It seems much more complicated ( to me) than most people make the problem of the
hole in the back scope to be. I have a little C 90 that has only a 25 mm hole in
it back and I have no problem using an 40 mm eyepiece with a 46 mm stop with it
and a 2" diagonal. There might be some vignetting but it does not seem that
noticeable visually. I have also used the same eyepiece with a focal reducer in my
12" lx 200 and got about a 1.2 Deg FOV. that does not quite fill the actual FOV of
the eyepiece. The farther you get a large eyepiece from the smaller hole the less
effect it has (sort of). Lastly without breaking the bank a 40 mm Orion Optilux
($130) and a 2" meade diagonal ($100) will get you just about the widest actual
FOV without a focal reducer.
Bob

Leroy Guatney wrote:

> 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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>
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