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Subject: [M]: LX-200 with f6.3FR NGF-S CFW ST-7
From: W. Miller
Reply To: mapug@shore.net
Date: Fri Feb 28 00:00:45 1997

Rick, I have added the "WM:" areas to answer your questions.

>Wed, 26 Feb 1997 15:28 From: rickn@ucolick.org (R. Nolthenius)
Thanks for your quick response... I've looked again at your drawing
and at the Mar 94 S&T article on the NGF-S and am still having
trouble seeing what we'll be required to do...Since this is College
money and it already looks like we'll have to find a little extra
to get everything done, I don't want to spend until I know what's what.

Fri, 21 Feb 1997 15:54:58 rickn@ucolick.org (R. Nolthenius) wrote:
> Has anyone been able to get the following arrangement of
> equipment to work successfully together?
> LX200 + (f6.3 focal reducer or MaxField) + JMI NGF-S focuser +
> Murnaghan filterwheel/flip mirror + SBIG ST-7 ?

Rick, I haven't combined all the above pieces, but I have done some
calculations to see how it might be done. Presently I am using the
following which gives me very good results at fl=1865mm, 8.5x12.7arcmin:
12in LX200, f6.3 focal reducer, 911N nebular filter, NGF-S, and ST-7.
My previous setup was fl=3048mm, 5.2x7.8arcmin:
12in LX200, NGF-S, CFW-6A, ST-7.
Even with this I did not need a flip mirror. The 8x50 finder scope works
just fine to center the first object for the night and then I sync TheSky
and move indoors and slew at the computer. I didn't like always having
the IR blocking filter (requried for leaks in those dichroic color filters)
in series between the CFW-6A and ST-7 since the ST-7 is sensitive to near
infrared. Why throw away all that IR information?

RN: I don't know of any commercially available affordable filters which
do not have IR leak which must be plugged...do you??

WM: My first thought was to buy a UBVRI set. Next I thought about
buying a BG-23, IR exclusion, and an IR pass filter to make tri
color that is spectrally shifted. The IR exclusion bandpass overlaps
the BG-23 so I would have to subtract these to get my "green". This
is a whole new subject. If anyone has some ideas I would be interested.
I'll get around to buying this combination from Murnaghan soon.

Here are my notes on optimum setup for a 12in LX200 f/10,
focal reducer (Meade), Crayford focuser (NGF-S), color wheel (CFW-6A),
and ST-7.

It is best to place the color wheel close to the ST-7. This prevents
vignetting due to the clear aperature of the filter being only 26mm.
Also distortion due to nonflatness or taper of the filter glass
would be minimized when the filter is close to the CCD.

Vignetting could be caused by insufficient clear aperture of either the
focal reducer or the filter. SBIG web pages show the layout of the
ST-7 CCD chips. Note that the tracking chip is offset from center.
I calculated the circle diameter that includes both chips to be 14.9mm.
My formulas for onset of vignetting:
A = Aperture = C + D(B-C)/F
B = Scope diameter 304.8mm 12in
C = CCD pair diameter ST-7 14.9mm track+image, 8.3mm image
F = Focal length 3048mm f/10, 1920mm f/6.3, 1545mm f/5.07
D = Distance CCD to aperture

Meade focal reducer has a focal length of 230mm. For the reduction to be
from f/10 to f/6.3 it must be positioned 85mm = 230(1-0.63) from the CCD.
Mag=0.63=1-(85/230). I assume it's design is optimized for this placement.

Setup "A"
Just simply screw the pieces together to get dimensions in mm:
FR NGF-S CFW-6A ST-7
|------50.5+23.5--|--16+23--| Mag=0.509=1-(113/230)
36.1 22.2 14.9
There is no vigneting due to either
Focal reducer dia 41mm > 36.1=14.9+113(304.8-14.9)/1551
Color filter dia 26mm > 22.2=14.9+ 39(304.8-14.9)/1551
I don't know if my scope has enough focus range to move it's primary mirror
this far toward it's secondary. This is like focusing on a close object and
spherical aberration will grow, p87, Telescope Optics, Rutten and Venroij.

Setup "B"
Mount the CFW-6A onto the ST7, drill and countersink 2 holes in the CFW-6A.
FR NGF-S CFW-6A ST-7
|------50.5+23.5--|--14+17--| Mag=0.530=1-(108/230)
34.3 20.5 14.9 34.3=14.9+108(304.8-14.9)/1617

RN: OK...from your last note, setup "A" is a non-starter. I assume
setup "B" is too, as it moves the FR closer to the CCD by only a little.
Right?

WM: You might try these especially if you have a CFW-8 which mounts closer
to the ST-7. Just screw the pieces together.

Setup "C"
Mount FR lens onto front of a 2in nosepiece of the CFW-6A. The f6.3FR lens
measures dia=44.4mm, clear=41mm, thickness=19.5mm. Fit into ID=47.8mm threads
for std 2" filter (clear=44mm). Black paper 1.9mm=(47.8-44)/2 fills diameter.
Use split rings made of 2in black plastic pipe to position lens.

<------- 42mm -------->
<5 x 18.5 x 19.5 x 4> < > x ^ v are arrowhead symbols
|---------------------| ^
---| |=========|--|
------------| | Drawing is flipped left to right
| | Front of scope this direction -->
| | This thing is a 2in diameter cylinder
T thread | lens | 2in OD that's 42mm long with a male T thread
male 42mm | d=44.4 | 51mm sticking out of one end.
| | The entire 42mm fits inside the NFG-S.
| | Build 6.5mm longer (48.5mm) to get f/6.3
------------| | and to give clearance when focuser is in.
---| |=========|--|
|---------------------| v

Here is the resulting setup distances and cone of light diameters:
NGF-S FR ..CFW-6A. .ST-7 27.7=14.9+85(304.8-14.9)/1920
|--------18.5+6.5+21--|--16+23--| 20.8=14.9+39(304.8-14.9)/1920
27.7 20.8 14.9

The last design causes less movement of the primary mirror. It's light cone
diameter at the FR is 6.6mm=34.3-27.7 smaller which is like being 66mm down
an f/10 but its physical placement is only 31.5mm=50-18.5 further down. Also
focus movement is less sensitive since it changes only f/10 dimensions.

Setup "D"
Purchase a filter wheel for 2" filters. Custom order filters with 1/4
wavelength surfaces. The loaded filter wheel would cost at least $2000:
12in LX200, filter wheel, f6.3 focal reducer, NGF-S, and ST-7.

Bill Miller, Los Gatos, CA

RN: I don't understand what setup "D" will solve... won't I still have the
problem of the FR being too far from the chip? Also, I think you're saying
that I'd need 2" filters to avoid vignetting with the filterwheel
so close to the secondary? We've only got $1400 for the loaded filterwheel.

WM: The filter wheel would go in front of, instead of inside the NGF-S.

RN: Setup "C" sounds like the only reasonable solution (we have $0 for
a MaxField right now, and we already have on hand the f6.3 FR, which
sounds like the right scale for the galaxies and clusters I'm most
interested in imaging). So. Let me try and understand setup "C"...

1. What "nosepiece" are you talking about for the filterwheel?
"Build 6.5mm longer to get f6.3"...build what 6.5mm longer? Is this
a nosepiece we'll have to hire a machine shop to make?
If so, this sounds like a jump up-and-down business opportunity for
someone with a modest machine shop!!!

WM: I wish I could just go out and buy a f6.3FR that is built inside
a 2 inch OD barrel. I understand the Optrex f3.3FR is available like
this, but then it can not be used with a color filter wheel, and
it requires a yellow filter. I don't want to cut out the blue and
green colors.

2. Does the nosepiece fit rigidly onto the filterwheel while the
NGF-S surrounds it without touching?

WM: My CFW-6A came with a 2 inch nosepiece which screws into it with
42mm T threads. The 2 inch OD nosepiece is held into the NGF-S with
2 set screws. Perhaps you are confused by the orientation of my drawing
since it is left right reversed. The LX-200 is on the right side.
I've added some notes onto my drawing above to clarify it.

3. I think I misunderstood the NGF-S before...there's only the 2" size,
not a 1 1/4" size; but you can put in an adapter to fit 1 1/4" right?

WM: Yes, but you would not use the 1 1/4" adapter for this setup.

4. So the black plastic rings, cut so they could compress and make a
snug fit, would be sufficient to position the FR exactly perpendicular
to the optical path (I'm not sure what kind of tolerance the chip
has, but I assume it's tight).

5. Did I understand the article right... the NGF-S mates onto the
back of the scope, and also comes with a mounting which makes the
downstream end of the NGF-S look like the back of the scope to
any other equipment, and that's standard equipment?

WM: Yes, but I don't use this piece on the downstream end.

6. The "T-thread male" at the left edge of your figure; is this
what's at the end of the NGF-S? I see two 42mm on your figure;
is the left one the diameter of the T-Thread?

WM: The NGF-S is 2 inch inside diameter. I use it this way.

Thanks again for your help... I'm hoping to get student labs doig
REAL astronomy by next semester! And have fun taking need pix for
myself and the Cabrillo Astronomy club to boot. BTW, we're having
a big starparty/lunar eclipse/comet bash party on March 23; you're
welcome to "Bopp" over the hill and join us!

Sincerely, Rick