Filters

[Michael O’Connell]

Hi Folks,

Couple of quick questions if you don’t mind please:

Is a V filter pretty much essential for CCD variable star work or can I just use a UVIR filter?

If V is required, is it possible to get a V filter in 2″ round mounted format to screw onto the 2″ adapter on the camera? Can’t seem to find them on the usual sites.

I have an SBIG ST-1001E camera and C14 scope. I don’t want to have to buy a new filter wheel and large filters unless absolutely necessary.

Thanks,

Michael.

[Jeremy Shears]

Hello Michael,

the requirement for a filter depends what type of variable star you are observing. I observe Cataclysmic Variables, in particular Dwarf Novae in outburst. I usually do unfiltered photometry to get the best signal-to-noise ratio. I’ll use a V-band comparison star sequence. Observing some eclipsing systems, where timing the eclipse is critical rather than the actual mag, can also been done unfiltered. For much other VS work a filter is highly desirable in order to be able to compare your photometry with other observers – and in the case of coloured variables (like Miras) a filter is essential. Whilst many amateurs use a V-band filter, some use a B, R or I filter. These are specific wavebands, based on the Johnson-Cousins system, which are designed for photometry and are thus different for standard coloured filters for tri-colour imaging (and different from UV or IR cut-off filters).

I am not sure where one can buy 2-inch V filters. The company Astrodon lists them, but they are unmounted. Maybe someone else can advise.

Best regards,

Jeremy

[Michael O’Connell]

Thanks Jeremy. I rang Custom Scientific – as the name suggests they do custom work. They can make a 2″ round mounted V filter for $400 + shipping. Although it is costs more than the 50mm round unmounted, it would save me the expense of a new filter wheel specifically for the larger unmounted filters. TBH, I have a preference for trying to keep the setup as simple as possible and avoid filters wheels if I can. I’ll think about it a little.

However, your comment re Cataclysmic Variables is interesting. I might look into that a little….. 

Ta,

Michael. 

[Jeremy Shears]

Great stuff Michael. Do let me know if I can be of assistance as your plans progress.

If you want to have a dabble with CVs, the VSS is running a campaign on one called NSV 2026, which is in Taurus. Unfiltered CCD obs (or filtered, if you get the V filter) are needed to spot outbursts of this enigmatic dwarf nova – then when an outburst occurs, time series photometry is required for as long as possible. It goes into outburst every couple of weeks, so you won’t have to wait long for something exciting to happen!

More details here: http://www.britastro.org/vss/NSV2026.pdf

Jeremy

[Michael O’Connell]

Thanks Jeremy. I’ll give NSV2026 a shot.

Michaelk.

[Jeremy Shears]

Excellent, Michael. The simplest way to start is to image the field of NSV 2026 every clear night to see if it’s in outburst or not (unfiltered is fine). If you spot an outburst, please post the news to the BAA VSS alert group: https://groups.yahoo.com/neo/groups/baavss-alert/info

Jeremy

[Gary Poyner]

But you have to join the group first 😉

Good luck,

Gary

[Michael O’Connell]

Thanks Guys.

Just joined.

[John O’Neill]

Hi Michael,

I would actually purchase at least two filters, so you can transform your data onto the standard system. I have BVI, but R is also a good option instead of I.

Welcome to Variable Star World!

John

[Gary Poyner]

Michael,

HT Cas has just entered outburst – it may be a superoutburst.  Good opportunity to have a go at time series.  HT Cas is an eclipsing UGSU star, so your light curve through an orbital period or two will look quite spectacular.  There is an eclipse ephemeris on my web page (http://www.garypoyner.pwp.blueyonder.co.uk/HTCAS15.txt). It’s quite old and needs a tweak, but it will give you a good idea as to when to expect minimum.

Good luck,

Gary

[Michael O’Connell]

Thanks John. 

Thansk Gary. I’ll give that a try.

Michael.

[Gary Poyner]

Latest news is that it could be a normal outburst, so no lovely superhump profiles on the light curve.  However sometimes these normal outbursts can trigger superoutbursts (never been seen in HT Cas though), so worth following after a return to quiescence.  Of course you still get eclipses during a normal outburst.   Last night (Oct 25.9 UT) it was still bright at 13.2.

Gary

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