(cba:news) (cba:chat) BW Scl colour index

Joe Patterson jop at astro.columbia.edu
Sun Oct 23 08:11:00 EDT 2011


Hi BW Scl fans,

I'd suggest no B, as far as time series are concerned.  B is the least 
desirable (OK, U is worse) of all the filters, because there will be 
significant differential extinction: a slow bend as BW Scl reaches high 
airmass.  This will slightly degrade the period search; but more 
significantly, it will introduce low-frequency periodic signals, 
probably around 1.0 or 2.0 c/d.  V, R, and I are much better in this 
respect (extinction coefficients low and similar).

Also, some B filters are a little dark.  Check.

Because I consider time resolution to be very important, I'd advocate a 
straight V time series.  When dwarf novae are bright, the various 
commonly obtained time series (B,V,R,I) tend to be extremely similar, 
with no useful information (usually) obtained from the filters.  So you 
lose time resolution, with no real benefit.  Very late in the outburst
(probably when V>13 for this star), a significant and still-unexplained 
red excess sometimes appears.  That would be great to study for BW Scl. 
  You could do it with VI time series... or possibly with a fairly 
extensive snapshot coverage.

But probably straight V is best for the next 2-3 weeks. Assuming this is 
a super, which now seems very likely.  When it gets faint, the 
time-series (rather than snapshot) choice will probably be *unfiltered*. 
  Probably.

Finally, can anyone take *very fast* time series?  Many dwarf novae in 
outburst show periodic signals ("dwarf nova oscillations", or DNOs) 
around 20 s or thereabouts.  This requires excellent time-keeping and a 
cycle-time not exceeding 7-8 s.  Don't knock yourself out to do this, 
though - remember the Tortoise and the Hare.

BTW since I'm advocating against B, B observations may end up being 
particularly valuable (because there might be a lot fewer of them).  I 
doubt it, but maybe!  I *really* doubt it for time series... but 
possibly for snapshots.


joe


On 10/23/2011 7:32 AM, Arne Henden wrote:
> Josch, you should be able to use the B filter on targets like this.  The red
> leak for your B filter is only important for stars with (B-Ic)>5; that is,
> very red stars; and for objects with huge Halpha emission, such as novae.
> For all other stars, it is fine.
> Arne
>
> On Sun, Oct 23, 2011 at 7:16 AM, Josch Hambsch<hambsch at telenet.be>  wrote:
>
>> Should we commence with BVI filters? So far I did V, I as the B has a
>> infrared leakage. But if the star is blue I could also add B filtered
>> images.
>>
>> Regards,
>>
>> Josch
>>
>> http://www.astronomie.be/hambsch
>> ----- Original Message -----
>> From: "Terry Bohlsen"<terry.bohlsen at bigpond.com>
>> To:<vss-members at googlegroups.com>;<cba-chat at cbastro.org>
>> Sent: Sunday, October 23, 2011 12:14 PM
>> Subject: (cba:chat) BW Scl colour index
>>
>>
>>> Dear All
>>> I took a short V time series last night before the weather made me stop.
>> I
>>> interspersed this with a few UBVRI images. The data is uploaded to the
>>> AAVSO.
>>> The star is very blue with a B-V = -0.20 and U-B= -0.40
>>>
>>> Cheers
>>>
>>> Terry Bohlsen
>>> Armidale NSW Australia
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