The Cave Nebula (Sharpless 2-155)

The Cave Nebula (Sh2-155)

Object: Sharpless 2-155 (Caldwell 9)

Type: Emission nebula
Constellation: Cepheus
Distance: 2400 light years

Date: November 2nd. 2018 (Ha), 17th (RGB)
Equipment: ATIK 460EX, Vixen 114mm f5.3 ED114 refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 40 x 250s H-alpha (2x2 binned), 20 flats, hot pixel removal in Astroart (no darks). 10 x 250s (2x2 binned) each for RGB

The Cave Nebula is part of an extensive region of ionised hydrogen gas associated with the Cepheus B giant molecular cloud.  The ionisation is caused by radiation from a group of young, hot stars hidden within the dust cloud, which knocks hydrogen electrons into "excited" states that re-emit radiation and cause the surrounding low-pressure hydrogen gas to glow.  Sh2-155 is just a small part of an extensive area of nebulosity in the Cepheus/Cassiopeia region that includes several notable emission nebulae.  It lies about 3.5 degrees south of iota Cephei...

The evening of November 2nd. was clear but the atmosphere was very unsteady. I decided to run the Atik camera in 2 x 2 "binned" mode, partly to make autoguiding accuracy less critical and also because the Cave Nebula is extremely faint and I didn't fancy spending hours getting enough 20 or 30 minute subs to beat down the background noise.

However, this loses a bit of resolution, but the poor seeing would have done that anyway. The seeing did make focussing a bit hit and miss however, and I should really have looked at the first couple of subs a bit more closely.  When I finished the session and downloaded the images next day, I was frustrated to find that the stars were out of focus, with some showing a "doughnut appearance".  At this point I would normally delete them and wait for another imaging opportunity, but clear, moon-free nights at reasonably civilised hours are an extremely rare occurence in my part of the world, so I decided to see if the situation was redeemable.

The Cave Nebula in H-alpha

Fortunately, a couple of software routines in Astroart (Digital Development followed by a few iterations of Lucy-Richardson deconvolution using a mild Gaussian point spread function, for the record) were able to pull the star and nebulosity details back into a reasonably decent shape.  A bit brightness and contrast adjustment (using "curves") and noise removal (mild edge-preserving smooth), both in PaintShop Pro, gave the Hydrogen alpha image above.

I did try and get some hydrogen beta data, but 300 second exposures at 2x2 binning revealed not a jot of nebulosity in the area, so I guess this object just doesn't emit in this part of the spectrum.

It was a couple of weeks before I was able to get some RGB data to make up a colour image.  250 second subs (2x2 binned) through Baader red, green and blue filters were stacked, aligned and RGB combined in Astroart to give this rather disappointing RGB image...

RGB - Cave Nebula area

Whilst the star colours were fine, there was hardly any trace of nebulosity visible. I guess a 60% waxing moon and shortish subs washed out the contrast.  Clearly, I would not be able to directly use the H-alpha frame as a luminance layer, as the lack of any nebula colour in RGB would just result in coloured stars on a grey nebula.

I decided to colourise the existing H-alpha frame by pasting it on to a red background in "Hard Light" mode in PSP.  This gave a bright red nebula image with white stars...

The brighter H-alpha regions tend to remain washed out and so I prepared a second mask as above, but combining in "multiply" mode. This gave a completely red nebula with red stars...

"Colourised" H-alpha frame...

Selecting the bright area (with a feather of about 20 pixels), I pasted and aligned the area over the "Hard Light" image.  With careful use of the eraser tool in PSP and a partial blending (around 50%) it was possible to greatly reduce the remaining washed-out H-alpha regions...











Pasting this over the RGB stars in "lighten" mode at around a 70% blend gave a reasonable mix of star and nebula colour,  A few tweaks to the final colour and contrast using curves gave the final result you can see at the top of this post.

OK, never a NASA APOD, but given the poor quality of data I don't think it turned out too bad.