From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi
 If you're not worried about tube currents and you don't
have a vignetting problem, go a head and build it. If the tube has a glossy
black surface, this may be better on the side away from the focuser. The
flocking may be better near the primary. You might only put the flocking
about 1/2 way up the tube. Look in the focuser and determine what parts of
the tube would be visible to the eyepiece. Also make sure there is no
direct path through the focuser from the aperture.
 As I stated before, you can use a larger diameter tube
and mount the focuser on a straight board that is mounted cordially. I've
see this done and it gets the best of both worlds. You get good clearance
for tube currents and the focuser is close to the secondary. Dwight


>From: "Dennis Woos" 
>
>Bob,
>
>We have modeled it in Newt2.5, and it indicates no vignetting of the 75% ray
>either at the front or at the focuser.  My son Tim wants a scope for
>planetary use (especially for the upcoming Mars opposition), and we don't
>need this scope to work well on anything else (we have built a number of
>other scopes, including a 10" f/5.6 newt and 6" f/6 mak-newt
for wide-field
>views).  All of the other scopes we have built used tubes around 2" in
>diameter larger than the primary.  However, in this case we wanted to use a
>small secondary (we got one .8125") and so we need to be closer to the
>optical axis than a big tube allows.
>
>Does this change your opinion about the vignetting?  If so, how about the
>issue of light reflecting in the tube - do you think flocking paper will
>eliminate it?  Thanks.
>
>Dennis
>
>----- Original Message -----
>From: "Bob May" 
>To: "atmlist" 
>Sent: Friday, June 13, 2003 1:09 PM
>Subject: Re: ATM Tube Question
>
>
>>
>> Dennis, I'd use that tube to make a larger tube of wood, fiberglass or
>other
>> good material and gain some additional diameter to the tube.
>> A nice project would be to get several different woods and make strips of
>> the wood the length of the tube about 3/4" wide (calculate out how many
>> strips of a particular width will go around the tube and then figure the
>> angle needed to make them mate without gaps - a hexagonal tube wants edges
>> cut at 30 deg.) and use the sonotube as a form for the real tube you will
>> use.
>> After making a larger tube the installation of baffles will make the
>scope's
>> performance supurb for light control.
>> That 5.5" tube is really too small as the edge of the tube
will limit the
>> amount of light that will be coming in from off-axis objects.  Go find
>> Newt2.5 and run the stuff that you have through the program to see what is
>> going on.  Many of us use a tube that is at least 2" larger in diameter
>than
>> the mirror just to make sure that the tube doesn't obstruct the light
>coming
>> in.
>> Bob May
>> http://nav.to/bobmay
>> bobmay{at}nethere.com
>> NEW! http://bobmay.astronomy.net
>>
>>
>>
>>
>
>
>
>

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