Secondary mirror on a 10" f/5

Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Any help will be appreciated.

VL

You could easily go to a 2.14" secondary. That'd be a 21% obstruction and
will still give you a good +/-1/2" diameter 100% illuminated field,
depending on how far above the secondary is the focal plane.

Howard Lester


"vL" wrote


Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Any help will be appreciated.

VL

On Thu, 25 Sep 2003 22:26:46 GMT, vL wrote:

Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

You can always go smaller, though you may not get desirable
results. If your concern is the amount of light loss, diffraction,
and lost contrast, you are likely to get very little gain for your
effort. It is quite likely that the diagonal size was chosen to
allow a large fully illuminated field.


I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Yes, that is one factor. There are utilities in the public
domain for calculating the percent illumination for various
newtonian configurations. You also need to consider how
you are going to mount and align the new diagonal in the
optical tube. Will you keep the existing spider, or get
a new one with a mount?


In all, if your current optics work well for you, you are
probably just as well off to leave it alone. In the Navy, I learned a
motto which may apply he If it ain't broke,
don't fix it!

Cheers,
Larry G.



Any help will be appreciated.

VL






--
Vote Republican - The Party of 'Spend and Squander'!

I have a 10" f/5 "Sky Mentor"
The secondary is 65mm (2.6") and I am wondering if I can go smaller.

Usual design for Newton 10" f/5 accepts even 1" diagonal (focal plane at
10"/2 from axis and no gap for field vegnetting)!

For the very first approach size of small axis of diagonal could be
calculated S = (D-d)*L/f + d, where D - main mirror diameter in mm, d -
diameter of field of view free of vegniting, L - distance from focal plane
to tube axis, f - focal distance of main mirror.

I have a homemade 10" f5, I put a 2.1" secondary on mine, and it's a
real beauty. I searched the net for Mel Bartel diagonal sizer program
and a few others. Based on my findings the 2.1 was optimal.

Luck,

On Thu, 25 Sep 2003 22:26:46 GMT, vL wrote:

Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Any help will be appreciated.

VL

In article , said...
Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

There are a number of considerations, but I used a 1.83" secondary on my
10" f5.51 back in the early nineties. I had a low profile focuser and no
intentions whatsoever of camera usage, but it worked fine. IIRC, the 100%
illuminated area will shrink with smaller diagonals, but for purely
visual work you'll hardly notice. In my scope, 1.83" was large enough,
but none too large.

To make sure it will work with a specific scope/primary/focuser, etc., I
recommended getting the "Newt" software (DOS, Google for it) and/or ray-
tracing the optical path as per the old Edmund Scientific book
instructed.

--

-- Len Philpot - --
------ ----- http://philpot.org/ --

VL,
I always suggest Mel Bartel's web site for anyone new,
http://www.efn.org/~mbartels/tm/tm.html
in this case, for the diagonal sizing program.

Factors that matter here include your aperture, focal length or f-ratio, and
secondary size (as noted below) plus the distance from your secondary to the
focal plane. Mel's performance criteria are based on a lot of observing; you
can believe him if he says you're fine or your in trouble. .

Should you decide to go smaller, you may need to make other changes to
maintain an optimized instrument. The most common change is a lower profile
focuser, so your eyepiece is closer to the secondary and thus better
illuminated. That change may require the primary to move back, the secondary
to move up, and a different focuser placement.

Before changing, consider this: you've got a 26% obstruction. Most SCTs are
33% so you're not that bad off.

On the other hand, field illumination is just seeing the entire primary
reflected in the secondary. Look down the empty focuser drawtube. If you see
a reflection of the whole primary when you're eye's at the center of the
focuser, it's a good thing. As you move away from the center, the primary
will eventually start to disappear. That's the limit of your fully
illuminated field. If you can move your eye to the edge of the drawtube and
still see the whole primary, your secondary is probably too big. You don't
really need more than a 1/2-3/4"

Have fun,
Frank

"vL" wrote in message ...
Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Any help will be appreciated.

VL

You need to carefully consider the quality of the diagonal. If the
quality near the edge is not good, then it will affect the on-axis
image if the diagonal is small enough.


vL wrote in message ...
Hi all,

I have a 10" f/5 "Sky Mentor" (Canada) aka Orion XT10.

The secondary is 65mm (2.6") and I am wondering if I can go smaller.

I have seen 10" (Mag 1 Portaball) with a 1.83" mirror.

I know there are other ascepts of tube design that I have to consider
but is it generally possible and what I have to do in order to use a
smaller secondary, (ie change the distance between primary and
secondary ???

Any help will be appreciated.

VL

Thank you to all of you for your input.

I am probably going to try a smaller secondary maybe 2.1" and with more
"research" 1.83 if it works.
It will also be an experience.


VL