Creating an image via spectral scans

It is possible to create line flux maps (2D images) of an object by scanning it with a spectroscope. Christian Buil described the method on his page and thanks to his sotware ISIS, amateurs are able to achieve such images.

This is my first attempt to create such maps. The observing conditions were not ideal, moon was almost full. I chose a really easy and well documented target : ngc 6720, the ring nebula.

I present here my results and compare them to a professional study made in 1994 :


The equipment I used is a 0,2 m F/5 Newtonian Telescope, a low resolution Alpy600 spectroscope with a 25 µm slit (R=600) and a CCD camera Atik414 EX cooled at -10°C. The dispersion is ~0,3 nm/pixel at 656 nm.

Data acquisition

In the context of a first attempt, the slit has been moved across the nebula each 3 pixels. That is not a smooth scan. An unique 300s exposure spectrum has been taken for each slit position. This short unique exposure won't help to get a good signal to noise ratio on faint line such as 4363 A or 5754 A. A total of 22 frames have been necessary to scan the nebula.

Data has been acquired on April the 5th 2020 from 02h00 UT to 04h00 UT.

All the raw frames have been bias, dark, flat corrected. The calibration of the instrumental response has been made on the A3V class star HD 174602 after acquisitions on NGC 6720.

Backyard observatory

Instrumental response calibration

Slit moving across the PN (1s exposure time on autoguider)

2D and 1D processed spectrum

The spectrum shows many lines compared the dozens of PN candidates spectrum acquired for the past 2 years. That is appreciable. The lines [OIII]λ4363 and [NII]λ5754 are unfortunately very weak due to the short exposure time. [SII]λ6716 λ6731 lines and Ha [NII]λ6583 λ6548 are partially blended due to the low resolution of the spectroscope.

2D Processed spectrum

1D Processed spectrum of the central part of n

Lines flux map and color images

Visualisation [NII]λ6583 Ha [OIII]λ5007

Visualisation [NII]λ6583 Ha HeIIλ4686.

Visualisation HeIIλ4686 [NeIII]λ3869 [NII]λ6583

Comparison with N.J. Lame et al. 1994

Line flux maps

The line flux maps acquired by N.J. Lame et al. 1994 are presented below. Mines are presented just after for the same wavelengths.

Maps look consistent. The resolution and the S/N of my maps are very poor, but can be slightly improved with a smoother scanning and longer time exposure.

Balmer decrement

The Balmer decrement is given by the ratio Ha/Hb. Comparison of my work and N.J. Lame et al. results is given below. My map is poorly spatially resolved. But the main features can be seen. The ratio values are also coherent. That confirms that the instrumental response is well calibrated. The outer part of the nebula presents a decrement equal to ~4, the inner part of the ring ~3. The mean value taken by N.J. Lame et al. is ~3.

Balmer Decrement - N.J. Lame et al. 1994

Electronic Density

The electronic density is proportional to the ratio of the intensity of the [SII] lines Iλ6716 / Iλ6731. These 2 lines are not well resolved on my data. The ratio of the maximum intensity of these 2 lines measured on the 1D spectrum gives R[SII]=1,06. According to Osterbrock formula, for Te=10 000 K it gives a electronic density of Ne=540 e/cm3. That is very close to the mean value measured by N.J. Lame et al.

I made a map of R[SII] (below left). My map is very noisy, but few measurements in higher and lower density regions confirm (maybe by chance !) the result given by N.J. Lame et al (below right).

Electron Density S+ - N.J. Lame et al 1994

Ionisation structure map

My ionisation structure maps (below left) are all coherent with N.J Lame et al. maps (below right). I didn't deredned the data as it has been done in the paper. The extinction is not very high and the correction will not affect significantly my (noisy) results.

Ionisation Structure - N.J. Lame et al 1994

Ionisation Structure - N.J. Lame et al 1994

Electronic temperature

The [NII] temperature has been estimated by N.J. Lame et al. They wrote : "The resulting [N ll] temperature map shows a smooth, fairly constant temperature distribution across the nebula. The mean temperature is 9643 K with a standard deviation of 230 K".

I can't assess the [NII] temperature due to the low resolution of the spectroscope I've used. But I tried to measure the [OIII] electronic temperature.

[NII] Electronic temperature - N.J. Lame et al 1994

The electron temperature in O2+ region is proportionnal to the ratio R[OIII]=(Iλ5007 + / Iλ459) / Iλ4363. According to the maximum intensities measured on the 1D sepctrum, the ratio is R[OIII]~135. The resulting temperature, using Osterbrock formula given below is ~11 500 K, maybe a bit higher if I would have derredned the lines. This value seems a bit high but consistent with temperature given by O’Dell et al 2013 (10 000 K - 11 700 K)

The bad S/N ratio of the [OIII]λ4363 line led to a noisy R[OIII] map. The temperature distribution seems constant across the ring. Few measurements in the ring indicate an mean R[OIII] of 114, Te~12 300K.

Up left : [OIII]λ5007+Iλ459 lines map ; up right : [OIII]λ4363 line map

Below left : R[OIII] map


I wasn't expecting much of this first attempt to create an image of NGC 6720 via spectral scans. But I'm quite impressed of the result. In less than 2 hours exposure time and a rough scan, the order of magnitude of the main physical characteristics of the nebula have been assessed successfully.

Additional attempts on well studied PNe will be undertaken with a smoother scanning and multiple raw frames. A total time exposure of 15 h will be a minimum for a PN such as NGC 6720. This should improve slightly the spatial resolution and noticeably the S/N ratio of weak lines. Low aperture telescopes will prevent amateurs from acquiring high quality maps. But I hope we can provide to professionals good first estimations on bright and yet understudied nebulae.

M57: The Ring Nebula

Image Credit: NASA, ESA, Hubble Legacy Archive; Composition: Giuseppe Donatiello