Aperture for extended target NGC 4151

Calculad

Affiliation

British Astronomical Association, Variable Star Section (BAA-VSS)

Sun, 03/17/2024 - 21:01

Is there any guidance on suitable aperture size for this object in particular and galaxies or comets in general please.

Thanks

Kevin

bskiff

Affiliation

American Association of Variable Star Observers (AAVSO)

Sun, 03/17/2024 - 23:01

Active galaxies measuring apertures

I'm assuming here that by 'aperture' you are referring to a photometric measuring aperture, not the telescope size/aperture. I'm no expert on the subject, but I know there is considerable literature about how one measures the variations in active galactic nuclei when the object is well resolved. The problem is that any reasonably small aperture will take in varying amounts of light depending on seeing, image-scale, and other factors. Various profile-fitting schemes (like a curve of growth) will be fraught with uncertainty. Even for an object like BL Lac, only just resolved, one is asked to measure it in a fairly large aperture (13" to 15" _diameter_) so that the underlying galaxy can be subtracted off in a consistent way. I think the only solution is to adopt some aperture that you can use in good and poor seeing, and do so consistently for a long time (years) so that systematic differences between you and others can be examined. One trial would be to get 'many' observations on some stable, clear nights over several hours to see how the photometry changes as a function of airmass, seeing, etc. One might try using a rather small telescope with long f/ratio to see if you could reduce the effect of the surrounding galaxy and maybe-almost treat the nucleus as a star. The NGC 4151 nucleus is like mag 12, so you don't need a big telescope for this.

I have some nights of UBV data on the NGC 4151 field using the Lowell 1.1-m telescope, intended to calibrate field stars. On two of the nights, nearby stars had fwhm of 2".65 while the nucleus of the galaxy shows fwhm of only 3".0, but with broad wings compared to stars. I'd wanna try (at least) an measuring aperture of something like 15" diameter to see if that was consistent. I would certainly also want to dig into the literature on variable-galaxy-nucleus photometry using SIMBAD to see what is common practice in this area, and what folks have done for this galaxy specifically.

Comets are treated similarly using curves-of-growth, where you ultimately calibrate in physical units based on the distance of the comet, and usually in narrowband filters that isolate specific emission bands for the gases and subtract off the underlying dust 'continuum' (reflected sunlight) using broadband filters. Some recent items related to Comet 12P Pons-Brooks include:

https://www.astronomerstelegram.org/?read=16508

http://www.cbat.eps.harvard.edu/iau/cbet/005300/CBET005369.txt

\Brian

Calculad

Affiliation

British Astronomical Association, Variable Star Section (BAA-VSS)

Mon, 03/18/2024 - 10:29

Thanks Brian,

Very helpful…

Thanks Brian,

Very helpful. Yes photometry aperture.

I have a few obs under my belt for which I did indeed use 15" aperture.(guided by VPhot Details section of star profile and signal circle radius).

Then there's the sky problem. When I used to do PEP I could just offset and take sky readings.

I think nearly all of the variation comes from deep within the nucleus so again as always consistency is the top priority.

I used 16 for inner radius of sky ring and 15 for its width. With my small 90mm refractor, f 600mm, this does get some of the outer galactic light but I can compare sky readings with comps to get a measure of the boost to sky.

The FWHM is also markedly different especially on one night with good seeing for my B (~2.50) and V(~3.0) obs.

Regards

Kevin

JBD

Affiliation

American Association of Variable Star Observers (AAVSO)

Mon, 03/18/2024 - 00:26

I feel a little challenged by your question

Hi Kevin, if we talk abour photometry...
I feel a little challenged by your question because I have asked myself it often...

For example, by special stars like R MON (NGC2261) or PV CEP, how to do photometry?
These stars are accompanied by constant cloudiness...

What I remembered is that of the three circles (apertures) which form the pointing of the star in photometry:

- The smallest is for reading the star, it should not necessarily be too tight either.
- The space between the small circle serves as a separation between the star or else, and the sky background.
- Then the space between the 2nd circle and the outer circle is for reading the sky background.

It is from these two readings that the photometry is made.

JBD