Fluorite vs. ED for film imaging

I was recently imformed that a fluorite refractor is bad for film imaging
because the focus changes during long exposures, whereas an ED refractor
will retain its focus at a constant during the entire exposure.

Any truth to this?

Thanks,
-Phil

I was recently imformed that a fluorite refractor is bad for film imaging
because the focus changes during long exposures, whereas an ED refractor
will retain its focus at a constant during the entire exposure.

Any truth to this?

Yes and no. They both change depending on design. Fluorite is an ED material,
by the way, so your heading should say ED vs. ED for film imaging.

Roland Christen

On Mon, 28 Jul 2003 23:51:03 GMT, "PJ"
wrote:

Hi Roland,

The design is my Tak FS-128. This will have to suffice until my name comes
up on a notification list for a certain 155mm ED refractor. Until that
glorious day arrives, I obtained the FS-128 for flim and CCD imaging. An
astrophotographer told me this was a mistake because fluorite won't retain
focus beyond 30 minutes.

Assuming a constantly dropping or rising temperture.
-Rich

This one looks like it is in focus
http://www.buytelescopes.com/gallery...oto.asp?pid=96
Thanks
Herb
ATWB


wrote in message
...
On Mon, 28 Jul 2003 23:51:03 GMT, "PJ"
wrote:

Hi Roland,

The design is my Tak FS-128. This will have to suffice until my name
comes
up on a notification list for a certain 155mm ED refractor. Until that
glorious day arrives, I obtained the FS-128 for flim and CCD imaging. An
astrophotographer told me this was a mistake because fluorite won't
retain
focus beyond 30 minutes.

Assuming a constantly dropping or rising temperture.
-Rich

Thanks for the explanation, Roland. Guess I need to take along a
thermometer.


"Chris1011" wrote in message
...
The design is my Tak FS-128. This will have to suffice until my name
comes
up on a notification list for a certain 155mm ED refractor. Until that
glorious day arrives, I obtained the FS-128 for flim and CCD imaging. An
astrophotographer told me this was a mistake because fluorite won't retain
focus beyond 30 minutes.

But if the ED has the same effect, as you are saying, then what scope can
be
deemed as worthy of long film exposures?

Depends on how critical you are. Lots of people use the 128 Tak for
excellent
photography. Why 30 minutes? Why not specify a change in temperature? All
scopes will change focus with dropping temperatures except for compensated
tube
assemblies (carbon fiber Cassegrains for example). Since I never used a
Fluorite refractor for imaging, I cannot tell you what that change would
be. I
would say that a 5 degree temp change would require you to refocus in any
ED
scope. This much change would probably also mean pretty bad seeing. For
the
sharpest astrophotos, you will need stable airmass above which inevitably
means
stable air temperatures.

Roland Christen

You mean this one?
http://www.buytelescopes.com/gallery...to.asp?pid=204
You may want to watch AstroMart tomorrow.
g

Herb
ATWB

"PJ" wrote in message
.. .
Thanks for the explanation and the link, Herb. I like your gallery
concept.
I'm just getting into film astrophotography, but managed to add a decent
one
to the Anacortes gallery for Nikon 105mm macro lens / NJP mount.

-Phil


"Herb York" wrote in message
news:385227ab37669c25fed477cb7f8971fb@TeraNews...
Nearly all telescopes require constant refocusing for things like - it
is
getting colder and the tube is shrinking - you change filters and they
are
not parfocal with each other - etc. This has almost or nothing to do
with
the optics substrate.
Thanks
Herb
ATWB


"Herb York" wrote in message
news:3751c5706f0a3fa3627f419801ddd94b@TeraNews...
This one looks like it is in focus
http://www.buytelescopes.com/gallery...oto.asp?pid=96
Thanks
Herb
ATWB

Yep...that's the one. If our photos help sell the products, do we get
commission checks? ;-)


"Herb York" wrote in message
news:1a9840c19cb6d95c4005af9906fb379b@TeraNews...
You mean this one?
http://www.buytelescopes.com/gallery...to.asp?pid=204
You may want to watch AstroMart tomorrow.
g

Herb
ATWB

"PJ" wrote in message
.. .
Thanks for the explanation and the link, Herb. I like your gallery
concept.
I'm just getting into film astrophotography, but managed to add a decent
one
to the Anacortes gallery for Nikon 105mm macro lens / NJP mount.

-Phil

"PJ" wrote in message
news:cEhVa.1420$Oz4.308@rwcrnsc54...
I was recently imformed that a fluorite refractor is bad for film imaging
because the focus changes during long exposures, whereas an ED refractor
will retain its focus at a constant during the entire exposure.

Any truth to this?

Thanks,
-Phil
Others have given the answer, but it is worth remembering, that you can
(assuming a 'backlash free' focusser mechanism is used), correct for this,
by having a focus controller that implements temperature compensation. It
may introduce other problems (how smooth is the focusser etc.), but does
allow longer exposures without this problem...

Best Wishes

Phil,
Any structure composed of materials having different expansion
coefficients will deform on temperature change. The index of refraction of the
glass will also change. Manufacturers make the best comprimise to hold both the
focus and color correction as constant as possible. You should be able to
obtain help from the manufacturer on both issues.
The Hubble is an extreme case of thermal problems, being exposed to
hundreds of degrees of heating on each orbit.
Bill Bambrick
41 N, 73 W, 95 ASL