Choosing a first ccd filter

Hi Albert,

While it is true that most of the modern big-telescope surveys are being done in Sloan filters, most of the time-domain studies are still using Johnson/Cousins filters.  The AAVSO does not yet have the means to display Sloan magnitudes in the VSP comparison-star photometry table; it does not plot Sloan magnitudes with the LCG; no other amateur organization that I know of supports Sloan photometry.  It is the wave of the future, but even there, the Johnson B&V filter passbands neatly divide the Sloan g' bandpass and therefore give more spectral information.  I think at least B&V have a long future.

Therefore, I'd recommend the Johnson V filter as your first choice.  Maybe in 3-5 years that recommendation may change, but I doubt it.  That said, if you are working with a professional who wants a specific bandpass, then that is what you should use - but in most of those cases, the professional will supply a filter and sometimes even a filter wheel if they want your photometry.

I'm a little surprised at the cost you are indicating for the filter - I thought Baader filters were less expensive than that.  A colored glass filter does just fine for V-band, and they can often be even cheaper.

Arne

It is a little sad to hear that AAVSO does not expect in the near future to display Sloan magnitudes in the VSP comparison-star photometry tables although the availability of APASS data and UCAC4 catalogues.

Probably the SLOAN data in APASS and UCAC4 are not perfect, but also are not perfect the UBVRI data in VSP comparison-star photometry tables.

Recently according то a request of my pro-am friends I had to follow for a while BH Vir in B and V filters. You can convince by yourself the VSP comparison stars precision in B and V filters that I had to use in the following lines:

Star        B             Berr       V             Verr

110        11.720   0.098     11.049   0.026    

103        10.886   0.093     10.317   0.031

I think that as soon as it possible the AAVSO has to include APASS and UCAC4 Sloan type magnitudes into the “games”.

I am imaging mainly in Sloan type g’r'í’ filters and for every target I had to prepare the comparison stars sequences manually by means of UCAC4 catalogue. I am doing ensemble photometry with lots of comparison stars (more than 12 for each target) and the task is not easy. Very soon (when the duty for confidentiality from my partners expires) I will start consecutively to contribute the photometric data for dozens of objects in Sloan type filters. The data are based on UCAC4 Sloan type magnitudes data.

Velimir

I agree with Arne. V is your best choice for your first filter.

And regarding filter prices, you should search widely on the internet, you can get more affordable prices.

Regards

Miguel

Thank you all for the advice and informative replies.

I'll search some more for a V filter online. I suspect the exchange rate for the Euro and Irish taxes may partly explain some of the prices I've seen so far!

Best Wishes, ~Albert

Hi Velimir,

I think rudimentary support of Sloan filters won't be long in coming at the AAVSO.  Before I retired, we had a beta version of VSP that was being tested.  This included the ability to customize the photometry table to include only those filters of interest to the researcher, and the list included Sloan.  We had a design specificiation for a new Light Curve Generator that included the ability to plot Sloan passbands.  There were plans to add the Sloan passbands to the comparison star database from one of the upcoming releases of APASS.  These are the important infrastructure changes that are necessary to start supporting photometry in both major filter systems.  The steps have understandably been delayed with a new Director, and may be modified along the way, but I expet to see Sloan support in the near future.  However, even with infrastructure changes in place to support observers who wish to use Sloan filters, I don't see a major thrust from the AAVSO to move from Johnson to Sloan on the commonly monitored stars.  Where you might see Sloan "inroads" are for hot-topic or campaign stars, where researchers may want Sloan photometry to support their professional data.

One of the big problems is going to be continuity.  We have 100+ years of visual data on a number of stars, and decades of V-band data on lots more. There is no V-band equivalent in the Sloan system, so transformation from one system to another will be imperfect.  It might be a good idea to always take a V-measure in addition to Sloan photometry if only for the reason of continuity.  This issue will be a good topic of discussion, once the infrastructure is in place to accept either kind of measurement.

Arne