The Head of the Jellyfish

[Trev S]

Attached is my attempt at imaging IC443, the head of the Jellyfish Nebula in the constellation of Gemini. This was taken on the nights of 26th January and 6th March 2024 from my back garden in Surrey and processed in the SHO palette. It consists of 7.5 hours of narrowband data only. Stars were created by extracting stars from the images and processing them separately in HSO.

IC443 is a galactic supernova remnant located about 5,000 light years away. It may be the remains of a supernova that occurred 30,000 - 35,000 years ago. The same supernova event likely created the neutron star CXOU J061705.3+222127, the collapsed remnant of the stellar core. IC 443 is one of the best-studied cases of supernova remnants interacting with surrounding molecular clouds. (source Wikipedia)

C9.25 SCT with 0.63 reducer/flattener

AZ-EQ6GT mount

ZWO ASI1600MM cooled camera

Antlia filters

PHD2 guiding, SGPro mount/camera control

Astropixelprocessor, PixInsight and PS2 processing

Sii (red) 30x300s

H (G) 30*300s

Oiii (blue) 30*300s

Thanks for looking

[tcos...@gmail.com]

Hi Trev

That’s another great image. Getting things like tracking and focus right is much more difficult with the SCT than a shorter focal length refractor but everything in your image is pin sharp. Your collimation and set up must be very good. Do you want to come and sort my Edge 11 out?! Btw what’s HSO?

Well done

Tim C

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[drja...@aol.com]

Hi Trevor

I agree with Tim.  I once tried imaging with an SCT and got nowhere at all.  Long exposures with narrowband must be the most difficult approach within an already hard task.  A remarkable image.  

I wonder if a darker black point would add contrast to the structure and bring out more detail.  The background looks a bit light to my eyes, though I do have a tendency to overdo it.

James

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On 11 Mar 2024, at 21:46, tcos...@gmail.com wrote:



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[Trev S]

Thanks for the kind comments.

Tim, I have recently purchased a tri-Bahtinov mask in an attempt to fine collimate my SCT.  It is a bit fiddly and first time took me about an hour, but collimation definitely improved and images are sharper.  Before, I collimated on out-of-focus stars but I think this is just a fairly rough method. The final collimation is ideally done on airy rings but the seeing is never good enough for this.  The tri-Bahtinov mask certainly made a difference imho.

By HSO I meant I used the HSO pallette (H=R, S=G, O=B) to create the stars whereas I used SHO for the nebula.

James, you are right, the background is a bit too bright. I processed this image in daylight and on my monitor it looked quite dark but when I look at it now in the evening it looks too bright.  Attached is a darkened version, hopefully an improvement.

[William Bottaci]

Hello Trevor, another excellent image. From focus, guiding, choice of palette filters and number of exposures with time and finally processing. It's a chain in that each item is a link of which if one is weak then it can't be made up elsewhere; it all has to be right. And I agree you did well with the guiding on the C9.25 SCT even with the 0.63 reducer.

On guiding, the focal length of your setups should be about:
  -  Esprit 80ED with Skywatcher flattener: 400 mm
  -  C9.25 SCT with 0.63 reducer/flattener: 1,480 mm
so nearly four times as long, about four times more critical - well done.

For anyone not knowing:

HSO palette; Hydrogen shown as red, Sulphur as green, Oxygen as blue.
Sulphur is ionised by having lost 1 electron (counted as two 'ii's), Oxygen missing 2 electrons (counted as three 'iii's) and Hydrogen is not ionised but its electron moves about the atom.

The Antlia filters: Each of the three gases shine at a very specific (thin slice of the spectrum) colour, and so the filter should let through only that light as anything else would not be the nebula, say street lighting or moonlight. The narrower the band of filter the better this is achieved, but the more difficult to manufacture so higher cost. Filters that let through only the nebula light would be prohibitive, even if they can be made, but then you'd be able to image in daylight, almost.

I hope this helps.

I do prefer the higher contrast version.

Thank you for sharing.
William

On Tue, 12 Mar 2024 at 18:25, 'drja...@aol.com' via croydonastro <croydo...@googlegroups.com> wrote:
Hi Trevor
I agree with Tim.  I once tried imaging with an SCT and got nowhere at all.  Long exposures with narrowband must be the most difficult approach within an already hard task.  A remarkable image.  
I wonder if a darker black point would add contrast to the structure and bring out more detail.  The background looks a bit light to my eyes, though I do have a tendency to overdo it.
James

On Mon, 11 Mar 2024 at 21:46, <tcos...@gmail.com> wrote:
Hi Trev
That’s another great image. Getting things like tracking and focus right is much more difficult with the SCT than a shorter focal length refractor but everything in your image is pin sharp. Your collimation and set up must be very good. Do you want to come and sort my Edge 11 out?! Btw what’s HSO?
Well done
Tim C

[drja...@aol.com]

Hi Trevor

Now that's a killer image! A great result that deserves to be published, Sky at Night Magazine.

The most commonly recommended method for collimating SCTs is to get to a good result on out of focus stars and then repeat progressively, starting closer to focus and with increased magnification. Seeing gets in the way of that and finding a long enough distance for an artificial star, eg 40m, is surprisingly hard. That's anyway assuming the rest of the optical train and the imaging sensor are in perfect collimation and not tilted.

For those into gadgets, the Ocal 3 electronic collimator shows you what the camera sees in great detail. But it still seems to be an art to get a good result, see the Youtube videos.

Moral? Better is the enemy of the good.

James

[Trev S]

Thanks for the kind comments.

James,  I have tried collimating on an artificial star. The best result was an LED torch shining on a ball bearing about 40 feet away at night. However, to get focus at this short distance meant using several extension pieces and the resulting collimation, while accurate with this configuration, was way out with the extensions removed and focussed on night sky objects. The tri-Bahtinov mask, while fiddly is the best I have managed to achieve so far.