Celestron Fastar - pros and cons

Good day

Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

From the above, it sounds like a great combination to hunt for minor
planets!

What are the disadvantages of this setup?

Thanks

Jack wrote:
Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

From the above, it sounds like a great combination to hunt for minor
planets!

What are the disadvantages of this setup?

One problem is that because it involves mounting your camera to a
corrector lens assembly that is on the front of the telescope, it
creates a risk of accidentally breaking the corrector plate.

Another is simply that the camera itself should be round, and small,
because it is going to be the new central obstruction, slightly larger
than the mirror.

Another is that, since it replaces the secondary, you won't see
anything by looking into the telescope at the back end any more...
and, again, since it's on the front of the telescope, off-axis guiding
is not possible (the corrector doesn't leave enough distance to the
camera to insert an off-axis guider or a diagonal anyways, but that
isn't a problem because in front of the telescope is no place for such
things). So you will only be able to guide your 'scope using the
finder. (Of course, you can always attach a bigger finder - also known
as a guide scope.)

As a result of the disadvantages noted above, the Fastar lens assembly
has been _discontinued_ by Celestron. Starizona makes its own
versions, apparently, but they're both fancier and more expensive than
Celestron's, I believe.

So much for the disadvantages. The advantages, as you've noted, are
shorter exposure times and a wider field of view; thus, I would almost
tend to say that if you have a suitably round-profiled CCD camera, the
Fastar lens assembly is almost as obvious a thing to get for a
telescope that can use one - as a Barlow lens. Which is why I think
it's a pity that it has been discontinued.

John Savard

Jack wrote:
What are the disadvantages of this setup?

Upon reflection, I realized that I left out the *most important*
disadvantage of that setup.

Given the (relatively minor) disadvantages I pointed out in my other
post, the most serious problem is that enduring those disadvantages is
unnecessary.

That's because the *detector* in most CCD cameras - until you get up
there to the *very* expensive kinds of detector - are pretty small.
Which means that one can, without problems of vignetting, simply
shrink the image produced by the telescope at the normal eyepiece
position for fast photography.

And, in fact, image reducers of this type (noted as for imaging use
only, since they would not work well visually with long focal-length
eyepieces) are available.

Meade has an f/3.3 reducer has a T-mount adapter, so it only works
with film cameras, but either Meade or Celestron has a strong reducer
billed as designed for one of their CCD cameras specifically.

John Savard

On Oct 11, 7:43 am, Quadibloc wrote:
Jack wrote:
What are the disadvantages of this setup?

Upon reflection, I realized that I left out the *most important*
disadvantage of that setup.

Given the (relatively minor) disadvantages I pointed out in my other
post, the most serious problem is that enduring those disadvantages is
unnecessary.

That's because the *detector* in most CCD cameras - until you get up
there to the *very* expensive kinds of detector - are pretty small.
Which means that one can, without problems of vignetting, simply
shrink the image produced by the telescope at the normal eyepiece
position for fast photography.

And, in fact, image reducers of this type (noted as for imaging use
only, since they would not work well visually with long focal-length
eyepieces) are available.

Meade has an f/3.3 reducer has a T-mount adapter, so it only works
with film cameras, but either Meade or Celestron has a strong reducer
billed as designed for one of their CCD cameras specifically.

John Savard

Well, depends on your definitial of "expensive," but larger detectors
are getting into the hands of more and more amateurs all the time.
Something along the lines of the ST2000 camera could definitely
benefit from Fastar,

Fastar itself? I don't think Celestron discontinued it specifically
because of the "disadvantages" you mention, but simply because they
were not selling many of the corrective optics packages. I suppose the
number of serious imagers and the subset of serious imagers interested
in trying Fastar was just not large enough.

As for the disadvantages? I've been surprised that people turn out
amazing images by hanging a DSLR on the corrector via Fastar. No, you
can't guide off axis, but most folks aren't doing that much anymore
anyway. The chance of damaging the corrector IS very real. Me? I'm
sticking with a Meade 3.3 reducer--even if I'm not too happy with it.

;-)

Originally Posted by Jack[_5_]

Good day

Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

From the above, it sounds like a great combination to hunt for minor
planets! What are the disadvantages of this setup?Thanks

I tested and reviewed the 8" Fastar at http://home.freeuk.net/m.gavin/fastar.htm

Nytecam

Quadibloc wrote:

Jack wrote:
Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

From the above, it sounds like a great combination to hunt for minor
planets!

What are the disadvantages of this setup?

One problem is that because it involves mounting your camera to a
corrector lens assembly that is on the front of the telescope, it
creates a risk of accidentally breaking the corrector plate.

Another is simply that the camera itself should be round, and small,
because it is going to be the new central obstruction, slightly larger
than the mirror.

Another is that, since it replaces the secondary, you won't see
anything by looking into the telescope at the back end any more...
and, again, since it's on the front of the telescope, off-axis guiding
is not possible (the corrector doesn't leave enough distance to the
camera to insert an off-axis guider or a diagonal anyways, but that
isn't a problem because in front of the telescope is no place for such
things). So you will only be able to guide your 'scope using the
finder. (Of course, you can always attach a bigger finder - also known
as a guide scope.)

As a result of the disadvantages noted above, the Fastar lens assembly
has been _discontinued_ by Celestron.

Total Nonesense.

On Oct 11, 5:47 am, Jack wrote:
Good day

Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

What are the disadvantages of this setup?

What are the disadvantages? The main one everybody forgets is that you
are wasting resolution of the telescope by imaging at F2. Ideally the
image should be sampled at some nominal rate, but if your pixel size
is many times larger than your theoretical Airy Disc size, then you
are severely undersampling the image. No matter what you do, you
cannot get max resolution of image details. Rule of thumb is that the
focal ratio should be about equal to the pixel size in microns,
therefore at F2 you would need a 2 micron pixel, which is just not
going to happen. With a 9 micron pixel camera you can shoot at F7 and
obtain 80 - 90% of the resolution that your scope is capable of.
Anything faster than that and the resolution begins to trail off
further. An 8" SCT shooting at F2 with a 9 micron camera has the
resolution of about a 2" lens.

The other thing people seem to forget is that you can bin your
detector and get the same result at long focal ratios as you would get
at much faster focal ratios with an un-binned detector. F10 binned 3x3
will get you the same image result as F3.3 binned 1x1 - same density,
same signal/noise and same resolution.

By the way, the images that I see posted with the Fastar have been
pathetic, nowhere near what an SCT is capable of. I don't understand
why people are happy with mediocre results when much better images are
possible with the same equipment in a more normal configuration.

Rolando

wrote:

On Oct 11, 5:47 am, Jack wrote:
Good day

Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

What are the disadvantages of this setup?

What are the disadvantages? The main one everybody forgets is that you
are wasting resolution of the telescope by imaging at F2. Ideally the
image should be sampled at some nominal rate, but if your pixel size
is many times larger than your theoretical Airy Disc size, then you
are severely undersampling the image. No matter what you do, you
cannot get max resolution of image details. Rule of thumb is that the
focal ratio should be about equal to the pixel size in microns,
therefore at F2 you would need a 2 micron pixel, which is just not
going to happen. With a 9 micron pixel camera you can shoot at F7 and
obtain 80 - 90% of the resolution that your scope is capable of.
Anything faster than that and the resolution begins to trail off
further. An 8" SCT shooting at F2 with a 9 micron camera has the
resolution of about a 2" lens.

The other thing people seem to forget is that you can bin your
detector and get the same result at long focal ratios as you would get
at much faster focal ratios with an un-binned detector. F10 binned 3x3
will get you the same image result as F3.3 binned 1x1 - same density,
same signal/noise and same resolution.

By the way, the images that I see posted with the Fastar have been
pathetic, nowhere near what an SCT is capable of. I don't understand
why people are happy with mediocre results when much better images are
possible with the same equipment in a more normal configuration.

Rolando

People totally forget about pixel sixe (and logic) and latch on
to "Im doing something novel!". They think they have a schmidt
camera. (In a Chrysler they think they are in a BMW!).
KLM

On Fri, 12 Oct 2007 09:08:47 -0700, wrote:

The other thing people seem to forget is that you can bin your
detector and get the same result at long focal ratios as you would get
at much faster focal ratios with an un-binned detector. F10 binned 3x3
will get you the same image result as F3.3 binned 1x1 - same density,
same signal/noise and same resolution.

Binning doesn't magically result in a physically larger chip. There's
absolutely nothing wrong with choosing to trade resolution for FOV by
working at a short focal length. An optimum pixel scale may result in a
suboptimum FOV. It all depends on your imaging goals.

And focal ratio doesn't matter much at all- not really worth considering
for imagers.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Oct 13, 12:14 am, Chris L Peterson wrote:
On Fri, 12 Oct 2007 09:08:47 -0700, wrote:
The other thing people seem to forget is that you can bin your
detector and get the same result at long focal ratios as you would get
at much faster focal ratios with an un-binned detector. F10 binned 3x3
will get you the same image result as F3.3 binned 1x1 - same density,
same signal/noise and same resolution.

Binning doesn't magically result in a physically larger chip. There's
absolutely nothing wrong with choosing to trade resolution for FOV by
working at a short focal length. An optimum pixel scale may result in a
suboptimum FOV. It all depends on your imaging goals.

And focal ratio doesn't matter much at all- not really worth considering
for imagers.

_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

If your goal is to obtain lots of crappy wide field images in a hurry,
then by all means shoot with an F2 focal ratio. If the idea is to
optimize the perameters of each item, i.e. the scope the camera, the
resolution, sig/noise, etc., then tying pixel size to F-ratio will
result in the best performance.

Remember this: shooting an image at F10 with a 1k x 1k CCD with 24
micron pixels will give you the exact same result (resolution, sig/
noise AND field of view) as shooting that same image at F2.5 with a 1k
x 1k 6 micron pixel CCD.

Rolando