Primary and secondary mirror relation

[Aditya Ravi]

Hi All,
I just wanted to clarify whether there is any relationship
between the size of the primary mirror to the secondary. Obviously
there should be one, could someone tell me what it is? Lets consider
an example of an 8" and 10" primary mirror, what should be the size of
their secondary?

Does the size of the secondary as change with variation of f-ratio of
the primary mirrors. In this case lets take examples of a 10" f/5 and
10" f/2.5 mirrors, how will their sizes vary?
Thanking you in advance.

Regards,
Aditya Ravi

[keerthi kiran]

Hello Aditya,
Size of secondary depends on many factors.
1. Design of the telescope: Newtonian, Cassegrain etc. Here I will assume Newtonian.
2. The size of secondary also depends on the aperture and focal length.
3. Another factor to consider is the "fully illuminated field of view you desire". Example: If you are using a camera with the telescope, the fully illuninated field should be at least the size of the sensor.
The formula for the "Minor Axis" of an elliptical mirror 

a = (D - d)*l/f + d

where,

a = Minor axis of secondary

D = Primary Diameter

l = Distance between secondary and focal plane

d = Fully illuminated field

f = focal length

Lets take an example.
Consider 200mm mirror with 1000mm focal length.
Consider the distance between secondary mirror and focal plane to be 200mm.
Consider Canon 500D to be the imaging camera.
Fully illuminated field should be the diagonal of the imaging sensor = sqrt(22.2mm^2+14.8mm^2) = 27mm

a = (200 - 27)*200/1000 + 27 = 62mm.

Regards,
Keerthi

--
You received this message because you are subscribed to the "The Bangalore Astronomical Society" mailing list.
To post to this group, send email to b-...@googlegroups.com
To unsubscribe from this group, send email to b-a-s-un...@googlegroups.com
For more options, visit this group at http://groups-beta.google.com/group/b-a-s?hl=en

News - http://www.bas.org.in/
Events - http://www.bas.org.in/Home/events_calendar
24X7 Chat on IRC - http://www.bas.org.in/Home/irc (#b-a-s on freenode)

[Akarsh Simha]

> d = Fully illuminated field

Thanks! I didn't know about this aspect!

Regards
Akarsh

[Chikur Sharma]

Hello all, 

Could anyone post information on the parameters that decides the qualify of telescopes ? Please post detail meaning of writings on binos like 10x50 etc.

Regards,

Suresh

Akarsh

[Ajay Talwar]

Although Keerthi has explained it nicely and fully, allow me to put the answer in different words:

The size of diagonal depends on ...
1. ... the primary's diameter - the larger the primary diameter, the larger the diagonal,
2. ... the primary's focal ratio - smaller the focal ratio, larger the diagonal,
3. ... size of tube and height of the focuser, the larger the radius of tube + height of focuser, larger the diagonal
4. ... the diameter of the field lens of the eyepiece, larger the field lens larger the diagonal required
 the above condition can also be applied to cameras - larger the size of camera's sensor, larger the diagonal if you need to fully illuminate the entire sensor.
5. ... how large do you want your fully illuminated field to be, larger the fully illuminated view, larger the diagonal. Usually the value is take to be of the size of a full moon, i.e. half a degree. This condition no 5 is in practice, intermixed with the previous condition no 4.

Conversly:
The larger the diagonal size ...
... more the diffraction
... more light loss
... brighter the image at the eyepiece

Some related experience:
Telescope manufacturers in India put in puny diagonals in their telescopes, if you buy a 6inch telescope with a smaller diagonal, you are essentially getting light gathering of a 5 or a 4 inch telescope. The focuser needs to be of lesser height for an ideal diagonal and fully illuminated field. But for photography, the focuser needs to be of greater height so that camera can reach focus.

All this is written keeping a Newtonian telescope in mind.

You may see this article for more information:
http://www.garyseronik.com/?q=node/8

Ajay Talwar

[Aditya Ravi]

Hi All,

          Thanks for the information Keerthi and Ajay Talwar for your answers.@Keerthi thanks for the equation, and it looks like you know my set-up really well ;) @Ajay Talwar in the article you provided,I performed the procedure for my scope. The value of  distance from the secondary mirror to the focal plane (denoted by L in the article) i got was approx 9.94" and value of f was 5. On dividing the two the value obtained for minimum diagonal size i got was  1.988" . When i measured the size of my secondary it was 2.67". Does this imply that I have a 0.682-inch fully illuminated field?

I now how a second doubt if i were to purchase a 10" f/2.5 primary mirror, how can i determine the size of the required secondary mirror if i were to purchase both(primary and secondary) at the same time. The value of L here would change here right? So am i to understand that i need to purchase the primary first and then perform the tracing paper on the focuser to find the value of L and only then calculate the size of the secondary and place the order?

I am slightly confused. Sorry if i'm not making sense.

Regards,

Aditya Ravi

[Harshad RJ]

On Thu, Feb 16, 2012 at 2:48 PM, Aditya Ravi <adityar...@gmail.com> wrote:

I now how a second doubt if i were to purchase a 10" f/2.5 primary mirror, how can i determine the size of the required secondary mirror if i were to purchase both(primary and secondary) at the same time. The value of L here would change here right? So am i to understand that i need to purchase the primary first and then perform the tracing paper on the focuser to find the value of L and only then calculate the size of the secondary and place the order?

L (Distance from secondary mirror to focal plane) is not directly dependent on the mirror but rather on the tube. Of course, if the mirror size changes you will change the tube-size but there is no direct relationship between the two. It will depend on what tube sizes are available, the size of mirror-cell and how much clearance you want to keep.

If you can't accurately predict tube size (that is typically true) you can wait to get the primary first before ordering the secondary.

Btw, this would make for a nice interactive drawing in GeoGebra. Anybody wants to take up the challenge?

--
Harshad RJ
http://lavadip.com

[Aditya Ravi]

if the mirror size changes
> you will change the tube-size but there is no direct relationship between
> the two. It will depend on what tube sizes are available, the size of
> mirror-cell and how much clearance you want to keep.
>
> If you can't accurately predict tube size (that is typically true) you can
> wait to get the primary first before ordering the secondary.

Thanks Harshad for clearing the doubt :) Starting to make sense now.


Regards,
Aditya Ravi

[keerthi kiran]

What is GeoGebra? (I can google for it but too lazy to google)...

[keerthi kiran]

Aditya,
Remember that, with such short focal mirrors, you will need a very good coma corrector... Coma increases as you move away from the center of the field. So, if you are planning to use the mirror for wide field imaging application, you will need a very good (and expensive) coma corrector...
Doc has a 8" f2.8 telescope. But the primary mirror in the scope is actually an f/4 mirror. Then there is a corrector which also acts as reducer... So the final setup would be a f2.8 telescope...
Also remember that, you will need very accurate tools to collimate such a telescope...

Regards,
keerthi

--

[Aditya Ravi]

Hi Keerthi,
The mirror mentioned here is something which i might consider in the
future not anything urgent or as of now.I am aware of the large coma
and that i would need to collimate the scope very often and
accurately. Based on the description you gave about Doc's scope, it
sounds like he has a power newt telescope. Is it possible to get a
corrector of some sort like what Doc has for an 8" skywatcher? And
about GeoGebra i got this from wikipedia: "GeoGebra is dynamic
geometry software. Constructions can be made with points, vectors,
segments, lines, polygons, conic sections, inequalities, implicit
polynomials and functions. All of them can be changed dynamically
afterwards. Elements can be entered and modified directly on screen,
or through the Input Bar. GeoGebra has the ability to use variables
for numbers, vectors and points, find derivatives and integrals of
functions and has a full complement of commands like Root or
Extremum."
The purpose of this post has been satisfied as i now know the relation
between primary mirror and secondary mirror.Thanks to all of you who
helped me out here :)

Regards,
Aditya Ravi

[Suresh Mohan]

Yes Keerthi is absolutely correct , way too demanding for a beginner , it will end up in frustration . You first need a howie glatter collimator, next the spacing has to be exactly 65 mm , not 64.9 or 65.1, next the cfs or critical focus zone is extremely small , hence the smallest error in the primary not exactly perpedicular to secondary will result in one side of pic in focus and the other not! I love to tinker so it is ok for me  and I don't want to invest in a TAK f2.8 which costs a bomb for 10 nights of my hobby

Suresh

Sent from my iPhone

[Harshad RJ]

On Thu, Feb 16, 2012 at 3:31 PM, keerthi kiran <info...@gmail.com> wrote:

What is GeoGebra? (I can google for it but too lazy to google)...

I could write some description but this video gives a good overview of its poer (watch till the end):
http://www.youtube.com/watch?v=3XqganitGMc

[Sunil G.R.]

hmm... nice software :).
So can this be used to demonstrate the basics of telescope?
i.e. how the final image will look like based on apperture, focal length, magnification etc..?

This way understanding of the basics will be more clear and helps in choosing the telescope to buy.

Regards,
Sunil.

[Harshad RJ]

On Thu, Feb 16, 2012 at 4:11 PM, Sunil G.R. <super...@gmail.com> wrote:

hmm... nice software :).
So can this be used to demonstrate the basics of telescope?
i.e. how the final image will look like based on apperture, focal length, magnification etc..?

Could be done. I had used this software to demonstrate apparent declination of geostationary satellites. (it should be in the archives).

This software is incredibly powerful; check out these demos:
http://www.youtube.com/watch?v=PP-gd4el7XI