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Re: [APML]: 2X converter
The Astro-Photography Mailing List
------------------------------------
When a Pentax 1.4X TC was used with my 6" f/7.1 AP and 100 field flattener,
I found severe astigmatism beginning around the 70% zone on 120 format
film. It needs to be confirmed, but I might conclude that the TC might
yield better results without the field flattener.
jg
----------
> From: Space Boy <astro@cu-online.com>
> To: astro-photo@nightsky.com
> Subject: Re: [APML]: 2X converter
> Date: Sunday, September 27, 1998 7:51 PM
>
> The Astro-Photography Mailing List
> ------------------------------------
>
> 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