Telescopes for astrophotography

wrote:
"In round numbers, if you look at a planet through a scope with a 28mm
eyepiece,"

An interesting piece of trivia.

I suspect that the 28mm EP is of Orthoscopic or Plossl design (45-50
dFOV) and that wider EP designs (like Naglers with 82dFOV) would have
the same FOV at 16-17mm.


You'd need to calculate out the field of view, and since the chip is
rectangular it wouldn't directly relate to a normal eyepiece.

Mostly, it's just a handy way to figure out what kind of magnification
you'll get on the chip.

"Mark S. Holden" wrote in message
...

I have three lenses for my D70, and naturally they each take a different
filter size. (I'd need 67,72, and 77mm)


Well then, you are obviously more into daylight photography than the average
astro-geek. :-)

On the other hand, if you are trying to convince an understanding wife that
an astro-imaging capable DSLR can also be used for vacation photos....

"Mark S. Holden" wrote in message
...

For time exposures, the mount will be a significant factor. You'll want
an equatorial mount with provisions for talking to an auto guider.
(You'll want an autoguider for exposures longer than about 2 minutes)

As for the type of telescope, I like apo refractors for deep sky
photography. No central obstruction, and the faster F ratios cut down
on the exposure time. I like them for planets too, but you need a
monster to get a good image scale.

The D70 is good for the Moon, but you're likely to prefer a webcam for
planets. (The web cam can be used with a laptop to create your
autoguider for deep sky)

Ok, with an eq mount I understand polar alignment and tracking. And I think
I understand the concept of autoguiding but I don't know technically how
it's done.

Does the software pick a couple of stars out of the frame and then somehow
automagically try to keep them in the same spot on the frame? And I've seen
mentions of manual guiding. How would one go about manually guiding? Seems
like that would be a difficult proposition.

Greg

"G.T." wrote in message
...

"Mark S. Holden" wrote in message
...

For time exposures, the mount will be a significant factor. You'll want
an equatorial mount with provisions for talking to an auto guider.
(You'll want an autoguider for exposures longer than about 2 minutes)

Ok, with an eq mount I understand polar alignment and tracking. And I
think
I understand the concept of autoguiding but I don't know technically how
it's done.

Does the software pick a couple of stars out of the frame and then somehow
automagically try to keep them in the same spot on the frame?

Somewhat depends on the brand and/or model of autoguider, but generally yes,
using a single star. Although it's not really all that magical since the
auto-guider camera is a grid array of pixels. The system tries to keep your
guide star on a pixel, issuing direction commands to the mounts RA and Dec
motors as necessary.

And I've seen
mentions of manual guiding. How would one go about manually guiding?

Using an eyepiece with a double cross hair arrangment, you place the star
either in the center square made up at the intersection of the two sets of
cross hairs, or you put it where two interesting lines meet and then
whenever the star appears to be moving off the lines or toward an edge of
the square, you press the appropriate direction button to keep the star
centered.

An auto-guider is much less hassle, once you have it set.

"Stephen Paul" wrote in message
...

"G.T." wrote in message
...

"Mark S. Holden" wrote in message
...

For time exposures, the mount will be a significant factor. You'll
want
an equatorial mount with provisions for talking to an auto guider.
(You'll want an autoguider for exposures longer than about 2 minutes)

Ok, with an eq mount I understand polar alignment and tracking. And I
think
I understand the concept of autoguiding but I don't know technically how
it's done.

Does the software pick a couple of stars out of the frame and then
somehow
automagically try to keep them in the same spot on the frame?

Somewhat depends on the brand and/or model of autoguider, but generally
yes,
using a single star. Although it's not really all that magical since the
auto-guider camera is a grid array of pixels. The system tries to keep
your
guide star on a pixel, issuing direction commands to the mounts RA and Dec
motors as necessary.

And I've seen
mentions of manual guiding. How would one go about manually guiding?

Using an eyepiece with a double cross hair arrangment, you place the star
either in the center square made up at the intersection of the two sets of
cross hairs, or you put it where two interesting lines meet and then
whenever the star appears to be moving off the lines or toward an edge of
the square, you press the appropriate direction button to keep the star
centered.

Right, but with manual guiding once that star is off-center haven't you
ruined the shot? Or since it's a webcam you just align the individual
frames once you're done with that particular exposure?


Greg

G.T. wrote:
"Stephen Paul" wrote in message
...

"G.T." wrote in message
...

"Mark S. Holden" wrote in message
...

For time exposures, the mount will be a significant factor. You'll

want

an equatorial mount with provisions for talking to an auto guider.
(You'll want an autoguider for exposures longer than about 2 minutes)

Ok, with an eq mount I understand polar alignment and tracking. And I
think
I understand the concept of autoguiding but I don't know technically how
it's done.

Does the software pick a couple of stars out of the frame and then

somehow

automagically try to keep them in the same spot on the frame?

Somewhat depends on the brand and/or model of autoguider, but generally

yes,

using a single star. Although it's not really all that magical since the
auto-guider camera is a grid array of pixels. The system tries to keep

your

guide star on a pixel, issuing direction commands to the mounts RA and Dec
motors as necessary.


And I've seen
mentions of manual guiding. How would one go about manually guiding?

Using an eyepiece with a double cross hair arrangment, you place the star
either in the center square made up at the intersection of the two sets of
cross hairs, or you put it where two interesting lines meet and then
whenever the star appears to be moving off the lines or toward an edge of
the square, you press the appropriate direction button to keep the star
centered.


Right, but with manual guiding once that star is off-center haven't you
ruined the shot? Or since it's a webcam you just align the individual
frames once you're done with that particular exposure?


Greg



For guiding, you normally guide through a second scope, or with an off
axis guider that swipes a little light from the side.

You normally don't guide with the same chip you're using to take the
image - but at least one outfit has developed a single chip
imaging/guiding system. Many of the sbig cameras have a small guide
chip next to the imaging chip.

Auto guiders are much better than manually guiding because the
autoguider can tell as soon as the light has shifted away from the pixel
the star is supposed to be on, and it doesn't get distracted.

If you catch the error quickly, it's not going to have much of an impact
on the image because if the light was bright enough to make a difference
that quickly, you wouldn't need to be taking a time exposure.

Regards,

Mark

"Mark S. Holden" wrote in message
...

Auto guiders are much better than manually guiding because the autoguider
can tell as soon as the light has shifted away from the pixel the star is
supposed to be on, and it doesn't get distracted.

If you catch the error quickly, it's not going to have much of an impact
on the image because if the light was bright enough to make a difference
that quickly, you wouldn't need to be taking a time exposure.


Excellent description.