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Comp Stars and Telescope FOV

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clkotnik
clkotnik's picture
Comp Stars and Telescope FOV

Hi,I am communicating with a researcher who is interested in observations of a bright star - 4V, 2.6I.  I know bright stars are one of the design goals for AAVSOnet, but I assume suitable "bright enough" comp stars must exist within the field of view.  I did not find a simple list of this characteristic so I put together the attached for all the AAVSOnet telescopes I am aware with recent observations.  I did a plate solve of an image to the plate scale and multiplied by the image size.  I pulled the site lat/long from the FITS header as well.

Do the entries in this list appear accurate?  Any advice for how bright comp stars must be?

thanks,

Cliff

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MZK
MZK's picture
Bright comps

Hi Cliff:

Yes, all the BSMs on the AAVSONet system are small refractors with a FOV of about 1.5 x 1.1 degrees. This does not apply to the three larger AAVSONet scopes which you should not use for this project. So, the easiest way of checking for the presence of nearby bright comps (say within  +- 1 mag) is to check the VSP for the targets of interest using that FOV. Remember, you can off-set the field slightly if that helps find a better nearby bright comp.

You might always get lucky but if not, keep the target below saturation (non-linearity) and use fainter comps with the expectation that the CCD is very linear. At some point this assumption may fail but you can test this by selecting another bright target where you can find a nearby bright comp and test the use of both the bright comp and a fainter comp star to compare the target magnitudes measured with both comps.

Ken

Mark Blackford
Mark Blackford's picture
Bright Comps

Hi Cliff,

I'm currently monitoring eta Car (mag ~4 V) for an Astronomer in Brazil, so similar situation as yourself.

There are no other bright stars in my field of view so we use a nearby mag 8 star as the Comp. Exposure time is 2 to 3 seconds maximum to avoid saturation but then scintillation becomes an issue. I take 60 images to make 4 average stacks (15 images each) then do aperture photometry on on them to get 4 magnitude estimates, taking the average as the final measured magnitude and the standard deviation as the error estimate (typically less than 0.01 mag).

Alternatively you could use the PEP technique of imaging the target variable then moving to a suitably bright Comp star (at similar altitude to minimise differential extinction). Repeated measurements would be advisable. But you would still have the problem of needing short exposures to avoid saturation.

Good luck,

Mark

HQA
HQA's picture
comp star brightness

The usual rule of thumb is that a comp star needs to be within 2.5 magnitudes of the target in order to be useful.  That means it has SNR=100 when the target is near saturation.  This rule can be exceeded by several techniques.  For example, you could use multiple faint stars in an ensemble, effectively increasing the SNR by averaging to reduce the noise.

What we are hoping to do with the next phase of BSMs using CMOS cameras is to stack potentially hundreds of subframes to both keep the target from saturating while increasing the dynamic range so as to use fainter comps.  In theory, this works, both reducing scintillation and increasing the number of possible comps.  As you note, bright stars often don't have decent comparison stars nearby.

If you use 9th magnitude as your faint limit, there are about 6 stars per square degree on average across the sky, of course concentrated towards that faint limit.  The number of stars increses by about a factor of 2.6 per magnitude, so at V=7, there are about 1 star per square degree - and that one is likely to be the target!  This means, on average, you can probably find a decent comp for, say, a 5-6th magnitude star if you have a 1-degree FOV.  

On the other extreme, at 11x11 square arcmin, I had very few fields where I couldn't find a comp star for the USNO-FS 1m telescope, when working at its "sweet spot" of V=13-17.  So the necessary size of the field depends greatly on the brightness of the star to be measured.

With the new stacking technique, I think the BSMs should be able to work with stars as bright as Vega, and still find comps.  In practice, we'll see whether the stacking technique retains linearity (a really big question) and reduces scintillation and scattered light sufficiently to be effective.

The other option is to pick your targets carefully.  Don't observe every 3rd magnitude variable - only observe those that have a 3-5mag comparison star in the same field.  There are lots of such fields.  Doing a catalog search for such fields would be a nice project and potentially could be used to modify the BSM background monitoring program.  A somewhat fainter example of this is the Cepheid variable EW Sct, which has a nearly identical magnitude and color comp within 5arcmin.

Arne

clkotnik
clkotnik's picture
defocus

Ken, Mark, Arne,

Thanks for the suggestions. 

I know an automated system like AAAVSOnet cannot allow for every option, but I wonder if defocusing to avoid saturation is something that a user can request in their proposal?

Arne,

In the project idea you tossed out might the Henry Draper catalog be a suitable starting point or would it be better to take a more modern catalog and just pull a subset based on, say V magnitude?

best regards,

Cliff

HQA
HQA's picture
defocus

Hi Cliff,

ACP (our scheduling/control program) does permit defocus on a per-exposure basis, so that you could defocus on VRI but have good focus on B, for example.  So what you suggest is possible and could be requested by the researcher.  You then have to use a larger measuring aperture for your defocused images, and then have to worry about blending/contamination from surrounding stars.  Not impossible, but you have to look at each case separately.

If I were to use a catalog to look at bright stars for close comparison stars, I'd probably use Tycho2, which has far better photometry than HD.  Be sure to use proper motion to get current positions before doing the selection, as bright stars tend to be nearby and therefore tend to move faster across the sky.

I'd like to see two searches done someday.  First, Richard Miles looked for red/blue pairs for measuring 2nd order extinction.  I'm not sure he did it for the entire sky (for example, southern hemisphere), and it would be nice to have a list of a hundred good pairs.  Second, look for bright variable stars that have a bright-enough comparison star within a reasonable field of view.  Reasonable to me is about a 60 arcmin radius from the variable, so that both the variable and its comparison would fit within a one-degree FOV, albeit offset from the center.

Good luck!  Both of those searches would be useful to people, in addition to the project on which you are working.

Arne

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