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Re: [APML]: 2X converter
The Astro-Photography Mailing List
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Greg Mueller wrote:
>
> I have the 4" focuser. The 2x converter I was referring to is the one
> Pentax sells for use between their lenses and the camera body. I figure,
> if it doesn't create any optical problems it would attach quite nicely
> as the field flattener has the "rear end" of a Pentax lens built onto
> it. All you'd have to do is unhitch the body put in the 2x converter and
> hook the body back on.
Chuck Vaughn wrote:
> > How much difference would there be between a 155mm f/7, and a 130mm f/8?
>
> This is an interesting question. Below is a comparison of the three
> popular AP scopes, the 130-f/8, the 155-f/7 and the 178-f/7. The
> differences are not as dramatic as you might think. See notes below.
>
> Scope F.L.(mm) 12um Star Size Exposure Magnitude Focuser(in)/
> (arcseconds) Factor(1) Increase(2) Field Deg.
>
> 130-f/8 1040 2.4" 1.3 0.0 2.7/3.78
>
> 155-f/7 1085 2.3" 1.0 0.38 2.7/3.62
>
> 178-f/7 1246 2.0" 1.0 0.68 4.0/4.67
>
> (1) The reference is f7.
> (2) Calculated on increase of aperture area compared to 130-f/8.
>
> Makes we wonder why AP makes a 6" scope.
I tried the Pentax 1.4x TC on my AP 5" f8 EDT (1040mm FL) for the last
solar eclipse. Note the 5" f8 is pretty similar to your 6" f7, in terms
of FL & f-ratio.
After shooting a bracketed-series @ prime-focus on P6x7 body #1 (to
get around 3 diameters of corona), I detached it & inserted P6x7 body #2
+ 1.4x TC (to get 2 diameters of corona). Note that I had a f8
field-flattener at the end of the focuser. On the REAL long exposures
(for outer corona), ~1-4 sec, I noticed ghost-images of the bright solar
limb (caused by internal reflections of the additional glass in the 1.4x
TC). On shorter exposures, the images were "artifact-free". The
negatives seemed did not seem to suffer from additional vignetting. (at
prime-focus & w/1.4x, the corners are chopped off a bit, due to the 2.7"
focuser). See the 2nd corona image at
http://www.comet-track.com/eclipse/secl98/secl98.html
<Can you *tell* there was a 1.4x TC in the lightpath? I can't> As a
matter of fact, ALL of the diamond-ring shots & the 3rd Corona image on
my eclipse page had a *high quality* 2x TC in the light path (Nikon
TC301, which has a field-flattener in the rear). <Can you *tell* there
was a 1.4x TC in the lightpath? I can't> Again, *only* on the REAL long
exposures (for outer-corona), I began to notice some artifacts.
*****[ Note on 2X TCs for astrophotography ]:
Mr. Kitahara uses a 2x (Nikon TC301) & 1.4x (I'm guessing Nikon TC14B?):
http://www.janis.or.jp/users/kitahara/ep-camera.jpg
This is why his images look so *GOOD*, because of the image-scale
_increase_ with the 1.4x/2x TCs. (his prime-focus system is 16" f6
~2400mm, which yields a pretty high FL & "manageable" f6 f-ratio,
allowing "reasonable" use of TCs). Shigemi Numazawa used a Pentax
100EDHF (4" f7 ~700mm) refractor+2X TC+Hassy201F for the last solar
eclipse. I wonder if he used the Pentax 6x7 2X TC, or was it Hassy?
I'm guessing he used a Hassy body, for the convenience of a motorized
back (to *efficiently* do a set of bracketed exposures). BTW, for last
eclipse, my ex-JPL optical engineer friend set someone up with a
motorized Hassy for his Brandon 5" f8 (Christen triplet lens. BTW, his
Ronchi tests indicated *poor* visual performer..)
*Note: the TC301 prices out at $549 (new), it is NOT cheap! (however, it
is WORTH it)
Chuck Vaughn uses a *matched* 1.4x TC on his Olympus 350/2.8 lens.
I've examined his "500mm f4" shots on TP2415, & they are REALLY GOOD (I
CANNOT detect any aberrations or vignetting).
For the '94 annular eclipse, using my AP 4" f6 (610mm FL) Traveler, I
used a AP 2x photo-visual barlow + extender, which made it ~3x. I.e.,
1800mm, to make the sun's image fill the 35mm frame. See
http://www.comet-track.com/eclipse/secl94/secl94.html
For the last eclipse, I had a 70mm f8 (560mm FL) fluorite refractor,
with a *custom-made* 3X TC (a 2x TC of simple design, with an macro
extension tube). I.e., a ~1700mm FL scope for _closeups_ of the solar
limb (2nd/3rd contact & during totality)
In fact, I considered taking the 4" f6 (610mm FL) + 1.4X TC (instead of
the bulkier 5" f8) to the eclipse for 3D of outer corona (854mm FL). I
even considered getting a P6x7 2X TC for 2D of outer corona (1220mm
FL). This would have been analogous to what S. Numazawa did (4" f7
Pentax refractor + 2x TC).
CONCLUSION: "Quality" astrophotographers use "quality" TCs
(Mr. Kitahara, S. Numazawa, C. Vaughn, Space Boy,...)
COMMENT: Why aren't you?
****[ Sorry about getting off-track ]****
To address your query on using the Pentax 2X TC, try it with your AP 6"
f7 & see what happens (f14!! on PPF400). (@f12 with 2x TC, Mr. Kitahara
goes ~5hr exposures on Super G400 film, as Chuck Vaughn exclaimed: "Did
you see those exposure times??!!") Your ST4 now becomes essential. If
there is a REALLY bright star in your field, you might get a ghost
image. The question is off-axis star images & vignetting.
I think you're query is addressing an *important* issue, and that is
*achieving *FOCAL LENGTH*. Typically, you do this by increasing scope
aperture: it will _increase_ FL & _increase_ photograpic efficiency
(manageable f-ratios): more image scale, better detail, "better looking
pictures" (there is an upper-bound, determined by seeing & film-grain).
However, why NOT use a *quality* 1.4x or 2X TC? Martin Germano states
on his homepage that he STILL had a use for his 8" f10 SCT (2000mm FL),
i.e. he knows the value of *focal length*. (in the "good-ole days" of
manual-guiding, he would be one of the few who could routinely do 2-3 hr
exposures. He & Mr. Kitahara are in the same "league"). He should try
using a *quality* 2x TC on his 14" f5, i.e. 14" f10 (3500mm FL), since
he LIKES challenging narrow-angle objects. He now has an ST4, so not a
big deal.. The *same* recommendation goes for everyone with scope
f-ratios ~5 or 6: Televue Genesis 4" f5, AP 4" f6, fast Newtonians, etc.
Side Note:
I did _simultaneous_ deep-sky shots @prime-focus with the AP 5" f8 EDT
& AP 4" f6 EDT (aka "Traveler") last week, & noticed significantly
*MORE* corner vignetting with the 6x7 negs for the 5". Exposure time
was 60 min on (unhypered) PPF. The 5" negs looked "thinner" than the 4"
negs (which were well exposed), so I think I might *try* a 60-90 min
shot using the 4" f6+FF+1.4x. This would effectively be a 854mm f8.4,
which would compete with the 5" f8 (1040mm f8). It would be interesting
to compare the two, in terms of field-illumination & off-axis star
"quality".
Space Boy