Spectrum of nebula: data reduction question

[Olivier Thizy]

Hello,


here is a question I received:

How can I delete the skyground with urban light when I try to capture a nebula with visual magnitude M ~ 10 (like M57) ? Do I need a UHC (or CLS) filter ?

Answer depends on the size of the nebula. If the nebula (or any extended object such as galaxy, comet...) size is too large, the best is to take another spectrum of the sly background separately and substract this background from your nebula spectrum. It takes time, but I do not think there are other option.

If the nebula is small enough so you have some background above and below (typically 50-100 pixels on each side), then you can substract the background directly from the image, just enlarge the binning area for spectra extraction.


Cordialement,
Olivier Thizy

[Thilo Bauer]

Hi Olivier,

I agree with the method to remove sky background. The size of the nebula on the slit may vary depending on the telescope and spectrograph you are using. Exposures to record bright skies will be very long to find. Larger than with conventional imaging.

There are a few problems to properly interpret sky contribution with a spectrum taken from a planetary nebula or similar object.

In the special case of certain planetary nebula, one should be a careful about the interpretation of sky portions very close to the bright nebula content. Certain planetaries are known to have outer shells, that are much fainter than the bright nebula recorded with short exposure times. One of the first reported in literature was M57. These outer shells so far are first reported by very, very long exposures using interference filters for H-alpha, O III, He or H-beta emission lines. Therefore, it could be a mess to tell if the dark sky close to the nebula will contain addiional emission lines of interest close to the brighter portions of the nebula or just contribution of the bright sky.

The ring nebula Messier 57 and also the famous Dumbbell nebula Messier 27 are two of these objects of special interest. These provide at least one to two faint outer shells that might be of interest to spectroscopists wanting to go to the very, very long exposures.

A typical example of the outer shell of M27 described by an amateur is shown here: http://www.juelich-bonn.com/jForum/read.php?8,276545. Please click on Links "Öffnen" to open the images showing the very faint outer shell of M27.

Another problem will be reflection inside the optics and especially the spectrograph itself, which can be easily found taking bright flatfields or stellar spectra of the very bright stars, like alpha Lyr.

Taking into account the exposure time reported for the above mentioned amateur image, exposures to retrieve spectra from the outer shells should be in the order of many hours to days. Therefore, I guess this cannot be done without stacking many spectra over days.

Best regards,

Thilo