RE: [APML] Takahashi BRC250 and E250 information

[Gaspar Bakos <gbakos@cfa.harvard.edu>]

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?

> There is only one commercial model (FLI) that will fit in the available
> back focus and it has a limited aperture.
>
> If you are gathering data of widely different wavelength bands the E250
> will very likely require refocusing between filter changes.  The good
> commercially available filters are not parfocal at f/3.2.  Even at f/5
> you may require focus changes.

We use Bessel-made V and I filters from Omega Optical, and they turned
out to be very similar in optical thickness: no discernible change at
f/1.8 focal ratio with Canon 200mm f/1.8 lenses. So I am more worried
about focusing in general that the filter changing.

> One saving grace is that the designs are temperature compensating and
> so the only refocusing required would be due to changes in filters.

Is this also true for E250?
At:
http://www.buytelescopes.com/product.asp?t=&pid=1554&m=
I read:
"The Epsilon-250 does not have a temperature compensating secondary.
The following Takahashi telescopes have a temperature compensation
secondary holders: BRC-250, FRC-300 and E-300. The E-350 uses an Invar
cage to keep precise focus."

> I have discussed off and on with a master machinist a couple of
> different replacement focuser designs that would replace the stock
> focuser and allow remove focusing without a loss of aperture.  I
> would be happy to discuss this with you.  I only hard bid I received

This might be actual in a couple of months, if we purchased the tube
(E250 or BRC250, stil up in the air and started building the system.

> With the BRC there was no flexure detectable in 1 hour exposures.
> With the E-210 there was no flexure detectable in 30 minute exposures.
> Chuck Vaughn worked with one of the smaller Epsilons some years back

I will use 5-10 minute exposures anyway. Tube flexure will affect
absolute pointing on the sky only.

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

> 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?

> Back focus	More in the BRC

The information I got for BRC is 106mm, which becomes 83mm with some
converter rings installed (or maybe it is measured separately from the
inner and outer shoulders of the focuser.

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.

Cheers
Gaspar
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