Color imaging of deep sky objects with interference filters

It is possible to achieve color imaging of deep sky objects with interference filters, especially images of planetary nebulae emitting most of their light at 6536A (H-Alpha) and 5007A (OIII). It has been tried to make image of the famous M27 object using two 10nm bandpass interference filter. The filter have the maximum transmission peak for the H-Alpha and the OIII band lines. Provisions concerning the peak transmission shifting have to be taken into account when using interference filter with fast converging beams.

An image of M27 has been recorded using the 500nm filter : it has dimmed all the stars and the sky background, and strengthened the contrast of the nebula. Some of the far outer shell of the nebula glowing at 500nm are visible (red arrows). This is not usual to see this, because usually it is drowned among the stars of the milky way !


Exposure of 540sec (9x60s), T250F3.5, OIII interference filter, high contrast view.


Same image as above with different cut level display, to see the inner part of the nebula.

Also an image with the H-Alpha filter has been recorded :


Exposure of 660sec (11x60 sec), T250F3.5, H-Alpha interference filter, high contrast view.

One hint : To be able to take images with interference filter, the CCD camera must be a low noise system. Here the main limitation is not the noise of the Sky background that had become far less prominent that it used to be, because of the interference filters, but the readout noise of the CCD that is made of dark current + amplifier + readout chain + intrinsic CCD noise. The camera used here has around 8e- of readout noise. In my case I could measure (OIII filter) with a single 60s exposure :

The total noise of a single exposure is 13 e-

Also, great care has to be achieved during dark frame subtraction.

In the end, we have a red image as the H-Alpha image. What are we doing to do of the OIII image ? If one puts a flash lamp through the OIII filter it look rather blue-green, so what I did I have used the OIII image as the blue image and as the green image. If the display cuts are set properly, it is possible to get an image that is close to the actual color image.

See yourself :


Combined image RGB image where R=Halpha filter, B=OIII filter V=OIII filter. The image has been log scaled.


Combined image RGB image where R=Halpha filter, B=OIII filter V=OIII filter. The image has been unsharp masking processed.

I think this is not so bad ! OK, the colors of the stars around the nebula are completely false, but the color of the nebula looks OK, especially the green-blue color of the OIII line.

 


Veil Nebula where R=Halpha filter, B=OIII filter V=OIII filter. Halpha filter is 1320s (11x120) of exposure and the OIII filter is 1560s (13x120).


Ngc281 where R=Halpha filter, B=OIII filter V=OIII filter. Halpha filter is 1320s (11x120) of exposure and the OIII filter is 1560s (13x120)

 


Ngc7320 where R=Halpha filter, B=OIII filter V=OIII filter. Halpha filter is 1320s (11x120) of exposure and the OIII filter is 1560s (13x120)

References

1. Articles from Astronomy and Astrophysics : 1, 2 about M27

By Cyril CAVADORE, All right reserved. Back to my homepage