adjust my secondary mirror with a shingle 
>spider?

I have heard good things about these thin wood spiders.  One idea is to use 
a piece of strapping steel.  Another is to create a wire spider.  Here are a 
few web sites of spiders that I know of:

My telescope, and a wire spider: 
http://www.xmission.com/~alanne/DS3SecondaryCage.html
My friend Jim's spider (from a binocular): 
http://www.lobo.net/~santiago/NewBino%20Project/November%2012.htm
http://www.lobo.net/~santiago/NewBino%20Project/November%2015.htm
http://www.lobo.net/~santiago/NewBino%20Project/December%209.htm

Commercial ones are also fine.

>
>4) I like the idea of having several different eye-pieces.  I was thinking 
>of buying a set with a 25mm, 12.5mm and maybe 8mm Plossl eyepieces.  The 
>instructions are not clear, is there a threaded piece that I attach to the 
>tube and the eyepieces just screw in and out?

Check out focusers at Orion.  www.telescope.com, accessories, telescope 
parts, focusers

>
>5) More on eyepieces, I though I had it figured out, but then I read this 
>on a web site:
>
>"Here is what happened to me. I talked to lots of people and decided to go 
>for 0.75 inches of in-travel. My mirror arrived marked as 59.0 inch focus. 
>I measured everything four times, cut the holes and found that I had lost 
>about 0.3 inches of in-travel! I figured I might be off by maybe 0.1 
>inches. I now suspect that the mirror was not what it claimed in this 
>regard. My advice is to not take any claim about the mirror for 
>granted...measure it! How, I am not sure, but it can be done. Better yet, 
>figure out a way to temporarily mount the mirror in a way that you can 
>adjust it. Find the focus of choice and then commit to boring holes.  And 
>make sure you understand in-travel vs. out-travel...these relative terms 
>can drive you crazy. "

Make an optical bench.  The first paragraph in the following link talks 
about how this is done:
http://www.xmission.com/~alanne/DS3TrussConstruction.html

Not sure if this helps - my scope is a truss, not a tube construction.


>
>(me again) I understand how to find the true focus length of the mirror, 
>but the "in-travel vs. out-travel" I don't understand.  Does each eye piece 
>have a specific "distance" it goes in and out, allowing you to slightly 
>miscalculate the focus length you need to mount your mirrors at?

All eyepieces need to be placed at a slightly different distance from the 
primary mirror.  This distance generally is very small.  What I did was take 
my eyepieces and make everything focus when the focuser was half way out.  
That way, as I change eyepieces, I have distance to move in/out.  So far, I 
haven't found any eyepieces that could not be focused.

>
>6)  Supporting the main mirror: this web page gives you two different ways 
>http://www.glpbooks.com/oyb/roscope/pri_cell.html does it really matter 
>which one I choose?
>

Mirror cells.  Once again, EVERYONE has an opinion.  Mine, although simple 
and not acceptable for a larger scope, works fine and is VERY easy.   Take a 
look at:
http://www.xmission.com/~alanne/DS3MirrorCell.html

>
>And I would just like to add:
>
>1) Yes, I am trying to meet someone local or hook up with a club so that I 
>can possibly have a mentor while I'm doing this.

What town are you living in?

>
>2) But I want to start now, and I am thinking that the best way to figure 
>some of these things out is to JUST BUILD IT!

Go to Mel Bartell's web site, and open every link that he has.  You will be 
amazed what you find.
http://www.bbastrodesigns.com/tm/tm.html

>
>3) I'm a little nervous, but then I remember, some guys were building these 
>things centuries ago, so how hard can it be??  :-)

Just remember what you need here.  A telescope is basically a thin film of 
aluminum (primary mirror) focusing light onto another thin film of aluminum 
(secondary mirror), with an eyepiece at the top of the light cone.  The rest 
is just "stuff" to hold those pieces of aluminum.  Telescopes are easy, are 
pretty forgiving, and if not done right, are easily fixed.  Have fun.  You 
will do fine.

>
>4) How can any curious person of moderate intelligence not be COMPLETELY 
>overwhelmed with how awesome and how awe inspiring the heavens can be?  
>Well, it only took 35 years for it to happen to me, and I'm glad it did...

>
>Thanks in advance for your advice and indulgence.
>
>Mike
>
>
>
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>ATM mailing list http://www.atmlist.net/

alan scott

http://www.xmission.com/~alanne/Astronomy.html
http://www.xmission.com/~alanne/DS3Main.html

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