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Night Vision experiments with the Fireworks galaxy NGC6946

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#1 alanjgreen

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Posted 10 August 2018 - 05:48 AM

This week, I have managed a few short sessions outside with the 20” dob and one of my recurring targets has been NGC6946 (Fireworks galaxy) which is well placed directly overhead.

 

Last night I managed to see two clear spiral arms curving back over the top of the galaxy and what looked like a circle of arms surrounding the galaxy core. I used the image stored within Sky Safari and oriented it to match the surrounding star patterns to confirm that I was not seeing things! There is another shorter arm that would come down underneath but this one was not seen.
The galaxy was pretty small at x38 magnification.

 

 

Tests attempted.

1. 55mm Plossl with no filter (f2 x38)
2. 55mm Plossl + 6nm Ha filter (f2 x38)
3. 35mm Panoptic + 6nm Ha filter (f3 x60)
4. 35mm Panoptic + 12nm Ha filter (f3 x60)
5. 35mm Panoptic with no filter (f3 x60)

6. Ethos10 without NV (f4.1 x200)

 

 

Actual Observations.

 

1. 55mm Plossl with no filter (WINNER)
Averted vision reveals two clear spiral arms curving back over the top of the galaxy and a circle of arms surrounding the core. No arm underneath visible. The galaxy was pretty small at x38 magnification.

 

2. 55mm Plossl + 6nm Ha filter (f2 x38)
No visible arms but surprisingly visible “nebula” style patch.

 

3. 35mm Panoptic + 6nm Ha filter (f3 x60)
View was too dark.

 

4. 35mm Panoptic + 12nm Ha filter (f3 x60)  (SPECIAL MENTION)
No visible arms but surprisingly visible “nebula” style patch. Better than the 55mm Plossl with 6nm filter. Surprise result!

 

5. 35mm Panoptic with no filter (f3 x60)
No visible arms but some arms hinted by scintillation dots in the right place. Galaxy has a better scale.

 

6. Ethos10 without NV

The hazy galaxy disc appeared much larger (x200) but no arms were visible within the patch.

 

Let me know if you have any success with NGC6946...

Alan


Edited by alanjgreen, 10 August 2018 - 05:51 AM.

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#2 Eddgie

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Posted 10 August 2018 - 06:12 AM

Good observations.  

 

I find the very best view of galaxies to be unfiltered.  Any filter I use almost always seems to cost some detail, and I think that I get my most detail on galaxies running fast.  Yes, they can get small, but I can often see details in the fast configurations that I miss in slower configurations.

 

So, speed or scale.   Use both, but don't be surprised to see that you will very often get the most detail in the smaller and brighter image.  

 

Sounds like you are having fun!



#3 TOMDEY

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Posted 10 August 2018 - 06:22 AM

Interesting, thanx for the tip! I've been viewing a LOT of galaxies lately, with NV on the JMI RB-16 binos (NV of both sides!)  My case, not afocal, just straight to the GaAs at prime focus F/4.5. I'll have to check out that one. Once the 36-inch telescope arrives (less than a month!) I'll have both direct and Afocal available! The 55mm Plossl will give me F/1.78 at 62x. Not THAT should certainly show something!  Tom



#4 alanjgreen

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Posted 10 August 2018 - 06:29 AM

Interesting, thanx for the tip! I've been viewing a LOT of galaxies lately, with NV on the JMI RB-16 binos (NV of both sides!)  My case, not afocal, just straight to the GaAs at prime focus F/4.5. I'll have to check out that one. Once the 36-inch telescope arrives (less than a month!) I'll have both direct and Afocal available! The 55mm Plossl will give me F/1.78 at 62x. Not THAT should certainly show something!  Tom

Yep, focal speed is the key to getting arms out of galaxies. Be prepared for the outer edges of the 55mm plossl fov to be crap but its whats in the centre of the FOV that counts.

 

The 36" will definately get the arms to show! Maybe you can also get that elusive smaller lower arm too?


Edited by alanjgreen, 10 August 2018 - 06:29 AM.

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#5 TOMDEY

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Posted 10 August 2018 - 07:25 AM

Yep, focal speed is the key to getting arms out of galaxies. Be prepared for the outer edges of the 55mm plossl fov to be crap but its whats in the centre of the FOV that counts.

 

The 36" will definately get the arms to show! Maybe you can also get that elusive smaller lower arm too?

Good to know, I had been wondering about that. Like, how can a lowly Plossl POSSIBLY accept an F/3.75 feed gracefully... Pass it to a GIANT exit pupil bla bla bla... So, I’m not at all surprised that the field gets crummy off-center... and that's OK, fine... long as I can be feeding the GaAs at F/1.8. I'm sure the 36 with NV will be awesome.

 

This also broaches the entire subject of Night Vision vs "regular" visual astronomy. The old purists (me included) discuss, ad nauseam, whether this or that 100-deg eyep forms an ever so slightly sharper image out at the edge. In the meantime, the NV guys are picking up stuff entirely invisible to the others!  Tom



#6 Jeff Morgan

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Posted 10 August 2018 - 11:51 AM

4. 35mm Panoptic + 12nm Ha filter (f3 x60)  (SPECIAL MENTION)

No visible arms but surprisingly visible “nebula” style patch. Better than the 55mm Plossl with 6nm filter. Surprise result!

 

I've always liked the 12nm. When h-alpha is called for 12nm is not always the best choice - but never a bad one.



#7 Jeff Morgan

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Posted 10 August 2018 - 11:52 AM

Yep, focal speed is the key to getting arms out of galaxies. Be prepared for the outer edges of the 55mm plossl fov to be crap but its whats in the centre of the FOV that counts.

 

Kind of like using a Brandon lol.gif


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