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Finest useful image scale?

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#1 Kendahl

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Posted 12 June 2019 - 04:06 PM

Image scale with my telescope and camera is 1.13 arc-secs/pixel. That makes M57 (Ring Nebula) only 90 pixels. By borrowing my wife's 8" SCT, I could reduce image scale to 0.44 arc-secs/pixel. But, with typical seeing at 2 arc-secs, would I accomplish anything other than to narrow my field of view?



#2 DuncanM

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Posted 12 June 2019 - 04:29 PM

Image scale with my telescope and camera is 1.13 arc-secs/pixel. That makes M57 (Ring Nebula) only 90 pixels. By borrowing my wife's 8" SCT, I could reduce image scale to 0.44 arc-secs/pixel. But, with typical seeing at 2 arc-secs, would I accomplish anything other than to narrow my field of view?

 

You would probably gain a bit in terms of resolution, especially because the resulting SCT image will be oversampled to a greater degree and consequently will be more amenable to image restoration via deconvolution. Additionally your light grasp is greater and it should record faint stars and nebula more readily.

 

Good focus will be critical.



#3 rgsalinger

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Posted 12 June 2019 - 04:29 PM

Yes but not as much as you would expect. Assuming that the optics are effectively the same, you will get dimmer details with the larger scope. You will get more stars with the larger scope. As you will be oversampled you will be getting as much detail as possible. Ideally, my bet is that with a focal reducer taking you down to more like .7 A/S pixel you be even better off - more signal per pixel with no loss of detail and the same number of dim stars. 



#4 sg6

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Posted 12 June 2019 - 05:33 PM

M57 is given as 230 arc sec, at 1.13 arc sec per pixel that means 230/1.13 pixels = 203 pixels.

Well that is my reckoning, just 90 seemed small, 203 isn't a lot either.



#5 Stelios

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Posted 12 June 2019 - 05:58 PM

Use an 0.63x reducer with your wife's SCT (0.7x if it's an Edge) and you will have approx 0.7 arcs/px scale which will give you about 330 px for the object. Not much, but it is what it is. 

 

I've seen people use lucky imaging techniques with this object, taking many hundreds of short (1-5") frames and stacking them. M57 is so bright it's visible even with quite short exposures. 



#6 Kendahl

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Posted 12 June 2019 - 06:57 PM

M57 is given as 230 arc sec, at 1.13 arc sec per pixel that means 230/1.13 pixels = 203 pixels.

Well that is my reckoning, just 90 seemed small, 203 isn't a lot either.

One would think that there would be agreement on the sizes of such well studied objects as the Messier catalog. Not so. I found the following:

  • Starry Night: 1.7'
  • Stellarium: 3.8' x 2.4'.
  • Wikipedia: 3.8'
  • messier-objects.com: 1.5' x 1.0'
  • constellationguide.com: 1.5' x 1.0'
  • Several sites: 1.4' x 1.0' (messier-seds.org, universetoday.com, freestarcharts.com, astropixels.com)

In my own photo of M57, the long dimension is 71 pixels or 63".

 

 

In any case, the point of my question is how much below seeing limits is it worth going before you are just pointlessly subdividing an amorphous blob? I can understand half of seeing to satisfy the Nyquist sampling rule. Maybe, one third. But no farther.



#7 TelescopeGreg

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Posted 12 June 2019 - 08:29 PM

One would think that there would be agreement on the sizes of such well studied objects as the Messier catalog. Not so. I found the following:

  • Starry Night: 1.7'
  • Stellarium: 3.8' x 2.4'.
  • Wikipedia: 3.8'
  • messier-objects.com: 1.5' x 1.0'
  • constellationguide.com: 1.5' x 1.0'
  • Several sites: 1.4' x 1.0' (messier-seds.org, universetoday.com, freestarcharts.com, astropixels.com)

In my own photo of M57, the long dimension is 71 pixels or 63".

 

 

In any case, the point of my question is how much below seeing limits is it worth going before you are just pointlessly subdividing an amorphous blob? I can understand half of seeing to satisfy the Nyquist sampling rule. Maybe, one third. But no farther.

Can't even agree on Wikipedia:

 

https://en.wikipedia...iki/Ring_Nebula says it's "apparent size" is 230" x 230", or am I looking at the wrong spec?



#8 rgsalinger

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Posted 12 June 2019 - 10:29 PM

The Wikipedia may be counting some of the very faint stuff that you don't ordinarily see when you image the ring nebula. That's still only just under 4 arc minutes. If you look closely (sorry about the jpg quality) you can see the outer shell. It may be that an even bigger telescope would show more nebulosity. M57_PSD.jpg  


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#9 bobzeq25

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Posted 13 June 2019 - 01:01 AM

Here's M57 at 1.0 arc sec per pixel.  Not too bad, I think.

 

https://www.astrobin.../full/355567/0/

 

Reducing that by 1/2 will either reduce signal to noise ratio or require 4X the imaging time to get the same SNR.  Resolution isn't everything.

 

Do you have an off axis guider?  Is your wife's SCT an Edge?

 

As stated above there are some advantages to a numerically smaller image scale.  It's worth a try, if you don't have to sink too much money into it.



#10 james7ca

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Posted 13 June 2019 - 06:54 AM

You need pretty good seeing to go under 1 arc second per pixel. However, given good seeing you can go significantly under that image scale to gain both detail and size.

 

Below are example images of the Ring Nebula that I took using 0.75 arc seconds per pixel with a five inch refractor and 0.21 arc seconds per pixel using a 9.25" EdgeHD. The image with the refractor was done in H-alpha so it shows more detail in the structure of the nebula and because the image was fairly sharp I enlarged it so that the reproduction scale is actually only 0.5 arc seconds per pixel. This image has 84 minutes of total integration time and it's just beginning to show the outer halo that surrounds the more commonly seen central ring.

 

Here is the link to the original post using the refractor (contains more information about the capture):

 

  https://www.cloudyni...dpost&p=7976663

 

The image done with the EdgeHD (at f/10) was captured at just 0.21 arc seconds per pixel and I reduced the size of the image for posting to 0.3 arc seconds per pixel. This was done in simple luminance and is just 8 minutes of total integration time (using a series of 4 second subs). The image doesn't show as much structure in the ring (the previous H-alpha did better), but it does clearly resolve the close double star that appears toward the upper left, close to a brighter member of another double (kind of a double-double).

 

I'm not sure what the separation is between this closer pair, but it is VERY seldom resolved in any images of the Ring Nebula. When I measure the separation it comes out to about 1.6 arc seconds, which isn't that impressive given that I've imaged under 1 arc second on double stars with both the 5" refractor and the EdgeHD (using lucky imaging techniques). However, for comparison the closest pair in the famous double-double star in Lyra is just about 2.3 arc seconds.

 

You can also see that this closer double is just barely suggested in the image done with the 5" refractor (appears as a faint line attached to one of the brighter stars).

 

Here is the link to the original post using the EdgeHD:

 

  https://www.cloudyni...l/#entry7325536

 

[UPDATE]

By measuring an image that had been taken by the Hubble Space Telescope I determined the separation of the tight double star as being 1.3 arc seconds. The brighter pair seems to be the double star HL 9001AB (a component of WDS18536+3303) which is listed as having a separation of 5.02 arc seconds. In my image that wider pair was measured at 4.95" and the Hubble image returned 4.9".

 

I've yet to find any mention of this closer double on the internet, although it certainly appears in the Hubble images and in some of the higher resolution images taken by amateurs. Also, I certainly don't know whether it is a real, physical double or just a chance alignment of two background stars. I also wonder whether it could be associated in any way with the multiple star system WDS18536+3303.

 

One last point, the magnitude of each component on this close pair of stars seems to be around 17 (could actually be between 17 and 18). Based upon WikiSky the image taken with the EdgeHD recorded starts well into magnitude 18, perhaps even close to 19. So, this double is probably beyond visual detection with just about any amateur telescope (this may be why it isn't mentioned anywhere on the internet, it's just too faint).

[/UPDATE]

Attached Thumbnails

  • Ring Nebula at 0.5 arc seconds per pixel.jpg
  • Ring Nebula at 0.3 arc seconds per pixel.jpg

Edited by james7ca, 14 June 2019 - 04:40 AM.

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