secondary mirror sizing

Hello all,
Could someone please direct me to a good explanation of proper secondary sizing, illuminated field diameter, etc?

I ask because I've long suspected my secondary is undersized. If memory serves me, I followed the advice of Tom Clark of Tectron Telescopes back in the early '90s. I used a 2.14" secondary for my 14.5" f/5.2 primary. If nothing else, it makes collimation harder for me as I can not see the periphery of the primary in the reflection of the secondary. I also suspect some vignetting. What do you all think?

Forgot to say that this is a Newtonian we're talking about... a typical Dobsonian.

Thanks,
Dan

You can download the program Newt and it will show you what size you need.
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

Dan Ritts said:

Hello all,
Could someone please direct me to a good explanation of proper secondary sizing, illuminated field diameter, etc?

I ask because I've long suspected my secondary is undersized. If memory serves me, I followed the advice of Tom Clark of Tectron Telescopes back in the early '90s. I used a 2.14" secondary for my 14.5" f/5.2 primary. If nothing else, it makes collimation harder for me as I can not see the periphery of the primary in the reflection of the secondary. I also suspect some vignetting. What do you all think?

Forgot to say that this is a Newtonian we're talking about... a typical Dobsonian.

Thanks,
Dan

Newt and Sec are excellent programs, and you should use them. But, keep in mind that secondary size is a compromise between field of full illumination and percentage of central obstruction (no perfect answer but some very good answers). I started to get a feel for using the programs by running the numbers on some telescopes I liked. The MN 56 for example had about 50% of full illumination at the edge of the 2 field--sounds awful, looks great (that's probably a little tighter that your dob, depending on the focuser height). Doesn't work for 35mm immaging though. (

Thanks for all the replies thus far... I am just arriving home late tonight but will measure the additional parameters in the morning. As I recall, I made my upper cage inner diameter 16" or a touch over, with a 3/8 thick focuser board, on top of which sits a JMI-NGF? focuser from about 1994 vintage. My first question is determining the precise location of the focal plane relative to the optical axis. Someone once suggested pointing the scope at the moon with the eyepiece out and finding where it focuses on a piece of paper? That should do it, correct? In any case, with a Tectron sight tube pushed all the way into the focuser and the focuser racked all the way in, I still cannot see the entire primary in the secondary's reflection. I don't have to rack my Naglers way out to get them to focus either, which would seem to suggest that I have not positioned the secondary unnecessarily close to the primary. Is that right?
More tomorrow... thanks ya'll.
-Dan

Alrighty, I got the measurements:

Inside diameter of upper cage: 16 1/8"
Focuser board: 7/16"
Height of Focuser racked all the way in: 1 5/8"
So the center of the OTA to the top of the focuser is 10 1/8"
I have not net determined how much further out the focal plane is from the top of the racked-in focuser, but from memory I would say my Naglers come to focus if I rack out the focuser 3/4" to 1" from the all-the-way in position. Perhaps that will tell you more experienced where the focal plane is exactly on my scope? If not, if it stops raining and I can get a look at the moon tonight I'll push the scope out and get the moon to focus on a piece of paper and report back.
...and to repeat the other specs...
Primary: 14.5" f/5.2 (75.4" focal length) according to mfg. specs (Galaxy Optics)
Secondary: 2.14" minor axis
Eyepieces used: 5, 7, 9, 13, 20 Naglers, 24 Panoptic
Also have Tectron sight tube, cheshire and autocollimator

And to repeat, I cannot see the entire primary in the secondary reflection even with the sight tube pushed all the way in and the focuser racked all the way in. So the pinhole of the sighttube would then be 10 1/4" from the OTA centerline.

Drawing the light cone in AutoCAD I see that if a 2.14" diagonal intersects the on-axis rays from the primary 11.128" from the focus of the primary, then you have a single point of 100% illumination. So the top of my focuser being 10 1/8" from the center of the light cone, and my eyepieces coming to focus just fine would seem to mean that if I'm not seeing the whole image of the primary through the sight tube, my secondary and focuser might be too close to the primary and consequently the focal point of the primary too far out beyond the top of the racked in focuser? As I said, I can't remember exactly how far out I have to rack the focuser to achieve focus with my eyepieces, but it's more than just a little bit and probably about 1". When I plug in all these numbers to NEWT for windows, it says I should have 100% illumination spot diameter of about .22" with a 75% illuminated spot dia. of 1.21". This would seem to be just fine in theory, wouldn't it (for visual only use)?

I took another round of measurements by two approaches. First, by Neil's method with a little light bulb at twice the parabola's focal length. Second, by projecting the moons image on the back of a business card and by very carefully measuring the distance from the mirror surface to the centerline of the focuser, and then from there to the observed focal plane. To the best of my ability, I concluded that my ACTUAL best estimate of focal length is 74-1/8", which makes this mirror f/5.11 and NOT f/5.2 as advertised.

As an aside, the two methods disagreed slightly, though I suppose barely within the estimated uncertainty of each measurement. The light-bulb-at-twice-the-focal-length method gives me f.l.= 73-7/8" (+/- 1/8 I'll assume due to measurement and focus error combined). I measured with both a tape measure, but also a Leica laser distance meter which is very accurate and doesn't suffer from floppy tape syndrome. The moon projection method and the direct measurement of the telescope part separations yields 74-3/16" +/- 3/16". So I suppose for now I can conclude that the focal length of the primary is almost certainly between 73-7/8" and 74-3/16". I'm going with 74-1/8 as my "final answer" based on my confidence in how I performed each method and oh-so-scientific gut feeling.

More interestingly, I put a mic on my secondary, or I should say the VISIBLE portion of the secondary's mirror surface, and it actually measures 2.05", NOT 2.14" This is because the secondary holder rolls over the surface of the mirror just slightly.

So my entries into NEWT are as follows:

Primary Dia: 14.5"
focal ratio: 5.11
diagonal minor axis: 2.05
tube inside dia: 16.0625
tube thickness: 0.47
focuser to front of tube: 6.875
focuser height: 2.4375 (exact height of focal plane for obj. at infinity measured above the surface of the focuser board)
focuser in-travel: .8125 (top of focuser racked all the way in)
add'l height for camera: 0
focuser inside dia: 2

All of which gives me a 100% illuminated circle at the focal plane of... drum roll please...

-0.296"

Time for a bigger secondary. Anybody need a 2.14 diagonal and secondary holder?