ATM Follies / problems

Hi.

I have just built my first telescope. Most of the parts I got from
University Optics, the secondary from Newport Glass and the primary from
ebay. It is a 6" f5 scope. I used Newt to get the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I can
see a large black blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of
the gurus agree? If not, what could be the problem?

The reason I think it might be the primary is it was only, like, 35 bucks.
It is the weakest link, except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

(This isn't my first scope, this is just the first scope that I built.)

Thanks for your help!
-Phil
www.pickupcentral.com

Phil wrote:
Hi.

I have just built my first telescope. Most of the parts I got from
University Optics, the secondary from Newport Glass and the primary from
ebay. It is a 6" f5 scope. I used Newt to get the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I can
see a large black blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of
the gurus agree? If not, what could be the problem?

The reason I think it might be the primary is it was only, like, 35 bucks.
It is the weakest link, except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

(This isn't my first scope, this is just the first scope that I built.)

Thanks for your help!
-Phil
www.pickupcentral.com




Hi

Sounds like you have the secondary set in the
wrong position here. It either needs to be moved
forward, or backward in relation to the primary.

I *think* it needs to be moved towards the primary.


--
AM

http://sctuser.home.comcast.net

CentOS 4.2 KDE 3.3

Point the primary at the horizon in the daytime. Use a business card to
find the focus, which will be an upside-down image of the horizon.
Measure the distance. If your are only a little off you may be able to
get away with moving the primary mirror cell fore or aft. Make sure
this distance is in the middle of your focuser range after allowing for
the position of your typical ocular (use your oculars like a magnifier
of your finger tip, will be up in the barrel).

wrote:
Point the primary at the horizon in the daytime. Use a business card to
find the focus, which will be an upside-down image of the horizon.
Measure the distance. If your are only a little off you may be able to
get away with moving the primary mirror cell fore or aft. Make sure
this distance is in the middle of your focuser range after allowing for
the position of your typical ocular (use your oculars like a magnifier
of your finger tip, will be up in the barrel).


You have a point here.
Moving the primary would be a whole lot easier !



--
AM

http://sctuser.home.comcast.net

CentOS 4.2 KDE 3.3

Ya, you got one of those china mirrors and they can range from f4 to f5+, I
don't know every little bit, but after building my scope I know a little.
You need to figure out just what the F of the mirror is, can be done in
daylight with the sun and a yard stick ot tape. That's how I figured what
mine was.


--

The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
Astronomy Net Online Gift Shop
http://www.cafepress.com/astronomy_net
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http://www.cafepress.com/ingarden
Blast Off Online Gift Shop
http://www.cafepress.com/starlords




"Phil" wrote in message
. com...
Hi.

I have just built my first telescope. Most of the parts I got from
University Optics, the secondary from Newport Glass and the primary from
ebay. It is a 6" f5 scope. I used Newt to get the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I
can see a large black blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of
the gurus agree? If not, what could be the problem?

The reason I think it might be the primary is it was only, like, 35 bucks.
It is the weakest link, except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

(This isn't my first scope, this is just the first scope that I built.)

Thanks for your help!
-Phil
www.pickupcentral.com

"Phil" wrote in message
. com...
Hi.

I have just built my first telescope. Most of the parts I got from University Optics, the
secondary from Newport Glass and the primary from ebay. It is a 6" f5 scope. I used Newt to get
the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I can see a large black
blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of the gurus agree? If
not, what could be the problem?

The reason I think it might be the primary is it was only, like, 35 bucks. It is the weakest link,
except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

(This isn't my first scope, this is just the first scope that I built.)

Thanks for your help!
-Phil
www.pickupcentral.com


Hi Phil, you might try setting up a quick Foucault tester (just a lit pin hole and a knife edge) to
get
1, an overall figure of your mirror.
2, the exact focal length.
F5 can mean, as others have pointed out, anything from F4.5 to F5.5 :-(
As even an inch in the wrong direction can stop you reaching focus it might be worthwhile.
Newt is great but it doesn't know the focal plane of your eyepiece/camera. I had to trim my F6.1
truss tubes by an inch to get things just right.
--
John Carruthers
http://mysite.freeserve.com/jc_atm/

Phil wrote:
Hi.

I have just built my first telescope. Most of the parts I got from
University Optics, the secondary from Newport Glass and the primary from
ebay. It is a 6" f5 scope. I used Newt to get the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I can
see a large black blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of
the gurus agree? If not, what could be the problem?

Almost certainly it is nominally f5 +- 20% ....

Easy way to measure radius of curvature by holding a pin on axis at
looking into the mirror. Inifinity focus is half this distance.

Or quick and dirty find where it focusses a distant horizon or streetlamp.

The reason I think it might be the primary is it was only, like, 35 bucks.
It is the weakest link, except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

Measure it!

Regards,
Martin Brown

SWell, I did the flashlight-and-a-piece-of-paper trick, and got a
measurement of about 42 1/2 inches. So, according to everything I have
read, my focal length is a shade over 21".

Swell. f3.5.

I wanted a rich-field scope, but this is a little extreme!

-Phil
www.pickupcentral.com




"Phil" wrote in message
. com...
Hi.

I have just built my first telescope. Most of the parts I got from
University Optics, the secondary from Newport Glass and the primary from
ebay. It is a 6" f5 scope. I used Newt to get the measurements.

Tonight I aimed it at the moon and could not get it to come to focus. I
can see a large black blob in the center of the FOV from the secondary.

I am thinking that the primary is not really f5 as advertised. Do any of
the gurus agree? If not, what could be the problem?

The reason I think it might be the primary is it was only, like, 35 bucks.
It is the weakest link, except for perhaps the builder. g

On a similar vein, how can I tell the focal ratio of the primary?

(This isn't my first scope, this is just the first scope that I built.)

Thanks for your help!
-Phil
www.pickupcentral.com

Phil wrote:
SWell, I did the flashlight-and-a-piece-of-paper trick, and got a
measurement of about 42 1/2 inches. So, according to everything I have
read, my focal length is a shade over 21".

Swell. f3.5.

I wanted a rich-field scope, but this is a little extreme!

-Phil
www.pickupcentral.com


Wow! That's remarkable. I'd be very curious to see how the figure on the
mirror is. You can make a slitless tester for only a couple of bucks
and a couple hours of work. Here's a URL that describes it:

http://www.jlc.net/~force5/Astro/ATM...ultTester.html

You can quickly get an idea of how good the mirror is by using this
thing. If you are interested and have any questions, drop me an email at
chills at mail dot utexas dot edu.

good luck - that's a mighty fast little telescope.

Chris

"Phil" wrote in message
om...
SWell, I did the flashlight-and-a-piece-of-paper trick, and got a measurement of about 42 1/2
inches. So, according to everything I have read, my focal length is a shade over 21".

Swell. f3.5.

I wanted a rich-field scope, but this is a little extreme!

-Phil
www.pickupcentral.com

That sounds rather short Phil, you are probably right but a typical mirror might be more toward F7,
but then you've measured it and I haven't even seen it. Many moons ago the 6" F7 was "the" amateur
telescope :-) they were made in their hundreds. An F3.5 sounds like it may have been made by an
amateur for a special purpose. Any idea how old it is ? sometimes they're etched on the back, I
always do mine, finish date, F length, fraction of wave etc.

--
John Carruthers
http://mysite.freeserve.com/jc_atm/