Re: [APML] Takahashi BRC250 and E250 information

["Bobby Middleton" <bobm@koyote.com>]

Hi Gaspar.
I have a Takahashi 250. It is one of the early ones which had the carbon
fiber tubes. It is VERY stable for holding focus. Focusing will not be
required except perhaps for cold season changing to hot season. I don't
believe the latter BRC's have this stability but I am not sure. The latter
models have metal tubes.
I have a web page with several film images made with it. You can look at the
web resolution for these but web resolution will not do justice for the
capability of the BRC. I can try to send some high-res files but I've had
quite a time sending large files over my dial-up connection lately.
Something in my system is not right and large files usually get corrupted in
transit.
Tec pan film images produce extrememly small stars across a 6x9 frame.
Here is my page:
http://www.koyote.com/users/bobm/astro2.htm
The tube and focuser are heavily built. The focuser is a helical focuser
with very fine adjustments. It also allows full rotation of the camera. it
will not loose collimation from any handling because it is built so well.
Collimation is not simple. I am close to good collimation on mine but it is
not exactly square. This is perhaps the one drawback of the design and I've
considered sending mine to takahashi for factory collimation. The manual is
only written in Japanese and following instructions by only looking at the
diagrams is not a good way to do such a complex task.
It is an f/5 instrument with almost a 50% obstruction. It will fully
illuminate a 6x9 frame provided the correct camera is used. I am now using a
Mitusibishi 6x9 vaccum camera with horsman vaccum back and the companion
mitusibishi off-axis guider. These allow full coverage of 6x9 except for the
very small edge of the corners.
I've never measured any star sizes. I believe Takahashi claims 3 microns??
It's definately the smallest star sizes I've had from any of my instruments
and one of mine is a very good 12.5" f/7 newt once owned by Tony Hallas with
a Jerry Wilkerson mirror. The tak beats it easily.
The tak also has a bit of a hot spot. At a 37mm image size you mention this
will not be much of a concern. I am able to digitally fix the uneven
illumination across all the 6x9 frame.
I have had some flexure when using a guidescope, but I would bet that was
from my guidescope mount not the Tak instrument. However, the OAG made for
this telescope might be adaptable to a large format CCD camera if one wanted
to pursue this.
I almost forgot but I do have a high-res crop from a Tak image here:
http://www.koyote.com/users/bobm/M31highrescrop.htm
You can see a bit of the mis-collimation in this image but the instrument is
in better collimation now.
Hope this helps,
Bobby Middleton

----- Original Message ----- 
From: "Gaspar Bakos" <gbakos@cfa.harvard.edu>
To: <astro-photo@seds.org>
Sent: Monday, August 23, 2004 6:14 PM
Subject: [APML] Takahashi BRC250 and E250 information


> Hi, folks,
>
> This is a slightly off-topic message. I am trying to contact people who
> are astrophotographers (thus they CARE about how the images look like),
> and they have experience with either the Takahashi BRC250 or the E250
> reflectors.
>
> In a nutshell:
> I would like to find out how the stellar profiles appear/compare in
> these two telescopes (quantitatively).
> And I would like to know about your experience with these optical tubes
> (focusing, stability, thermal expansion, tube flexure, various
> comments, etc).
>
> ---------------------------------------
> And here it is in a bit longer version:
>
> As a predoctoral fellow at the Harvard-Smithsonian Center for
> Astrophysics, I am in charge of designing a fully robotic astronomical
> instrument for the Smithsonian Astrophysical Observatory and Harvard
> University. We are contemplating about using the Takahashi E250 epsilon
> astrograph, or the BRC250 Ritchey-Cretien system coupled with our own
> robotic telescope mount and large format CCD detectors.
>
> Unfortunately, I found very scarce technical information on the web on
> these telescopes, and I am turning to the astro-photographer community
> for help.
>
> 0. I read that the list is limited to traditional film photography, but
> as i see no other source, let me ask: does anyone have FITS files taken
> with the E250 or BRC250 telescopes? High res scans of photos would be
> also informative.
>
> [[ Maybe even TIFF or other commercial format pictures would help. Raw
> images would be the best, i.e. not ones that have been combined/stacked
> from several individual images. ]]
>
> Takahashi claims a 2 micron stellar size in the center for BRC250,
> which is hard to believe. I can imagine maybe a 5 micron size. How is
> E250 compared to this?
>
> 1.  Maybe you even have a spot diagram?
> We would use the inner 37mm area centered on the optical axis in the
> wavelength range of 500 to 900nm.
>
> 2. Another related issue: how did your find the appearance of the
> out-of-focus profiles? Maybe you have images that were taken out of
> focus? They would be very useful.
>
> 3. Do you have an idea what fraction of the light is blocked by the
> secondary mirror for both E250 and BRC250?
>
> 4. My understanding is that BRC250 is very well corrected against
> thermal changes. Is this correct?
>
> 5. Have you ever experienced tube flexure?
>
> I am looking forward to hearing from you. It would be of GREAT help.
> Please send images, or show me a link from where I could download
> them.
>
> With best regards,
>
> Gaspar A. Bakos
>
> Predoctoral Fellow, Solar, Stellar and Planetary Sciences Division
> Harvard-Smithsonian Center for Astrophysics
> 60 Garden Street, Cambridge, MA 02138 (USA)
> Phone(office): 617-495-7410 Fax: 617-495-7049
> email: gbakos@cfa.harvard.edu or bakos@konkoly.hu
> homepage: http://cfa-www.harvard.edu/~gbakos
>
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo