| |
 |
|
|
Science Forum Index » Astro - Amateur Forum » Plossl ??
Page 1 of 1
|
| Author |
Message |
| Mark F. |
 Posted: Sun Apr 27, 2008 10:57 pm |
|
|
|
Guest
|
Thanks for your help with my question about my new scope.
So I have played with my 10" hardin now for a few nights.
I have a few obserations.
1. The X2 barlow may work on the moon but make mars look blury.
2. It looks like there is a light coating or haze on the primary mirror 10"
and none on my old 4.5 does the 10" need cleaning?
3. Rotaion bearing way to easy-- i will need to put felt down to stop this.
4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the upgrade
for a backyard duffus like me.
PS this is the kit the scope came with -- Celestron #94304. It is OK or
junk?
Mark F.
--
Fly Lexx! |
|
|
| Back to top |
|
| SkySea |
| Posted: Mon Apr 28, 2008 1:40 am |
|
|
|
Guest
|
Quote: On Mon, 28 Apr 2008 03:57:53 GMT, "Mark F." <res049nn@gte.net> wrote:
1. The X2 barlow may work on the moon but make mars look blury.
2. It looks like there is a light coating or haze on the primary mirror 10"
and none on my old 4.5 does the 10" need cleaning?
3. Rotaion bearing way to easy-- i will need to put felt down to stop this.
4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the upgrade
for a backyard duffus like me.
PS this is the kit the scope came with -- Celestron #94304. It is OK or
junk?
Mark F.
Pardon if I'm butting in...
1) It's typical that with more magnification, you magnify whatever
inherent blur exists. If you're lucky, it's just a night of bad
seeing. Less lucky, you're in an area that has a lot of local thermal
disturbance (like if you're observing from a large paved area), or
your optics may need cleaning or collimation. If you're very unlucky,
it's a poorly manufactured barlow or worst of all, something poorly
made in an optical component of the scope.
2) Sounds like your primary could use a good cleaning. Maybe the
secondary, too.
3) OK
4) It depends on the actual light path. If the smallest opening it
clears is 2" (so you'd be using a 2" to 1¼" adapter for your
eyepieces), then yes, a 2" eyepiece would help. If your light path
goes through a 1¼" constriction and you're thinking of geting a 1¼" to
2" adapter, then I'd be less inclined to think it would help much. It
would a bit, because the 2" lenses are (rule of thumb) higher quality
anyway. But you probably wouldn't gain much if anything by way of more
light. Please note, there are eyepieces that have both 1¼" and 2"
barrels (Televue 12mm Type II Nagler, e.g.), and in these cases a 2"
mount would not help.
Dunno for the Celestron #94304.
Good luck! |
|
|
| Back to top |
|
| Martin Brown |
| Posted: Mon Apr 28, 2008 3:45 am |
|
|
|
Guest
|
Mark F. wrote:
Quote: Thanks for your help with my question about my new scope.
So I have played with my 10" hardin now for a few nights.
I have a few obserations.
1. The X2 barlow may work on the moon but make mars look blury.
Moon is very high contrast and plenty of light.
Quote: 2. It looks like there is a light coating or haze on the primary mirror 10"
and none on my old 4.5 does the 10" need cleaning?
Unlikely. It is amazing how dirty a mirror can be and still give prefect
images. Cleaning it can risk surface damage and additional wear if done
improperly.
Quote: 3. Rotaion bearing way to easy-- i will need to put felt down to stop this.
4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the upgrade
for a backyard duffus like me.
It gets you a wider field of view for the same focal length eyepiece
assuming that the 2" exploits the larger diameter to full effect.
You are probably better off with a couple of decent 1.25" eyepieces to
start with unless you already know you want widest possible field for
deep sky and are prepared to pay the price.
Quote:
PS this is the kit the scope came with -- Celestron #94304. It is OK or
junk?
Probably a reasonable mid range eyepiece of around 1" focal length.
If you have a 2x Barlow it is worth avoiding eyepieces exactly 2x
ratio apart. A 17mm focal length eyepiece is often a good choice.
These days the better zoom eyepieces are worth considering too.
Regards,
Martin Brown
** Posted from http://www.teranews.com ** |
|
|
| Back to top |
|
| Mike Ruskai |
| Posted: Mon Apr 28, 2008 5:11 pm |
|
|
|
Guest
|
On or about Mon, 28 Apr 2008 03:57:53 GMT did "Mark F." <res049nn@gte.net>
dribble thusly:
Quote: Thanks for your help with my question about my new scope.
So I have played with my 10" hardin now for a few nights.
I have a few obserations.
1. The X2 barlow may work on the moon but make mars look blury.
If anything looks blurry, you're overmagnifying. The ceiling magnification
for a 10" scope is about 600x, but it's quite unlikely viewing conditions will
ever be good enough to support that. More reasonably, you're better off
staying at 300x or less. Assuming 1250mm as the focal length, that means you
shouldn't use less than a 4mm EP, or an 8mm EP with a 2x barlow.
It may look OK with the moon simply because it's a lot bigger, and there's
plenty of large-scale detail. Mars, however, is tiny.
Quote: 2. It looks like there is a light coating or haze on the primary mirror 10"
and none on my old 4.5 does the 10" need cleaning?
Unlikely. I'm assuming it's not something as obvious as dew. In general,
don't clean the mirror. As someone else pointed out, they can get quite
apparently filthy before you notice any impact on performance.
Quote: 4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the upgrade
for a backyard duffus like me.
In general, a 2" EP barrel simply allows a larger focal length without
reducing the apparent FOV. For a Plossl, that means if it's about 40mm or
higher, you'll want a 2" barrel, so the AFOV can stay around 50-60 degrees.
Quote: PS this is the kit the scope came with -- Celestron #94304. It is OK or
junk?
Probably OK or better. Very doubtful that it's junk.
If that 12.5mm EP is what you're trying with the barlow, that should give you
about 200x magnification. If that's blurry, the most likely culprit is bad
seeing. |
|
|
| Back to top |
|
| Mark F. |
| Posted: Mon Apr 28, 2008 10:53 pm |
|
|
|
Guest
|
Thank for your wise help and wisdom.
And getting me off the hook about cleaning the mirror. (grin)
Mark
"Mark F." <res049nn@gte.net> wrote in message
news:5XbRj.6156$E77.3509@trnddc05...
Quote: Thanks for your help with my question about my new scope.
So I have played with my 10" hardin now for a few nights.
I have a few obserations.
1. The X2 barlow may work on the moon but make mars look blury.
2. It looks like there is a light coating or haze on the primary mirror
10" and none on my old 4.5 does the 10" need cleaning?
3. Rotaion bearing way to easy-- i will need to put felt down to stop
this.
4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the
upgrade for a backyard duffus like me.
PS this is the kit the scope came with -- Celestron #94304. It is OK or
junk?
Mark F.
--
Fly Lexx!
|
|
|
| Back to top |
|
| David Weinshenker |
| Posted: Mon Apr 28, 2008 11:06 pm |
|
|
|
Guest
|
Mark F. wrote:
Quote: Thank for your wise help and wisdom.
And getting me off the hook about cleaning the mirror. (grin)
Mark
There are ways to clean the surface if needed, but it's best
to do so as little as possible. Frequent cleaning can eventually
degrade or wear away the mirror coating (some types are more
durable than others), and a bit of scattered dust doesn't really
cause too much trouble.
-dave w |
|
|
| Back to top |
|
| Andrew Smallshaw |
| Posted: Wed Apr 30, 2008 11:55 pm |
|
|
|
Guest
|
On 2008-04-29, Kickin' Ass and Takin' Names <PopUlist349@hotmail.com> wrote:
Quote:
Do not attempt to clean the primary mirror -- you will -- without fail
-- screw it up.
Leave it alone.
That seems a bit of an overgeneralisation. There are many people
who perfectly successfully clean their primaries and regard it as
nothing more than routine (if irregular) telescope maintenance.
I've done it myself. Sure you have to obey a few simple rules when
doing it but it is hardly a difficult operation.
--
Andrew Smallshaw
andrews@sdf.lonestar.org |
|
|
| Back to top |
|
| Chris L Peterson |
| Posted: Thu May 01, 2008 9:12 am |
|
|
|
Guest
|
On Mon, 28 Apr 2008 17:38:34 -0700 (PDT), "Kickin' Ass and Takin' Names"
<PopUlist349@hotmail.com> wrote:
Quote: Do not attempt to clean the primary mirror -- you will -- without fail
-- screw it up.
Hardly. Cleaning a mirror is no big deal. These days, nearly all have a
hard coating over the aluminum, and with a modicum of care (and use of
the correct cleaning materials) cleaning will not result in any damage.
This whole business about leaving the mirror alone and rarely if ever
cleaning it is nonsense. It is left over from days when mirrors were
more delicate, and is a message promulgated by telescope manufacturers
who don't want to deal with the results if their customers don't do it
right.
Mirrors have been damaged by contaminants left on them too long, and
critical observers can see the difference in a few percent loss of
light- something a thin layer of dust can result in.
There's no need to be fanatical about cleaning, but it's not a bad idea
to keep the primary relatively clean. Most Newts can use a cleaning
every year or two.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com |
|
|
| Back to top |
|
| Margo Schulter |
| Posted: Thu May 01, 2008 9:44 pm |
|
|
|
Guest
|
Mark F. <res049nn@gte.net> wrote:
Quote: 4. Does a 2" plossl do more for viewing then a 1.25. Is it worth the
upgrade for a backyard duffus like me.
Please let me agree with what someone said earlier in this thread: a 2"
eyepiece would be most useful if your telescope has a 2" focuser, in
which case you would use a 2"-to-1.25" adapter with the eyepieces you
have now. There's a point which I thought might be useful to clarify,
just to be sure that you're aware of the range of 2" options if you
have a 2" focuser.
A plossl or super-plossl is one type of eyepiece, which typically has
an apparent field of view or AFOV of about 50-55 degrees. This is fine.
As someone suggested, when people go in for 2" eyepieces, one reason
is to be able to have a wider AFOV than this (often somewhere from 65 to
82 degrees or so) at longer focal lengths, which together yield what we
call a wide true field of view, or TFOV. This means the ability to see
a larger portion of the sky in a single eyepiece field.
For example, I first saw the open cluster M25 in Sagittarius with a
25mm super-plossl having an AFOV of something like 52 degrees, if I'm
might. My scope is a SkyWatcher 200mm (7.9") f/6 Newtonian on a Dobsonian
mount, which means that the focal length is 1200mm. To find the
magnification, we divide the scope's focal length by the eyepiece focal
length: 1200mm/25mm, or 48X.
Now to find TFOV, a quick method is to divide the AFOV by the
magnification: 52/48, or a bit more than a degree. With this perfectly
good eyepiece M25 looked beautiful: I could about fit the cluster itself
into a single field.
Why do some of us enjoy wide-angle 2" eyepieces? -- although, as has been
said, you can do just fine with 1.25" eyepieces.
With my Garrett Optical 30mm Ultra-Wide, the AFOV is 82 degrees, giving
what has been called a "spacewalk" feeling. The magnification with my
scope is 40X (1200mm/30mm), so the TFOV is 82/40, or a tad more than
2 degrees. This "frames" the clusters with surrounding stars, another
view and a quite impressive one.
The obvious advantage of a big TFOV is that you can fit "extended
objects" like the Pleiades (M45) more or less in a single field. It
can also be easier to starhop directly through the eyepiece, moving
between nearby objects or locating an object once you're in the right
general vicinity looking only through the eyepiece, rather than shifting
between it and a finder of some kind. Also, with a scope like a typical
Dobsonian that doesn't automatically track, even a smaller object such
as a planet can be easier to track manually if you have a wider TFOV.
If you have a 2" focuser and do at some point consider the option of
a 2" eyepiece with a wide TFOV, one caution is that such eyepieces can
either be expensive (e.g. a Nagler), or have imperfections which can be
more or less of an issue depending on things like the focal ratio of
your scope. Thus my Garrett 30mm is quite acceptable to me at f/6, with
a bit of "field curvature" (focus changes somewhat as you move from the
center to the edge of the field); but at f/5 it might be a less ideal
compromise, let alone at f/4.5 or the like. The lower the f-ratio or
"faster" the scope becomes, the more critical these issues can become.
With a scope at f/8 or slower, someone might in contrast notice the
quirks of the eyepiece less than I do at f/6, all things being equal.
As long as we're looking at possible options for the future, I might
mention that there are also 1.25" eyepieces with wide AFOV's for
"picture window" or "spacewalk" effects. As has been remarked, the
special advantage of a 2" eyepiece with a wide AFOV is that the focal
length can get longer with the same AFOV -- so that the magnification
is less, and thus the TFOV greater, nice for those extended objects.
If you're curious, and give me the f-ratio of your 10" scope, we could
see what kind of TFOV you might be getting from your current Celestron
eyepieces, etc.
With many thanks,
Margo Schulter
mschulter@calweb.com |
|
|
| Back to top |
|
| |
|
Page 1 of 1
All times are GMT - 5 Hours
The time now is Tue May 13, 2008 2:50 pm
|
|
