In­tro­duc­tion to comet pho­tom­e­try

Mea­sure a comet’s bright­ness by cal­i­brat­ing and stack­ing your im­ages

Sky at Night Magazine - - IMAGING FOR SCIENCE -

Pho­tom­e­try is the science of mea­sur­ing an ob­ject’s bright­ness and there are a num­ber of im­por­tant as­pects to con­sider when per­form­ing pho­to­met­ric mea­sure­ments on comets. Firstly, im­ages need to be pre-cal­i­brated us­ing match­ing flat and dark frames. As sky bright­ness makes a con­tri­bu­tion to the comet’s bright­ness, this too needs to be mea­sured. Some pro­grams,

such as Max­imDL, per­form this func­tion us­ing an aper­ture and an an­nu­lus. The aper­ture mea­sures the comet’s bright­ness while the an­nu­lus is used to mea­sure the back­ground sky bright­ness.

The BAA pro­gram Com­phot (see Hard­ware & Soft­ware, p72) can be used to per­form pho­tom­e­try on comet im­ages. This re­quires two FITS im­ages as a source, one stacked on the back­ground stars and the other on the comet. This is a com­mand line pro­gram nor­mally ini­ti­ated by typ­ing: com­phot off­set.fit fixed.fit x y

Here off­set.fit is the im­age stacked on the comet, and fixed.fit is the im­age stacked on the stars. Both files are in the FITS (Flex­i­ble Im­age Trans­port Sys­tem) file for­mat, a tech­ni­cal for­mat pre­ferred for as­trom­e­try and pho­tom­e­try.

The x and y val­ues are the co­or­di­nates of the comet’s coma cen­tre and can be de­rived by us­ing a po­si­tion-mea­sur­ing pro­gram such as Astro­met­rica. The UCAC-4 star cat­a­logue is rec­om­mended for po­si­tional mea­sure­ments. As the coma con­tains re­flected sun­light as well as light gen­er­ated by ex­cited coma atoms and mol­e­cules, spe­cial­ist pho­to­met­ric fil­ters are nor­mally used for imag­ing. If you’re us­ing FITS im­ages from a CCD with­out a fil­ter, R-band mag­ni­tude val­ues should be se­lected in Astro­met­rica. If us­ing a green fil­ter, V-band mag­ni­tudes should be used. An R-band fil­ter is a red fil­ter typ­i­cally cen­tred on 658nm, while the V-band fil­ter is cen­tred on the visual wave­length at 551nm.

The Com­phot soft­ware is ca­pa­ble of out­putting its mea­sured find­ings in the cor­rect for­mat re­quired by the In­ter­na­tional Comet Quar­terly (ICQ). Like the MPC sub­mis­sions men­tioned in project 2, the ICQ re­quires that this data fol­lows a strict sub­mis­sion for­mat. De­tails of this for­mat are avail­able from www.icq. eps.har­vard.edu/ICQFor­mat.html.

One is­sue when mea­sur­ing a comet’s bright­ness in an in­ter­ac­tive pro­gram such as Max­imDL is de­ter­min­ing an ‘aper­ture’ size to en­com­pass the en­tire comet head

V-band (visual) 551nm +/–88nm R-band (red) 658nm +/–138nm Com­mon pho­to­met­ric fil­ters used for comets are V- and R-band fil­ters. They help to iso­late re­flected sun­light from ex­ci­ta­tion ra­di­a­tion from within the coma

Outer an­nu­lus Ob­ject aper­ture In­ner an­nu­lusAn ob­ject aper­ture should con­tain the comet’s head. The in­ner an­nu­lus is a gap, ad­justed un­der soft­ware to ide­ally con­tain no stars

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