«

On Sat, 24 Oct 2009 01:43:38 +1000, Troy Piggins <usenet-(E-Mail Removed)>
wrote:

>The astrophotography has been keeping me occupied lately. This
>is my first attempt at planetary imaging. Lots to learn, I know.
>Don't see much astrophotography here so thought I'd share.
>
>Taken with a 8" f/10 scope with a 2.5x powermate (like a
>teleconvertor) giving it an equivalent focal length of around
>5000mm. Camera was a DBK21 CCD camera.
>
>The dark spot is the shadow of one of the moons, and you can just
>make out the Great Red Spot at the top.
>
>http://piggo.com/~troy/photos/2009/2...er091023_1.jpg
>
>All up I'm pretty happy with it. Suspect the scope needs some
>tweaking of the collimation which should give a sharper image.
>Will have to try that next time, haven't done it before.

Much depends too on "seeing" conditions. The atmospheric stability. Most
times you just have to wait and hope for the best days. The very same
perfectly collimated optics can provide a draw-dropping 3D-looking view of
Saturn one day, and an irregular mushy blob the next. Look into the
sharpening techniques that web-cam astrophotographers use, by combining
details from many many frames to virtually look through the turbulent
atmosphere, capturing and combining those bits of each image that are
stable and sharp.

You might also try stopping down the aperture of your telescope during bad
seeing conditions. A larger aperture means that your telescope is trying to
image through larger lower-frequency areas of atmospheric turbulence. If
the turbulence that night is mostly of the lower-frequency variety it will
help to filter it out. I keep a 6" mask handy for those times to put on my
16" scope. Apodizing masks also cure things on some days for planetary
imaging.

On Sat, 24 Oct 2009 07:21:39 +1000, Troy Piggins <usenet-(E-Mail Removed)>
wrote:

>
>
>How does one stop down the aperture of a fixed aperture scope?
>The bare scope is f/10. With the 2.5x powermate it becomes an
>equivalent f/25. I haven't heard of people using those masks
>you're referring to. I'll look into it. Thanks.

A simple round hole cut into some opaque plastic, cardboard, or thin
aluminum sheeting will suffice. Placed over the opening of your telescope.
For a refractor this is easy (no central obstruction). Just place it
concentric with the optical axis.

For a reflector the choice is not so easy. The secondary mirror's size is
optimized for the light path and f/ratio.

Larger reflector telescopes can use an aperture mask offset to one side, so
as to use an unobstructed region of the mirror between the outside diameter
of the primary and the outside diameter of the secondary, and situated
between the spider-vanes. Consider too the number of spider-vanes you have.
If 4 vanes you will have to cut your mask smaller so its diameter fits
within an open quadrant between any two spider-vanes.

The huge plus of this for planetary imaging is that now you have an
obstruction-free telescope. Of reduced aperture but for bright subjects and
due to "seeing" problems this can be a huge plus too. Many people buy 12"
or larger reflectors with the intent to only use it as a stopped-down
off-axis planetary imager. (8"-10" telescopes too, but you then start to
lose resolution due to primary size alone when stopped-down off-axis.)
There is a huge cost-savings in buying pre-fabricated easy to make
manufactured telescopes much greater than the size needed, as opposed to
buying or building an off-axis (asymmetric) reflecting telescope design
(see below), or prohibitively expensive refractor of those diameters which
is now fraught with CA problems.

With the aperture offset you are no longer plagued with diffraction from
secondary mirror and its spider supports. Since this is a reflector, you
now have a telescope that is free of all chromatic-aberration, making it
much better than a refractor of the same size (large and astronomically
expensive refractors bought with planetary imaging in mind). Special
asymmetric reflector telescopes are designed this way, but grinding and
figuring the offset curvatures are extremely difficult and many ingenious
methods were tried and found to try to circumvent this fabrication problem.
One of the more ingenious is to grind an achromat corrective lens for use
with a standard parabolic mirror set at an angle. This achromat ground to
the proper figure by using a creative method found for the home telescope
builder, but then you introduce CA problems. Often, to simplify things,
they'll just buy a much larger pre-figured mirror and then cut it up into 2
or 3 smaller offset-telescope primaries. (I don't think I could bring
myself to do that, even though I have the means. It would be like cutting a
favorite child into 2's or 3's.)

By using an offset aperture mask on a large telescope you now have the best
of 3 worlds. An exceptional planetary imager (the same as a prohibitively
expensive asymmetric reflector telescope), no CA problems as exists in all
refractors, and when the mask is removed you now have a very very nice
deep-sky light-bucket.

Aside: This is precisely why I chose the size telescope I now have (16"
dia.). The 16" also not chosen arbitrarily due to costs nor other issues.
When researching I found that due to even the most pristine seeing
conditions (unless I am on a mountain-peak), that without adaptive optics
the resolution of this size telescope is the same as that of Mt. Palomar's
200" telescope. The weakest link now being caused by the atmosphere itself.
There was no appreciable gain in resolution by buying larger. Light-grasp
yes, resolution no. (Keep in mind too, this was before image-stacking
became popularized to increase resolution. And since I was going to
primarily use it for visual astronomy this didn't enter into my
decision-making equations. Then, nor now.)

Another plus to an offset mask is that you can rotate the aperture-mask to
find a "sweet spot" of your mirror where the figure is the most pristine.
This can greatly improve on its 1/8th to 1/20th wavelength of light
tolerance across its whole surface.

For smaller telescopes you can try an aperture reducing mask placed
concentric with the axis of the telescope, but then the smaller you stop
down the aperture the more that diffraction becomes an issue due to the
larger percentage of central obstruction vs. the useful light path.

Experiment.

On Sat, 24 Oct 2009 22:17:05 +1000, Noons <(E-Mail Removed)> wrote:

>Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>eh?

Inland is worse, but then you'd know this if you knew the least bit about
photography and astronomy. Another **** poor attempt of yours to try to
look like you knew something. Trolls never do.

Troy Piggins wrote,on my timestamp of 24/10/2009 7:21 AM:
> * Damn 35 F Rain - Staying Warm Inside Is Winning Today wrote :
>> * Troy Piggins wrote :
>>
>> [---=| Quote block shrinked by t-prot: 11 lines snipped |=---]
>>> http://piggo.com/~troy/photos/2009/2...er091023_1.jpg


Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
eh?
Sometimes I wish I could retire in a place like Maree or Oodnadatta and enjoy
clear, cloudless skyes all year round. I do recall reading a newspaper in the
campsite by starlight alone, no moon! Beer (Red Back) ain't half bad over there
either...


> Yes, this image was stacked from around 2500 frames of an avi
> file using Registax. Suspect that's the technique you're
> referring to.

Did you get that size image from the 8" scope and sensor alone or did you add a
converter and/or digital resize?

I'm toying around with the idea of a 8" or 10" dobsonian, want to get a feel for
what's possible and what's needed. Kids have been bugging me to get back into
this stuff...

Outing Trolls wrote,on my timestamp of 24/10/2009 9:24 PM:
> On Sat, 24 Oct 2009 22:17:05 +1000, Noons <(E-Mail Removed)> wrote:
>
>> Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>> eh?
>
> Inland is worse, but then you'd know this if you knew the least bit about
> photography and astronomy. Another **** poor attempt of yours to try to
> look like you knew something. Trolls never do.
>

What an idiot...

Troy Piggins wrote,on my timestamp of 24/10/2009 11:06 PM:


> I've been to a few dark sites this last year, at new moon, not a
> cloud in the sky. Got a sore neck from constantly gazing up at
> the sky.

The dry air inland makes quite a difference: very little haze.
Never cease to be amazed how clear the sky is in the desert: I've got photos of
Mt Connor at nearly 30Ks that look like the blessed thing is just 5 away.


> The C8 8" f/10 schmidt cassegrain I have had a 2.5x powermate
> (like a teleconvertor) on it, which gave focal length of around
> 5000mm. Plus the image was slightly cropped to square it up from
> the sensor size of 640x480.

Cool. Good luck with convincing the other half for the better camera!


> "Aperture rules" - 10" lets in almost twice the amount of light
> the 8" does

Yeah, I know. But it also increases the size and weight of the thing a lot!



> Do you want it for visual observing or taking photos? If visual,
> all good. If photos, slippery slope. Dobs/Newtonians might be
> fine for planetary imaging, but no good unless you mount them on
> equatorial mount for deep sky, long exposure shots.
>
> If you're really keen, email me for more chats.
>

Mostly visual to start with: can't afford all the imaging stuff at the moment.
There are a few suppliers of eq mounts I can use later on once I can afford the
photography side. Figured the Dobsonians are a good priced entry point for wide
aperture visual and can be used as a base for more advanced stuff.
Thanks, I'll definitely ping you later on.

On Sat, 24 Oct 2009 23:56:56 +1000, Noons <(E-Mail Removed)> wrote:

>Outing Trolls wrote,on my timestamp of 24/10/2009 9:24 PM:
>> On Sat, 24 Oct 2009 22:17:05 +1000, Noons <(E-Mail Removed)> wrote:
>>
>>> Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>>> eh?
>>
>> Inland is worse, but then you'd know this if you knew the least bit about
>> photography and astronomy. Another **** poor attempt of yours to try to
>> look like you knew something. Trolls never do.
>>
>
>What an idiot...

Inland *is* worse, because inland air is more unstable, has higher
fluctuations in humidity levels, often contains more dust and particulate
pollutants, and is downright turbulent compared to areas near ocean air.
All are enemies of "seeing" conditions. The weakest link when you have a
decent telescope optics.

Some of the most stable pristine skies can be found in less-inhabited
regions of places like Florida, where any part of the land is only a couple
hundred miles from either coastline. The skies deep in the Everglades for
example, easily rival the night-skies you will see in some remote national
forest at the very top of the Rocky Mountains. (Viewed and photographed the
night-skies at both, so I have first-hand experience with these locations
for night-sky seeing conditions.) Ocean water has generally laminar
air-flows, most of the pollutants have dropped out of the sky--any coming
from other land-masses when airflow direction is inland. The fluctuations
in humidity levels (a killer of air quality and seeing for astronomy), are
usually much more gradual when dealing with ocean air as opposed to inland
continental air.

This is why the most favored large telescope installations are built
furthest from large land masses, as high as possible (when possible), and
surrounded by or very near the largest bodies of ocean water with
prevailing inland air-flows. This is precisely why they choose the Hawaiian
Islands for some of the larger and more advanced observatories not too long
ago. The upcoming mega-telescopes now in construction are being built near
the ocean in places like the coastal deserts in Chile near the Pacific
shore.

http://en.wikipedia.org/wiki/Very_Large_Telescope

Since he is doing planetary imaging, light-pollution is not much of a
concern, unless he gets into the outer planets (which won't show much in a
telescope of that size anyway). Or if he'd be trying to do long exposures
in place like downtown Times-Square New-York City.

Had you said, "Pity you can't move all that gear to a coastal region
further from light pollution." Then you might have been perceived as having
an iota of credible experience with either subject, photography or
astronomy. Since you gave him the worst advice possible concerning this
field of interest, there's only one conclusion possible.

Did you learn anything today? You useless **** of an ignorant troll.

On Sat, 24 Oct 2009 09:29:35 -0500, Look! Another Troll!
<(E-Mail Removed)> wrote:

>On Sat, 24 Oct 2009 23:56:56 +1000, Noons <(E-Mail Removed)> wrote:
>
>>Outing Trolls wrote,on my timestamp of 24/10/2009 9:24 PM:
>>> On Sat, 24 Oct 2009 22:17:05 +1000, Noons <(E-Mail Removed)> wrote:
>>>
>>>> Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>>>> eh?
>>>
>>> Inland is worse, but then you'd know this if you knew the least bit about
>>> photography and astronomy. Another **** poor attempt of yours to try to
>>> look like you knew something. Trolls never do.
>>>
>>
>>What an idiot...
>
>Inland *is* worse, because inland air is more unstable, has higher
>fluctuations in humidity levels, often contains more dust and particulate
>pollutants, and is downright turbulent compared to areas near ocean air.
>All are enemies of "seeing" conditions. The weakest link when you have a
>decent telescope optics.
>
>Some of the most stable pristine skies can be found in less-inhabited
>regions of places like Florida, where any part of the land is only a couple
>hundred miles from either coastline. The skies deep in the Everglades for
>example, easily rival the night-skies you will see in some remote national
>forest at the very top of the Rocky Mountains. (Viewed and photographed the
>night-skies at both, so I have first-hand experience with these locations
>for night-sky seeing conditions.) Ocean water has generally laminar
>air-flows, most of the pollutants have dropped out of the sky--any coming
>from other land-masses when airflow direction is inland. The fluctuations
>in humidity levels (a killer of air quality and seeing for astronomy), are
>usually much more gradual when dealing with ocean air as opposed to inland
>continental air.
>
>This is why the most favored large telescope installations are built
>furthest from large land masses, as high as possible (when possible), and
>surrounded by or very near the largest bodies of ocean water with
>prevailing inland air-flows. This is precisely why they choose the Hawaiian
>Islands for some of the larger and more advanced observatories not too long
>ago. The upcoming mega-telescopes now in construction are being built near
>the ocean in places like the coastal deserts in Chile near the Pacific
>shore.
>
>http://en.wikipedia.org/wiki/Very_Large_Telescope
>
>Since he is doing planetary imaging, light-pollution is not much of a
>concern, unless he gets into the outer planets (which won't show much in a
>telescope of that size anyway). Or if he'd be trying to do long exposures
>in place like downtown Times-Square New-York City.
>
>Had you said, "Pity you can't move all that gear to a coastal region
>further from light pollution." Then you might have been perceived as having
>an iota of credible experience with either subject, photography or
>astronomy. Since you gave him the worst advice possible concerning this
>field of interest, there's only one conclusion possible.
>
>Did you learn anything today? You useless **** of an ignorant troll.

Ignorance is mutual.

Here is the lanscape not far from Oodnadata
http://www.flickr.com/photos/eric_th...69197/sizes/l/

Hardly ever any wind. Hardly ever any rain (those clouds are
exceptional). Hardly anyone or anything for many many miles in any
direction. Lovely flat terrain over which the air flow stabilises.
Oodnadata is not an entirely silly suggestion, except for the thought
of living there.



Eric Stevens

On Sat, 24 Oct 2009 09:29:35 -0500, Look! Another Troll!
<(E-Mail Removed)> wrote:

>On Sat, 24 Oct 2009 23:56:56 +1000, Noons <(E-Mail Removed)> wrote:
>
>>Outing Trolls wrote,on my timestamp of 24/10/2009 9:24 PM:
>>> On Sat, 24 Oct 2009 22:17:05 +1000, Noons <(E-Mail Removed)> wrote:
>>>
>>>> Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>>>> eh?
>>>
>>> Inland is worse, but then you'd know this if you knew the least bit about
>>> photography and astronomy. Another **** poor attempt of yours to try to
>>> look like you knew something. Trolls never do.
>>>
>>
>>What an idiot...
>
>Inland *is* worse, because inland air is more unstable, has higher
>fluctuations in humidity levels, often contains more dust and particulate
>pollutants, and is downright turbulent compared to areas near ocean air.
>All are enemies of "seeing" conditions. The weakest link when you have a
>decent telescope optics.
>
>Some of the most stable pristine skies can be found in less-inhabited
>regions of places like Florida, where any part of the land is only a couple
>hundred miles from either coastline. The skies deep in the Everglades for
>example, easily rival the night-skies you will see in some remote national
>forest at the very top of the Rocky Mountains. (Viewed and photographed the
>night-skies at both, so I have first-hand experience with these locations
>for night-sky seeing conditions.) Ocean water has generally laminar
>air-flows, most of the pollutants have dropped out of the sky--any coming
>from other land-masses when airflow direction is inland. The fluctuations
>in humidity levels (a killer of air quality and seeing for astronomy), are
>usually much more gradual when dealing with ocean air as opposed to inland
>continental air.
>
>This is why the most favored large telescope installations are built
>furthest from large land masses, as high as possible (when possible), and
>surrounded by or very near the largest bodies of ocean water with
>prevailing inland air-flows. This is precisely why they choose the Hawaiian
>Islands for some of the larger and more advanced observatories not too long
>ago. The upcoming mega-telescopes now in construction are being built near
>the ocean in places like the coastal deserts in Chile near the Pacific
>shore.
>
>http://en.wikipedia.org/wiki/Very_Large_Telescope
>
>Since he is doing planetary imaging, light-pollution is not much of a
>concern, unless he gets into the outer planets (which won't show much in a
>telescope of that size anyway). Or if he'd be trying to do long exposures
>in place like downtown Times-Square New-York City.
>
>Had you said, "Pity you can't move all that gear to a coastal region
>further from light pollution." Then you might have been perceived as having
>an iota of credible experience with either subject, photography or
>astronomy. Since you gave him the worst advice possible concerning this
>field of interest, there's only one conclusion possible.
>
>Did you learn anything today? You useless **** of an ignorant troll.

I should have give
http://farm4.static.flickr.com/3338/...93aa1704_b.jpg as the
URL in my previous post.



Eric Stevens

Look! Another Troll! wrote,on my timestamp of 25/10/2009 12:29 AM:

>>>> Goos stuff as usual, Piggo. Pity you can't move all that gear 300 miles inland,
>>>> eh?
>>> Inland is worse, but then you'd know this if you knew the least bit about
>>> photography and astronomy. Another **** poor attempt of yours to try to
>>> look like you knew something. Trolls never do.
>>>
>> What an idiot...
>
> Inland *is* worse, because inland air is more unstable, has higher
> fluctuations in humidity levels, often contains more dust and particulate
> pollutants, and is downright turbulent compared to areas near ocean air.
> All are enemies of "seeing" conditions. The weakest link when you have a
> decent telescope optics.


Moron. Inland Australia is an inhabited desert, you twerp.
He lives in Australia, not your precious backwater in Florida.
You really don't have a clue what you are talking about, do you?

(rest of your crap deleted, no point in wasting time)