Re: [APML] Takahashi BRC250 and E250 information

[Wei-Hao Wang <whwang@gmail.com>]

On Tue, 24 Aug 2004 22:59:30 -0400 (EDT), Gaspar Bakos
<gbakos@cfa.harvard.edu> wrote:
> Dear Gene, and others,
> 
> I am cc-ing this email to the astrophoto list as well, as it might be
> interesting for others.
> 
> RE: BRC250 and E250
> > Focusing - Both use a high precision indexed helical focuser which is
> > lockable.  This is both a blessing and a curse with CCD.  This means
> > spinning the camera to change focus unless you add a secondary focuser.
> 
> Auch. Is this the case? It is absolutely necessary to keep my CCD
> aligned as it was, and change the focus at the meantime.
> 
> I received a diagram from Texas Nautical on the cross-section of the
> helical focuser, but from this 1d view it is hard to reconstruct what
> is happening. As i am adjusting the focus, the camera turns around? How
> can one make nice shots of e.g. Andromeda Galaxy with the proper camera
> angle on the sky and good focus simultaneously? The helical focuser is
> so fine that you can rotate it a +-90 degrees around without seeing
> noticeable focus change?

You can rotate the camera/CCD without changing the focus.  On the other
hand, if you change the focus, the camera/CCD will be rotated as well.  So
people just focus first, then rotate the camera to the right orientation.  This 
sucks. 

> > It has been some years but IIRC I did measure a 9u star on film
> > from the BRC.
> This sounds reasonable...

The 2um star that Tak claims only accounts for geometric optics, I believe.
Just the diffraction alone is much more than 2um.  By the way, I may be able 
to find the spot diagram as well as Ronchi test results for BRC250.  Let me
try.

> 
> > These are the material difference IMO:
> >
> > Speed         E-250 f/3.2 (3.3?)      BRC f/5
> > Weight        BRC is lighter
> 
> I see that E250 is 34kgs, whereas BRC250 is 15.6kgs = half of E250!
> Hard to believe. Maybe a carbon fiber BRC tube and an aluminum E250 are
> compared? Or maybe the E250 weight is with finder + other stuff?

34kg might be the weight for metal tube version of E250.  On the website
of Tak, the weight of E250 is 18.9 kg.  This should be the carbon-fiber tube
version.

> 
> I have not found information on E250 back focus. I hope it is
> reasonable: 60mm or longer. If you have information on this, let me
> know.

I'm afraid it's very close to 60mm, if not shorter.  E250 doesn't allow the 
use of Pentax 67 body because of its very short back focus.

Let me try to see if I can find the spot diagram and Ronchi test results.

What kind of observation are you going to do with these Takahashi?
Superna or GRB?

Cheers,

Wei-Hao

-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       
2680 Woodlawn Drive         Personal Website:
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