newbie homebuilt distortion

I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.
I read somewhere where some of the mirrors from this source were not
the right grind - spherical instead of parabolic or vice versa. This
mirror turned out to have a focal length of 23.5 inches (shorter than
advertised). I have another larger mirror from them (a 10") and its
exactly the focal length they advertised... (dont have it in front of
me but its 40 something). I have not built a mounting for the larger
mirror yet, kinda wanting to figure out whats wrong with this before I
dive in deeper.

Any ideas?

Yep, that has been found in LOTS of their mirrors. Only thing I know of is
to regrind it after removing the coating and get it to meet parbloic shape.
You'll have to figure if you want a slow one (like my f8 Babylon 8) or a
faster f5 one.

Or reurn it and get your money back.


--
The Lone Sidewalk Astronomer of Rosamond

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wrote in message
oups.com...
I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.
I read somewhere where some of the mirrors from this source were not
the right grind - spherical instead of parabolic or vice versa. This
mirror turned out to have a focal length of 23.5 inches (shorter than
advertised). I have another larger mirror from them (a 10") and its
exactly the focal length they advertised... (dont have it in front of
me but its 40 something). I have not built a mounting for the larger
mirror yet, kinda wanting to figure out whats wrong with this before I
dive in deeper.

Any ideas?

Thanks for the quick reply

So I take it my 'odd' distortion on distant objects is the mirror shape
then? Is there any easy way to tell if the larger mirror I have is the
right grind before I take time to make a mount for it and put it
together?



Starlord wrote:
Yep, that has been found in LOTS of their mirrors. Only thing I know of is
to regrind it after removing the coating and get it to meet parbloic shape.
You'll have to figure if you want a slow one (like my f8 Babylon 8) or a
faster f5 one.

Or reurn it and get your money back.

wrote in news:1158202193.640794.266360
@p79g2000cwp.googlegroups.com:

I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.
I read somewhere where some of the mirrors from this source were not
the right grind - spherical instead of parabolic or vice versa. This
mirror turned out to have a focal length of 23.5 inches (shorter than
advertised). I have another larger mirror from them (a 10") and its
exactly the focal length they advertised... (dont have it in front of
me but its 40 something). I have not built a mounting for the larger
mirror yet, kinda wanting to figure out whats wrong with this before I
dive in deeper.

Any ideas?

If the focal length is really 23.5", then for an 8" mirror that is an F
ratio of about 2.9, which will result in severe comma. If the mirror is
spherical then throw in severe spherical abberation as well.

Klazmon

wrote in
ps.com:

Thanks for the quick reply

So I take it my 'odd' distortion on distant objects is the mirror shape
then? Is there any easy way to tell if the larger mirror I have is the
right grind before I take time to make a mount for it and put it
together?

You can use the knife edge test invented by Leon Foucault.

http://www.stellafane.com/atm/atm_fo...er_plans_1.htm

There is also the Ronchi test which is good for detecting a turned down
edge. Explore the above site for details.

Klazmon.






Starlord wrote:
Yep, that has been found in LOTS of their mirrors. Only thing I know of
is to regrind it after removing the coating and get it to meet parbloic
shape. You'll have to figure if you want a slow one (like my f8 Babylon
8) or a faster f5 one.

Or reurn it and get your money back.

I would think you'd need the useage of a slit / Foucker tester, others will
know the right name, but you can set up the mirror and use the tester to get
an idea of your mirrors figure. I've got one, but due to living in a trailer
I've not had enough room to use it right.


--
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
In Garden Online Gift Shop
http://www.cafepress.com/ingarden
Blast Off Online Gift Shop
http://www.cafepress.com/starlords
Astro Blog
http://starlord.bloggerteam.com/




wrote in message
ps.com...
Thanks for the quick reply

So I take it my 'odd' distortion on distant objects is the mirror shape
then? Is there any easy way to tell if the larger mirror I have is the
right grind before I take time to make a mount for it and put it
together?



Starlord wrote:
Yep, that has been found in LOTS of their mirrors. Only thing I know of
is
to regrind it after removing the coating and get it to meet parbloic
shape.
You'll have to figure if you want a slow one (like my f8 Babylon 8) or a
faster f5 one.

Or reurn it and get your money back.

wrote:
I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.

If we knew the scope was well colimated, this reads like astigmatism
and a turned edge.

However, if the scope is not well collimated, then a good colimation is
in order before jumpting to any conclusions.

I would start by having you Google up Mel Bartels and reading about
collimation. You can also Google up Newtonian Telescope Collimation. It
may take a while for you to get the scope into a well collimated state
(several nights). But do come back once it is well collimated and tell
us how the image looks then.

wrote:
but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.

If we knew the scope was well colimated, this reads like astigmatism
and a turned edge.

However, if the scope is not well collimated, then a good colimation is
in order before jumpting to any conclusions.

Maybe, or badly pinched optics ? loosen the screws, put a thickness of
paper between the screw and the glass, do up light finger tight, remove
the paper.
Same for the secondary.
jc

wrote:
I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
woul

d expect for out of focus.

snip

Any ideas?


When I first read this my first thought was collimation, and I still
think that's part of the problem, but, bottom line, if I were you I'd
get my money back if you still can.

An 8" mirror at f/3 is extremely hard to figure, we're talking about
digging out a lot of material in the middle of the mirror. And if you
manage to get a decent figure on it, the coma will kill you, and it will
be very hard to get into collimation.

I'm usually a big one for DIY stuff, but I'd run from this one. There
are better cheap mirrors out there to play with.

'luck-
Chris

On Wed, 13 Sep 2006 21:49:53 -0500, wrote:

I recently built my first homebuilt in probably 40 years, a simple
Dobsonian with (for now) a 8" mirror I bought from Sylvan on eBay. The
scope seems good on focusing on near objects and does a beautiful image
on the moon, but in looking at say Jupiter tonite I get an odd
distortion. The viewed image at its smallest and brightest is nowhere
near round. It looks like a weird scrunched up bowtie. One side is
much larger. Focus either way and it goes to a dimmer circle like I
would expect for out of focus.
I read somewhere where some of the mirrors from this source were not
the right grind - spherical instead of parabolic or vice versa. This
mirror turned out to have a focal length of 23.5 inches (shorter than
advertised). I have another larger mirror from them (a 10") and its
exactly the focal length they advertised... (dont have it in front of
me but its 40 something). I have not built a mounting for the larger
mirror yet, kinda wanting to figure out whats wrong with this before I
dive in deeper.

Any ideas?

To summarize what others have pointed out individually,
the problem could be a number of things, or even a
combination of them:

1. Poorly figured mirror (f/3 is difficult, and you won't
find a good & cheap one anyplace but an estate sale.)
2. Coma and/or astigmatism. Common in fast (low f/number)
mirrors.
3. Pinched/strained optics. VERY LIKELY. You didn't say
how they were mounted. And this tends to produce non-
symmetrical images.
4. Other possibilities:
- defective diagonal
- poor collimation (also VERY LIKELY)
- wrong choice of eyepiece for fast (f/3) system
- defective eyepiece
- defects in the eye

Hope this helps.
Larry G.




--
Freedom is a terrible thing to waste !
- turn OFF your TV
- turn ON your MIND.

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