Hi Pete,

>You should review Doc G's site on the limitations of the Schmidt thread
>adapter on your telescope. It only has a 1 1/2" internal diameter, so
>any eyepiece glass over that size is pretty much wasted on your setup.
>
>Pete P


I've been quantifying the use of 2" eyepieces for the 8" SCT over on sct-user
recently and your reference to Doc G's site is a good one to review on this
question. Reading it, I realized that even though I have had a 12" for eight
whole months now, and I have a had a 10" for more
than a dozen years, my viewing has been as limited as the 8" SCT is.

Ironically, someone was asking for a visual description of the view through an
8" with two inch eyepieces (compared with 1.25") and even though I have and
use and love my 2" eyepieces, it never occurred to me that it was an
equivalent view to that of the 8". I can say that you definitely improve with
2" over 1.25" and I'll get to an attempt at quantifying that below.

That of course will soon change. With the Eyeopener and a 2" drawtube
diagonal on the way, I will finally be able to see what I have been missing!

After reading Doc's page in the archives, I began to tinker with a slightly
different approach mathematically that may have more merit to understanding
this problem. The question is compounded because not every eyepiece's field
stop exceeds the limitations that are typically discussed.

That is, if an eyepiece only has a 10mm field stop, it does not matter whether
it is an 8", or 10-16" because its field stop is still smaller than the 38mm
of the 8" baffle/cassegrain exit hole. Please correct me if I am wrong here.

OTOH, if you look at the Meade 56mm eyepiece with it's 49mm field stop
(probably maximal for 2" accessories), the 2" diagonal will make a difference.
Since we are typically trying to maximize the true FOV, the tendency is to go
with longer focal length, wider AFOV eyepieces.

The 1.25" eyepiece will be similarly maximal for the best case 1.25"
comparison. I will use the maximum possible field stop of 31mm (31.75mm is
the barrel outer diameter). The other numbers are 38mm for the 8"
baffle/cassegrain exit hole diameter (inside the stand SCT rear-cell threads).
Finally, I'll use the 49mm given on Doc's page for the Meade 56mm eyepiece.

I have two ways of analyzing this numerically. The proportional field stop
sizes are:

8" SCT (or back) with 2": 38mm/31mm = 1.225 -> 22 1/2 percent increase

10"+ SCT w/o obstruction: 49mm/31mm = 1.581 -> 58 1/10 percent increase

Now, let's try to quantify the apparent views on the basis of these field stop
numbers. The basic formula for the area of a circle is pi*r*r
(pi*r-squared):

8" SCT with 1.25" eps: 3.14 * 15.5 * 15.5 = 754.385
8" SCT with 2" eps: 3.14 * 19.0 * 19.0 = 1133.54
10"+ SCT with 2" eps: 3.14 * 24.5 * 24.5 = 1884.785

so increase in viewing area through the eyepiece is:

8" SCT (or back) with 2": 1133.54/754.385 = 1.503 -> 50% increased area

10"+ SCT w/o obstruction: 1884.785/754.385 = 2.498 -> 150% increased area

Now, numbers may still be deceiving. This analysis does not take into account
for factors such as improved eye relief for 2" eyepieces, or the fact that you
can't magically snap your fingers and make your 8" SCT a 10"+ overnight.

I would not recommend the high-end accessories for the 8", such as the
eyepieces Bill lists below.

If it is not in your budget to move up from an 8", I would recommend buying
the standard Meade 2" diagonal (there could be more available
after reading this <G>), and go with 2" eyepieces under $200 or whatever
reasonable limit you set for yourself. *Most* of my eyepieces are under $200,
and several are under $100. The standard Meade 2" diagonal is
under $100.

It is a way to optimize a few more percentage points out of your telescope.
Ultimately, we are all responsible for deciding the value of the cost of such
optimizations. Hopefully, I've helped put some numbers to that intent.

If the numbers are easy to visualize, arrange for a side-by-side comparison.
We do that at our dark sky site all the time.

I am still learning about optical characteristics, so please let me know if I
have goofed somewhere.

Leroy


>-----Original Message-----
>Der, Bill
>ent: Wednesday, February 28, 2001 12:10 PM
>Subject: [M]: Wide FOV EP Reco
>
>
>I'm just starting out in this fascinating, yet highly addictive hobby.
>I got my LX200 8" just over a month ago and am updating to
>a 2" diagonal. I'm seriously considering either the Panoptic 27,
>Nagler 22 T4 or Nagler 31 T5 as low power, wide FOV EPs for
>the 8". Adequate eye relief is important since I wear glasses.
>
>I apologize if this has been hashed out before, but would appreciate
>any real life experiences. This group has been super as a source of
>info.
>
>Thanks in advance.
>
>
>


Leroy Guatney
12" #126548
N 39 39'/W 103 56'
http://home.rmi.net/~lwlg/SCI/astronomy.html

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