NGC 4236 - Barred spiral galaxy in Draco

[ONIKKINEN]

668 x 30s - 5h34min

NGC 4236 is a low surface brightness (magnitude 24/arcsec according to stellarium) barred spiral galaxy in the constellation of Draco close to the naked eye star Kappa Draconis. Plenty of PGC and other distant galaxies everywhere in the field of view. Haven't really seen this one before, and it doesn't seem to be a very popular target, so why not go for it myself?

On acquisition:

Shot with the gear in the signature: OOUK VX8, RisingCam IMX571 OSC, EQM-35, mini-pc etc. from a Bortle 6-ish area.

Shot on 2 different nights. On the first night conditions were really not great, bad seeing and very humid but moonless. Second night was also moonless but very good transparency and seeing, in fact i don't think i have seen better yet. Contemplating whether i should scrap the first night and add more of the good stuff, well maybe one day since this target never sets low enough in the sky to be troublesome. Guiding was great (RA only), as it is for this patch of the sky for me. No subs over 1 arcsec RMS stacked, and i believe the average could be as low as 0.7. Some subs on the better night were as good as 0.3 arcsec RMS for the duration of the exposure, safe to say i never expected to see numbers like this from the mount. Perversly as the target gets higher in the sky guiding gets worse, since the tube is more vertical and my mount really doesn't like that, so most of this is just after the meridian on the north side. left side of the shot has some tilt/spacing/collimation issues. Not sure at all which one or all of them is the cause, working on that.

Processing in: DSS - SIRIL - PS. Starless and starlayer stuff with StarXterminator, also some optical aberration surgery since the mag 5.8 star left a rainbow reflection in the bottom left corner of the shot. The background is quite "nuked" and remains a bit spotty/noisy, but it works for now.

I will probably return to this target one day, i feel like 5h34min is not quite enough for this target and there is probably more to see.

Feel free to comment/advice if you want, still getting to grips with, well everything in astrophotography.

[alan potts]

Not one I have seen done before, and to my eye it looks very nicely done too. Might just try this one myself

Alan

[ONIKKINEN]

4 hours ago, alan potts said:

Not one I have seen done before, and to my eye it looks very nicely done too. Might just try this one myself

Alan

Thanks, although more southern photographers might have to wait a few months for a better opportunity.

NINAs sky atlas told me about it, otherwise i doubt i would have just stumbled upon it. At 20 arcmin should be shootable for many kinds of telescopes!

[ONIKKINEN]

I think i actually prefer the non-altered background version. Actually i definitely prefer it, so here it is. Trying to remove the rainbow reflection is probably what killed the background in the first round.

[The Lazy Astronomer]

I like the little rainbow, it can be your signature on the image 😁

[ONIKKINEN]

4 hours ago, The Lazy Astronomer said:

I like the little rainbow, it can be your signature on the image 😁

It adds some flair for sure. What im not sure if its from the HD-something star in the image or Kappa Draconis a few degrees to the bottom that caused this. Havent seen one like this before so there must be a tight spot where it can occur at a specific off angle.

[Spitfire]

I like your images but I prefer the first without the rainbow.

You're right, it is quite faint at Mag 10, but what is interesting is that there is another galaxy that sits on top of it (NGC4236-2) at mag 15. I took an image of them both some 11 years ago with my Vixen R200SS. It took 23 hours in Luminance and my processing then was even poorer than it is now! But just to show how technology has improved since then, I have attached my image from then. I think yours with just 5 hours is far better then mine, and you're right that some more data will improve it further.

Geoff

[ONIKKINEN]

45 minutes ago, Spitfire said:

I like your images but I prefer the first without the rainbow.

You're right, it is quite faint at Mag 10, but what is interesting is that there is another galaxy that sits on top of it (NGC4236-2) at mag 15. I took an image of them both some 11 years ago with my Vixen R200SS. It took 23 hours in Luminance and my processing then was even poorer than it is now! But just to show how technology has improved since then, I have attached my image from then. I think yours with just 5 hours is far better then mine, and you're right that some more data will improve it further.

Geoff

Thanks Geoff. I noticed this technology gap while browsing astrobin for various targets and it looks like comparing today's images to even a few years back is a very uneven playing field. I wonder how astrophotography looks like 10 years from now if the same improvements keep on coming.

23 hours is a LOT of exposure for a single target, not sure i have 23 hours in total 😅.

Edited November 11, 2021 by ONIKKINEN

[GalaxyGael]

Lovely image and an interesting galaxy. I've added it to my to-do list, thanks.

The rainbow anomaly in curious, since it is reminiscent of dispersion through a glass element. The perfectly circular but thinner rainbow would confirm this. this type of issue can be tracked down and eliminated.

Are you sure it was definitively light form the mag 5.8 star? It is odd that it would cause dispersion like this. If it off-axis light from that star, it might be dispersing through a very fine mist of water droplets on that part of the sensor in order to see the circular feature.

 

Very off-axis light through a thick highly curved coma corrector could approximate a large single raindrop, but that is more typical of light hitting the curved side, which is facing the camera. these anomalies are curios to me  Did you see them before?

 

 

[ONIKKINEN]

5 hours ago, GalaxyGael said:

Lovely image and an interesting galaxy. I've added it to my to-do list, thanks.

The rainbow anomaly in curious, since it is reminiscent of dispersion through a glass element. The perfectly circular but thinner rainbow would confirm this. this type of issue can be tracked down and eliminated.

Are you sure it was definitively light form the mag 5.8 star? It is odd that it would cause dispersion like this. If it off-axis light from that star, it might be dispersing through a very fine mist of water droplets on that part of the sensor in order to see the circular feature.

 

Very off-axis light through a thick highly curved coma corrector could approximate a large single raindrop, but that is more typical of light hitting the curved side, which is facing the camera. these anomalies are curios to me  Did you see them before?

 

 

Thank you!

I am hesitant to try and "fix" the issue since there are so many possible surfaces to reflect from. All coated of course, but still possible. 3 glass elements in the coma corrector, both the telescope side and camera side are visibly curved (0.95x TS maxfield corrector), UV/IR cut filter, AR-coated sensor window and of course the sensor surface itself. I can probably improve the process of removing it in post if i practice, so its only a critical issue if it obstructs the actual target itself. Here is a picture showing my CC in the focus position, it does not obstruct and is sufficiently covered to not have really any "direct" reflections. (dew shield on while imaging also). The comacorrector is open only to the opposite side where there are no bright stars. Dew in the sensor would be bad news, but i don't think that's the case ( i hope not). The sensor is sealed with a desiccant tube thingy in there and the number of stars/median ADU/HFR readings from NINA i use to determine sub quality show no jumps. If there were jumps, the subs were scrapped. But very fine mist not big enough to notice? Could be, ill have to check the desiccant if its saturated.

 

I am not so sure at all it was the top left star in the image. It could also be 3 stars just below the image that are also quite bright. In fact it probably is, since i have not seen on-axis stars produce something like this before.

I have seen something like these reflections during go-tos to bright stars for focus. The first go-to is always a couple of degrees off and can show some reflections, but once centered they disappear. The Moon obviously also causes these when anywhere in the general direction, but not rainbow coloured.

[GalaxyGael]

Its curious. the fact that is is a rainbow and a fully circular one, is very characteristic of dispersion through tiny droplets or from a glass element. not likely happening on the mirrors unless there is a small patch of dew forming on the underside of the corrector maybe?