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


HI Scott
 The manufacture focal length is usually +/- some. It is
always recommended to measure the actual mirror. ROC ( Radius of Curvature
) is 2 times the focal length. This can be easily measured, even while
inside the OTA.
 Place the OTA on it's side. Take a flash light and make
a mask with a hole in it. I like making star shapped ones, like you'd make
a paper snow flake, except you throw away the snow flake and keep the rest.
 Put this mask over the flash light but have some of the paper
hang off to the side to make a return screen. Shine the flash light through
the mask, to the mirror, such that the image of the mask hits the screen.
Adjust this for best image of the mask by moving towards or away from the
telescope.
 Measure the distance to the edge of the mirror with a tape
measure. Divide this number by two to equal focal length. To be exact, one
end of the tape measure would be exactly half way between the mask and
returned image. Don't try to measure to the center of the mirror or you
might damage it. Measuring to the edge will be accurate to a few micro
inches.
 This method works nice for coated mirrors. Since both the
mask and screen are at the same distance, when focused, it will be at ROC.
Dwight


>From: "Kevin H Williams" 
>
>scott
>
>I am assembling the tube tonight, followed at least some of your advice
>
>I installed the focuser about 10" from the front of the tube, installed the
>mirror cell at the measured distance from the focuser (as specified by
>newt),  installed the secondary and aligned it so my eye thru the focuser
>was centered in the secondary, and the primary reflection
>
>I took the scope outside and tried to look at the moon, and yes I can see
>it, but no detail what so ever, unable to achieve focus, so I concluded the
>mirror cell is in the wrong place, is this a valid assumption?
>
>data that has changed from below, the focuser is 3", tube is
8", newt say
>everything is OK, but I have installed wrong, or the stated focal length of
>the purchased mirror is not correct
>
>you recommended moving the mirror cell until the focal plane is the desired
>distance above the fully racked in focuser
>how do I do this, ie how do I know when the focal plane is the desired
>distance above the focuser, and how do I know what the desired distance is?
>
>thanks for your help
>kevin
>
>
>----- Original Message -----
>From: "scottythefiddler" 
>To: "Kevin H Williams" 
>Sent: Thursday, March 06, 2003 11:31 AM
>Subject: Re: ATM focuser placement calculations and tube lenght
>
>
>> Hi Keven,
>>
>> good decision ...to build your own scope.
>>
>> Regarding your question with Newt, it is a good thing that you ran your
>> numbers through the software.   Try running your specifications with a
>> smaller tube diameter, perhaps 8", which is more typical for
a 6" mirror.
>> The advice, "Diagonal too small to admit 100% ray, means that you are
>> locating your focal plane too far from the diagonal, or too look at
>another
>> way, the diagonal is too close to the primary to catch the light from the
>> edge of your primary mirror.
>>
>> If you look at the ray diagram in Newt, you will see that there is a cone
>of
>> light reflected up the tube, to the diagonal, where the diagonal folds the
>> light path out the side of the scope.   The strategy is to locate your
>> diagonal as far up that cone of light as possible, yet to 'fold' nough of
>> the top of that cone, to allow the focal plane to be located outside the
>> tube.
>>
>> Solutions:1)  smaller diameter tube, 7.5" if you can find
one, 8" approx.
>if
>> you are using sonotube.
>>                 2) lower focusser.  3" is quite high.   If
you must have a
>> rack and pinion focusser, a 2" one may serve you better.   A
low profile
>> helical focusser can shorten this distance a little bit further.   Be
>aware
>> that the lower the focusser, the more challenges you face in baffling the
>> focal plane from stray light coming in over the top of the tube.   i.e. if
>> you can look through your focusser, at an angle, and see daylight through
>> the top of the tube, you have a problem,.....which can be rectified by
>> extending the top of the tube further.
>>                3)  locate the focal plane only 1/2" from the top of the
>> focusser.
>>
>>
>> General help in assembling the components.... install the focusser,
>perhaps
>> 8" from the top of the tube, (you may still need to add an additional
>light
>> baffle on the top of the tube opposite the focusser, depending on focusser
>> height.   Install the secondary spider, cell and mirror so that the
>> secondary is across from focusser.     After this part is done, install
>the
>> primary in its' cell, and position the cell/mirror in the tube, so as to
>> bring the focal plane the desired distance above the fully racked in
>> focusser.   Mark the position of the cell, and THEN cut the excess off the
>> bottom of the tube.   Leave extra on the tube, until you are satisfied
>with
>> the position of all the optics.   You can also leave extra on the top of
>> your tube, and trim it as you deem fit, at a later time.     Bear in mind
>> that the stated focal ratio of a mirror is only an approximation of the
>> focal length.   To know the focal length accurately, it may be necessary
>to
>> measure it, if you haven't already done this.   It is not always a good
>idea
>> to lay out  the different parts of the tube based upon measurements.   It
>is
>> far safer to install the fixed components (secondary and focusser), and
>then
>> to actually place the primary and cell to get the focal plane in the
>desired
>> location.
>>
>> Just a little tip.... before trying to put the primary in the tube, take a
>> little bit of excess tube and make a cut in it, so that you can open it up
>> to the shape of a letter 'C'.    Cut about 1/2" off one of
the ends, and
>> slip it inside the bottom of the tube.  If it fits snuggly, you can slide
>it
>> into position in the tube so that the cell will rest up against it.   As
>you
>> check the position of the focal plane, it sure helps to have something to
>> give the cell a little bit of stability.  You can adjust this ring back
>and
>> forth until you know exactly where you want the cell, then remove before
>you
>> finally fix the cell into position.
>>
>> So, ultimately, you can go back to NEWT, and try juggling numbers.  You
>most
>> definitely want the 100% rays to reach the focal plane.   Without them,
>you
>> are actually discarding light from the edge of the mirror.
>>
>> Best of luck, and give me a shout with any further questions.  Do you
>> already have the parts on hand?   If not, the first thing to consider is a
>> smaller diameter tube, then a lower focusser.   If you would like, send me
>> the specs for your scope, and I can run them through newt, and try a few
>> different combinations for you.   I usally spend a fair bit of time with
>> Newt, trying different scenarios, attempting to optimize different aspects
>> of the scope.
>>
>> Scott Donaldson
>>
>>
>> ----- Original Message -----
>> From: "Kevin H Williams" 
>> To: 
>> Sent: Thursday, March 06, 2003 9:14 AM
>> Subject: ATM focuser placement calculations and tube lenght
>>
>>
>> >
>> > I am building my first telescope
>> >
>> > 6" f/5
>> > diagonal with a minor axis of 1.6"
>> >
>> > I was hoping the list participants could review my
calculations before I
>> > make any cuts
>> >
>> > from barry's "build your own telescope" I get this
formula, all length
>are
>> > in inchs
>> >
>> > (distance from diagonal to front of mirror) = (focal length)
- (diagonal
>> to
>> > focal plane distance)
>> >
>> > diagonal to focal plane distance = (collapased focuser height
+1) + (1/2
>> > diameter of tube)
>> > with my numbers = 3 + 1 + 4.5 = 8.5
>> >
>> > focal length = (f/x * mirror diameter)
>> > with my numbers focal length = 5*6 = 30
>> >
>> > (distance from diagonal to front of mirror) = (focal length)
- (diagonal
>> to
>> > focal plane distance)
>> > with my numbers = 30 - 8.5 = 21.5
>> >
>> > so I get distance from the front of the mirror to the center of the
>> diagonal
>> > of 21.5
>> >
>> > now I need to make a decision about tube length, if I want to put the
>> mirror
>> > cell 1 inch from the back end of the tube, and the mirror cell is 2.5
>> inchs
>> > long
>> >
>> > the distance from the back of the tube to the front of mirror is 3.5
>> > I want the distance from the focuser to the front of the tube
to be 6.00
>> >
>> > so tube length is (distance from front of the mirror to the center of
>the
>> > diagonal) + (distance from the back of the tube to the front of the
>> mirror)
>> > + (distance from the center of the diagonal to the front of the tube)
>> >
>> > so I get tube length to be 21.5 + 3.5 + 6 = 29 inchs
>> >
>> > any reason to back the tube shorter or longer? would it hurt to make it
>> > longer?
>> >
>> > when I plug this into Newt 2.5, I get a YES on "Diagonal
to small to
>admit
>> > 100% ray" - how bad is this, and what are the ways to fix this?
>> >
>> > hope this is not too confusing, I have searched the internet for
>software
>> to
>> > do these calculations, but could not find anything, it probably sounds
>> > simple to someone more experienced, but not to me, and I want to do it
>> right
>> > the first time
>> >
>> > thanks for any help
>> > kevin
>> >
>> >
>> >
>>
>>
>
>
>

--- BBBS/NT v4.00 MP