Posts Made By: K. Michael Malolepszy

As someone who has "big guns" (11 to 20 inch dobs), I can tell you from first hand experience that a good wide field refractor view can still be a real joy even after you've gotten used to lots of aperture.

I still get a real kick out of my 4" Vixen Flourite - which also does great on planets. Right out of the box without collimating it after being shipped to me in August '03, the first time I pointed it at Mars I was able to immediately tell visually that there was some heightened dust activity in/near Hellas.

K. Michael M.

Assuming XT10 has a focal length of 49.4 inch (1,255mm) (f/4.9) and assuming you want to use a 4mm exit pupil for nebula and using

exit pupil for telescope = focal length of eyepiece / focal ratio of telescope

Solve for Exit Pupil:

Exit Pupil= Ocular focal length / 4.9

So, for a desired 4mm exit pupil:

Ocular focal length =4mm*4.9 = 19.6mm

So, a 20mm eyepiece of some sort will work. If you have the money, a 20mm Nagler is as good as it gets (I've read the type 5 are better suited for f/5 and faster scopes than type 4 Naglers - the 22mm Nagler T4 is said to show a bit of coma in scopes as fast as yours, and it's much heavier than the 20mm T5). A 22mm Orion/Vixen LVW, 19mm or 22mm Panoptics may work too, or the Pentax 20mm XW.

Sometimes you may want to decrease the exit pupil to 2mm for deep sky objects. If so (for desired 2mm exit pupil);

Ocular focal length = 2mm*4.9 = 9.8mm.

So an eyepiece around 10mm is what you'd need. A 9 or 11 mm Nagler will work, or a 10mm Pentax XW or 9 or 13 Orion/Vixen LVW (I really like the 13mm LVW).

I don't know how well these eyepieces work for a f/4.9 scope, though. You may need a paracorr if you want pinpoint stars to the edge of your field of view. I do know that my f/5.4 11" newt works very well with all my Naglers (22 T4, 17 T4, 12 T4) and Panoptics. On my f/4.5 16" newt, I usually use a paracorr, though it's not really necessary - how much coma you want to put up with is a matter of taste.

Try before you buy, if you can. If not, used high end eyepieces on astromart can usually be resold for close to what you bought them for used if they are in good shape.

See

An Eyepiece Primer ‑ by Al Nagler

http://www.skyandtelescope.com/equipment/basics/3311076.html

Exit pupils Article at

http://www.astronomics.com/main/category.asp/catalog_name/Astronomics/category_name/J34KLLT872239NV4HBW7FSRMB1/Page/1

or http://tinyurl.com/yv6c96

Harry Vanderburg said:

Hi, I want to buy a 2 inch eyepiece for an Orion Xt10 Dob for Nebula and other dso's etc. What do you recommend is the best all around starter 2" eyepiece for the first upgrade?

As usual Dave, thanks for posting your DSO list.

What criteria do you use when making up these lists?

K. Michael M.

Dave Mitsky said:

Here are my DSO lists for this month.

Dave Mitsky

Seventy-five deep-sky objects for October: NGC 7640, NGC 7662, NGC 7686 (Andromeda); NGC 7180, NGC 7183, NGC 7184, NGC 7293, NGC 7392, NGC 7585, NGC 7606, NGC 7721, NGC 7723, NGC 7727 (Aquarius); Cz43, K12, M52, NGC 7635, NGC 7788, NGC 7789, NGC 7790, St12 (Cassiopeia); B171, B173-4, IC 1454, IC 1470, K10, Mrk50, NGC 7235, NGC 7261, NGC 7354, NGC 7380, NGC 7419, NGC 7510 (Cepheus); IC 1434, IC 5217, NGC 7209, NGC 7223, NGC 7243, NGC 7245 (Lacerta); NGC 7177, NGC 7217, NGC 7320 (the brightest galaxy in Stephan's Quintet), NGC 7331, NGC 7332, NGC 7339, NGC 7448, NGC 7454, NGC 7479, NGC 7619 (the brightest member of Pegasus I), NGC 7626, NGC 7678, NGC 7742, NGC 7769 (Pegasus); NGC 7541, NGC 7562, NGC 7611 (Pisces); IC 5156, IC 5269, IC 5271, NGC 7172, NGC 7173, NGC 7174, NGC 7176, NGC 7201, NGC 7203, NGC 7214, NGC 7221, NGC 7229, NGC 7314, NGC 7361 (Piscis Austrinus); NGC 7507, NGC 7513, NGC 7713, NGC 7755, NGC 7793 (Sculptor)

Top ten binocular deep-sky objects for October: M52, NGC 7209, NGC 7235, NGC 7243, NGC 7293, NGC 7510, NGC 7686, NGC 7789, NGC 7790, St12

Top ten deep-sky objects for October: K12, M52, NGC 7209, NGC 7293, NGC 7331, NGC 7332, NGC 7339, NGC 7640, NGC 7662, NGC 7789

Challenge deep-sky object for October: Jones 1 (PK104-29.1) (Pegasus)

The objects listed above are located between 22:00 and 24:00 hours of right ascension.

Unfortunately, wide-field eyepieces that work well with f/4.7 scopes are gonna cost at least a little bit of money. If you can find a used 13mm or 21mm Orion Stratus, that may work for you. You can find them on Astromart for $75-80. Comments on excelsis seem to indicate these work well with Orion XT10 scopes:

http://www.excelsis.com/1.0/entry.php?sectionid=22&entryid=629

http://www.excelsis.com/1.0/entry.php?sectionid=22&entryid=648

The 21mm will put your exit pupil right at 4.46mm.

The 17mm will give you 3.47 mm, the 13mm 2.76 mm

Harry Vanderburg said:

I tried my GSO 30 in my XT10i and loved it. Only negative thing is that it doesn't have enough focus and I must lift the ep. Nebulas looked nice in it. What eyepiece should I get so I could see star cluster's and galaxy's better? I was guessing around a 15mm for a 1200mm scope? At higher magnification, will a wide angle work good or am I better of just buying plossls(I can't spend too much)? Will any inexpensive wide angle(like the 1.25" 15mm GSO wv) in a fast scope(4.7) be ok or will I still run into distortion problems similar to the cheaper 2" 30ish wide angles?

From Spaceweather.com (10/24/07):

HOT NEWS: Astronomers in Japan, Persia and Europe report that Comet 17P/Holmes is undergoing a spectacular eruption. The 17th magnitude comet has brightened by a factor of five hundred thousand or more during the past 24 hours becoming a naked-eye object in the evening sky. Look for a yellow 2.5th magnitude fuzzball in the constellation Perseus after sunset. [sky map] [ephemeris] [3D orbit]

"This is unbelievable!" says Iranian astronomer Babak Tafreshi. "I was amazed to find Comet Holmes so easily with the naked-eye in the light-polluted skies of metropolitan Tehran." Click on Tafreshi's photo to pinpoint the comet:

The outburst may signify a breakup of the comet's core or a rich vein of ice suddenly exposed to sunlight--no one knows. At present the comet looks more like a star than a comet; it does not have a discernable tail, but it might grow one as the outburst continues. Amateur astronomers are encouraged to monitor developments using binoculars and backyard telescopes. Stay tuned for updates!

Charts at http://www.cometchasing.skyhound.com/

17P/Holmes: An all-night comet visible to the naked eye

This comet is in super outburst, having brightened by many magnitudes on the night of October 24. It is currently (as of 17:00 UT October 24) in Perseus at magnitude 2.8 (and brightening!). Look for a nearly stellar coma in the telescope. It appears as a new star in Perseus to the naked eye. How long this outburst will last is anyone's guess, but according to Gary W. Kronk's Cometography 17P was discovered during a similar outburst in November of 1892. During that outburst it was a naked-eye object for several weeks.

17P is in naked-eye super-outburst! On October 24 this comet brightened to naked-eye visibility! This comet is currently well placed for northern observers in Perseus. It appears very star-like with a reported DC of 9. As of 17:00 UT Seiichi Yoshida (Japan) reports it has brightened to magnitude 2.8. See below for more info and a finder chart.

http://www.cometchasing.skyhound.com/comets/17P.gif


-------- Original Message --------
Subject: Re: [AAVSO-DIS] [cvnet-outburst] Comet Holmes is brilliant object in Perseus
Date: Wed, 24 Oct 2007 20:38:17 +0100
From: Keith Geary
To: , ,
"Baavss"
CC: vsnet alert
References:

Wow!!!!! This is the single most strangest comet I have ever seen in my Life, earlier today, an alert went out that this comet had suddenly brighened from a predicted magnitude 17 to magnitude 3 - incredible, I wonder how this has happened? I quickly set up my 15x70mm binoculars and I easily found it in the constellation Perseus, just about 5 degrees from Alpha Persei at magnitude 1.8, also the star Delta Persei is at magnitude 3.0, from these two stars, I can see the comet at about magnitude 2.5 !!!!! I could immediately see that it is very much yellow in colour, just like the planet Saturn, and it is definitely non star like, with a slighty elongated shape east-west, with no sign of a tail, especially since the near full moon is shining brightly.

This can easily be mistaken for a nova, and is easily visible with the naked eye ! Here is a quick shot that I took.....this will grab many headlines over the next few days...

Keith....

http://farm3.static.flickr.com/2277/1731958125_87c03db8e8_o.jpg

----- Original Message -----
From: cnj999
To: [email protected]
Sent: Wednesday, October 24, 2007 7:25 PM
Subject: [cvnet-outburst] Comet Holmes is brilliant object in Perseus


It is perhaps appropriate to warning observers that the 2nd magnitude object visible in northern Perseus is not in fact a new nova but rather Periodic Comet Holmes, which is undergoing an enormous photometric outburst. Undoubtedly, there will be multiple reports of it as a nova (since it appears stellar) as some unsuspecting European observers spot it dramatically altering Perseus' appearance when darkness falls over the continent.

JBortle

Comet 17/P Holmes is now an *easy* object to see - even from the city.

I can see it naked eye with a nearly full moon in the sky from my heavily light polluted urban location. At first I couldn't believe my eyes - Perseus has an extra star! Then I used 10X50 binoculars and even while hand holding them, I could tell it wasn't quite stellar in
appearance. I got a 3" refractor out and saw what reminded me of a very, very bright planetary nebular such as the Eskimo or Blue Snowball (NGC 2392 or NGC 7662, respectively ), except MUCH brighter and more detailed - with an off-white color in the 3". Just to confirm
that it indeed it bright enough to be seen with the naked eye, I used a green laser to point to it while my girlfriend looked at it through the 3" and saw the laser in the field of view. Yes, it is bright enough to see naked eye under horrible conditions!

Of course, I had to get something bigger on it and setup an 11" dob and .... WOW! I could see it had a very bright, almost stellar pseudo-nucleus, a very bright surrounding coma, and a noticeably dimmer (but still very bright!) outer hood, with no real hint of a tail. A slight elongation of the inner coma could be seen once it got higher in the sky. I see it with a very, very slight tinge of yellow, others see it a more distinct yellow hue.

To find it, see the finder charts ("skymaps") at
http://www.spaceweather.com/

Semi-dark sky naked eye view is similar to this:

http://www.aerith.net/pictures/aguirre/IMG_0957procesada.JPG

This is what kind of view a tripod mounted binocular or small scope will show:

http://www.spaceweather.com/comets/holmes/24oct07/becky-ramotowski-Comet4930_1193289142.jpg?PHPSESSID=36i648fmr2q56klfs70snl56f4
http://tinyurl.com/ynlbxh

Telescope views of this object very much like this Oct. 24, 2007 image by R Richins:

http://www.aerith.net/pictures/richins/Comet_Holmes.jpg

A medium to large sized scope at higher magnifications will show something very much like this view of it as seen by Eric Allen who recorded this view through a 16-inch telescope at the Observatoire du Cégep de Trois-Rivières in Quebec:

http://www.spaceweather.com/comets/holmes/24oct07/Eric-Allen1.jpg?PHPSESSID=36i648fmr2q56klfs70snl56f4
http://tinyurl.com/25gte7

Gary Kronk page on 17P/Holmes:

http://cometography.com/pcomets/017p.html

Wikipedia page on it:

http://en.wikipedia.org/wiki/17P/Holmes

On Tuesday night (Oct. 30), I was able to see it's coma naked eye. I've been able to see it as a "star" since the 24th, but now it's coma is large enough that it is unmistakable the naked eye, even my heavily light polluted urban sky.

K. Michael M.

Dave Mitsky said:

The periodic comet 17P/Holmes has undergone a major outburst and has increased 14 magnitudes in brightness to naked-eye visibility. It is currently near Alpha Persei.

http://www.aerith.net/comet/weekly/current.html

Dave Mitsky

Yes, while Comet Holmes has been getting all the attention,
I've been wondering what kind of display LONEOS is putting on,
especially after reading the "Oct 26/27th 2007 -
Evening Tail Surprise!" sketches an Irish amateur
Martin Mckenna made of comet C/2007 F1 LONEOS posted
at

My C/2007 F1 LONEOS Observations

http://www.nightskyhunter.com/My%202007%20F1%20LONEOS%20Sesions.html

Mckenna writes:

"...On this crisp evening I decided to focus my
attention on C/2007 F1 LONEOS which is due to vanish
from UK skies on Oct 27th. I have not seen the comet
for some time and despite several evenings of
unsuccessful attempts due to poor sky conditions I had
considered the comet gone. This evening while
conducting a vertical search through the western
twilight with a 8.5" F/7 reflector with 32mm I swept
up LONEOS at 19.34 BST. The coma was of moderate size
and well condensed, circular in shape, grey-white in
colour with bright disk-like central condensation. The
coma was viewed through gaps between distant tree
branches which shared the telescopic field. What
caught my attention was the wonderful colourless
straight Ion tail extending to the E of N for a
minimum distance of 2 degrees although I suspected it
to be much longer!. I followed the tail across the sky
with the telescope and could see many streamers. The
tail was very bright and looked like a beam of light.

After several minutes the coma set leaving a wonderful
search light beam extending up from the tree tops!. It
looked like a minature 'Great Comet'!. Also seen in
B50. This is a very beautiful comet and one which I
think is currently under observed by many people which
is a shame. I had LONEOS in view for 9 minutes before
it set but before it did I glanced to the NE sky and
seen 17P/ Holmes with the naked eye - two bright
comets visible at the same time!. My 11th observation.

Mag: 5.0 Dia: 3' D.C:7 Tail: + 2 degrees. Two
sketches. One original and another inverted."

There's a good image of it with a background galaxy
at:

http://www.aerith.net/pictures/jager/2007F120071008lrgbnet.jpg

Orbit simulation at:

http://ssd.jpl.nasa.gov/sbdb.cgi?orb=1;sstr=2007+F1

Chart at

http://www.cometchasing.skyhound.com/comets/2007_F1.gif

but it's getting close to being out of date.
Heavens-above has finder charts at

http://www.heavens-above.com/

Mike

David Brokofsky said:

Hi all,

With all the commotion over 17p/Hlolmes, I was wondering if anyone has had a look at any additional comets over the last week or so? I hear that C/2007 F1 Loneos is pretty bright assuming one can catch it before it sets shortly after sunset. I haven't had a look as the W. horizon from where I'm at is pretty cluttered. Anyone else?

The important questions are:

Is this real?

If so, does the process produce more energy than it consumes? If not, it is nothing more than a curiosity at this point.

I think the maximum FOV of a scope is usually calculated with the same eyepiece FOV calculation that you mentioned, assuming a 7mm exit pupil - the average maximum for most peoples eyes. Refracting telescopes have no upper limits on exit pupil sizes due to central obstruction issues.

To answer the obvious question, what if you don't care about exit pupil size? from

http://www.mapug-astronomy.net/astrodesigns/MAPUG/MaxField.htm :


"... The actual field of view of the sky is determined only by the focal length of the telescope and the diameter of the most restrictive field stop in the optical system. The field stop is usually at or near the real aerial image position in the system. For a telescope this is just outside the back plate of the telescope at a position called the back focus focal plane.

The ultimate field stop is the hole in the back plane of the telescope. The practical field stop is that of the eyepiece. The size of this field stop is limited by the diameter of the eyepiece tube. On a given telescope a 2 inch eyepiece has greater maximum field of view of the sky than a 1 1/4 inch because it can intercept more of the real aerial image that the telescope objective produces.

The purpose of the eyepiece is similar to that of a fancy magnifying glass. It enables the human eye to inspect the real aerial image in fine detail. The focal length of the eyepiece determines the magnifying power of the whole system. A so called wide field eyepiece is designed in such a way that for a given focal length of the eyepiece the field stop can be made larger. The field stop can never be larger than the diameter of the eyepiece tube. (obviously) The larger diameter field stop of the wide field eyepiece actually intercepts a larger portion of the real aerial image and thus gives a view of a larger circle of the sky. It also gives what is called a larger apparent field of view. This is the size of the circle seen when one looks into the eyepiece."

------

K. Michael M.

Michael Tencza said:

I know how to calculate the FOV for an eyepiece but every so often I see a written specification for a telescope for a maximum FOV. How is this determined? By the eyepiece calculation I get 4.95 degrees with my Paragon 40mm in my 80mm f/550mm refractor. Am I actually seeing this wide a FOV or something less?