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M87 jet seen at F10 but not F2

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#26 Rickster

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Posted 16 May 2019 - 06:33 PM

adg,

You raise an interesting question.  There are so many variables at play that an analytical answer is beyond this forum (although there is probably someone somewhere that can do it).  But I am willing to go out on a limb and say that I think an 80mm ED APO might be able to resolve the jet with longer exposure.  Resolution ultimately comes down to signal to noise ratio.  A larger aperture gathers signal (photons) faster and in doing so, drives up the signal (numerator).  On the other hand, a smaller aperture is less challenging to the mount and typically has better corrected optics.  This reduces the noise component (denominator).  Both increase the signal to noise ratio.  The final analysis would be a field test.  It would be interesting to see if there is someone on this forum that can resolve the jet with a small high quality refractor.

 

Having said all that, I should add that although it may be possible to resolve the M87 jet with 80mm of aperture, it would no doubt be easier with more aperture (as you recognized).

 

This subject, David vs. Goliath aperture, comes up periodically on CN.  And, as luck would have it, there is a thread currently running on the subject.  https://www.cloudyni...ng-vs-aperture/


Edited by Rickster, 16 May 2019 - 06:34 PM.

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#27 adg

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Posted 16 May 2019 - 11:04 PM

Rickster,

 

I find your comments and the thread that you point to extremely interesting!

 

Also, it appears that your hunch that an 80mm ED APO can detect the M87 jet with long exposure is correct: I just found the following by search, which seems to confirm your intuition:

 

https://www.astrobin...400997/?nc=user

 

Just barely, but quite amazing! (The image is taken using an 80mm f/6 and a 183MC camera, which give an image scale of 1''/px.)


Edited by adg, 17 May 2019 - 01:44 AM.

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#28 adg

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Posted 17 May 2019 - 02:15 AM

To summarize the results of this thread so far regarding detection of the M87 jet.  The M87 jet:

 

(A) Could not be detected when the image scale was 2.36''/px (undersampling) on a 200mm scope (post #1, Dave)

 

(B) Could be detected for the following image scale and aperture combinations: 0.47''/px on 200mm (post #1, Dave); 0.9''/px on 200mm (post #11, saguaro); 1.22''/px on 200mm (post #16, Dave); 1''/px on 80mm (see post #27).

 

From these, I am concluding (again) that on an 80mm f/6 scope, the 294MC will most likely be unable to detect the jet (due to undersampling, given the 2''/px image scale).

 

Question: If an 80mm ED APO f/6 is fitted with a 2x barlow (high quality), would the 294MC then, with the improved 1''/px image scale, be able to detect the M87 jet?

 

Again, an answer will help me choose my EAA gear.  The real answer may need a field test, but some experts here may be able to provide good insight (as Rickster did in post #26!).


Edited by adg, 17 May 2019 - 04:14 AM.


#29 descott12

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Posted 17 May 2019 - 06:09 AM

Just another wrinkle to throw into your calculations is that fact the local seeing conditions and the level of light pollution will likely affect whether you can see fine detail  or not.  I think I am in a  Bortle 6 zone.


Edited by descott12, 17 May 2019 - 09:49 AM.

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#30 mclewis1

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Posted 17 May 2019 - 09:33 AM

I find the jet "easy" with a C11 operating at f3.5 (1000mm focal length) using an IMX224 camera (Altair Hypercam) - this combo is delivering an image scale of around .85 arc/sec/pixel, and somewhat less "easy" (ok I really have to look carefully) with an ICX418 camera (analog Mallincam Xtreme) - this combo is delivering an image scale of over 2 arc/sec/pixel. Visibility certainly helps with an increase the size of the image (digitally zooming). I assume it would be a piece of cake with the IMX224 camera on the C11 at f10.

 

Remember that because the 418 based camera is an analog video setup the image scale calculation isn't entirely accurate for what I'm seeing on the video monitor where the scan lines in the NTSC signal are what determines the resolution per "pixel". The bottom line is that the resolution I'm seeing on the monitor is likely not as good as 2 arc/sec/pixel.

 

"Easy" to me is readily apparent live in 15s exposures (with just a little histogram and brightness/contrast tweaking, and some digital zooming). This is under relatively good suburban skies (mag 5 or so).

 

If the weather cooperates over the next month I'll try it with the IMX224 camera on the following shorter focal length and smaller aperture combinations ...

 

C6 @f3.5/f5/f10 - 500/750/1500mm (1.5/1/.5 arc/sec/pixel)

TMB115 refractor @f7 800mm (1 arc/sec/pixel)

80mm ED refractor @F7 560mm (1.4 arc/sec/pixel)


Edited by mclewis1, 17 May 2019 - 03:02 PM.

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#31 adg

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Posted 17 May 2019 - 01:44 PM

Just another wrinkle to throw into your calculations is that fact the local seeing conditions and the level of light pollution will likely affect whether you can see fine detail  or not.  I think I am in a  Bortle 6 zone.

Thanks Dave, I am in a Bortle 6 zone too, and certainly understand that when things are so close, conditions will determine the result.

 

As a newcomer to EAA/OAP investing in initial gear, I was about to settle on a 294MC camera plus a short focal length (480mm) scope, when I came across this thread.

 

The upshot of this thread seems to be this:  An image scale greater than 2''/px is too course to see the jet (first post of this thread), but finer image scales of ~1.2''/px or less are able to detect it.  Edit: This conclusion seems incorrect now.

 

I am still thinking of getting the 294MC + the 480mm focal length scope.  It will limit the image scale to 2''/px and so the jet cannot be seen with this combo, but my guess is that adding a high quality 2x barlow, which will bring down the image scale to 1''/px, will be able to detect the jet (under right conditions).

 

For example, your Evo 8 + HyperStar + 294MC combo could not see the jet (post #1), but I am guessing that throwing in a 2x barlow to this combo (yes, strange) would push it just across the boundary where the jet becomes visible.


Edited by adg, 18 May 2019 - 06:41 AM.


#32 adg

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Posted 17 May 2019 - 02:18 PM

Mark, it is very interesting that you can (barely, but still) detect the jet with the analog camera.  But, as you noted, it is difficult to interpret the image scale in this analog case.

 

If you are able to field test the jet with your smaller scopes and the IMX224 camera and post the results here, that will be very valuable.

 

I find the jet "easy" with a C11 operating at f3.5 (1000mm focal length) using an IMX224 camera (Altair Hypercam) - this combo is delivering an image scale of around .85 arc/sec/pixel, and somewhat less "easy" (ok I really have to look carefully) with an ICX418 camera (analog Mallincam Xtreme) - this combo is delivering an image scale of over 2 arc/sec/pixel. Visibility certainly helps with an increase the size of the image (digitally zooming). I assume it would be a piece of cake with the IMX224 camera on the C11 at f10.

 

Remember that because the 418 based camera is an analog video setup the image scale calculation isn't entirely accurate for what I'm seeing on the video monitor where the scan lines in the NTSC signal are what determines the resolution per "pixel". The bottom line is that the resolution I'm seeing on the monitor is likely not as good as 2 arc/sec/pixel.

 

"Easy" to me is readily apparent live in 15s exposures (with just a little histogram and brightness/contrast tweaking, and some digital zooming). This is under relatively good suburban skies (mag 5 or so).

 

If the weather cooperates over the next month I'll try it with the IMX224 camera on the following shorter focal length and smaller aperture combinations ...

 

C6 @f3.5/f5 - 500/750/1500mm (1.5/1/.5 arc/sec/pixel)

TMB115 refractor @f7 800mm (1 arc/sec/pixel)

80mm ED refractor @F7 560mm (1.4 arc/sec/pixel)



#33 mclewis1

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Posted 17 May 2019 - 03:18 PM

https://www.cloudyni...ures/?p=7732888

 

Comments and image from an earlier thread on capturing the jet. I used a C6 at ~f5 (750mm) with the Mallincam Xtreme. . The image was captured at 640x480 resolution (very close to the native resolution of the camera) so an image scale of somewhere around 2.5 arc/sec/pixel. It's pretty close (a bit better) than the live image on the video monitor.


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#34 adg

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Posted 17 May 2019 - 06:46 PM

Mark,

 

Your image shows that the jet can be captured at greater than 2 arcsecs/px image scale.

 

That disproves my previous conclusion (posts 28, 31) that 2 arcsecs/px may be too course to capture the jet.

 

Great thread, by the way.

 

[Note: 750mm f.l. and ~9um pixel size (avg, from Sony ICX418AKL datasheet) give ~2.5 arcsecs/px.]


Edited by adg, 17 May 2019 - 06:55 PM.


#35 adg

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Posted 18 May 2019 - 08:13 PM

In addition to Mark's analog video image, I found images of the M87 jet by AP folks bobzeq25 and schmeah in this related CN thread, which used small scopes and low magnification with image scale courser than 2 arcsecs/px (bobzeq25 used a 70mm scope at a Bortle 7 location, and schmeah's image scale was 3.4 arcsecs/px, see "example" at end of this post).  I also measured the distance between the nucleus of the M87 galaxy itself and the brightest spot of the jet closest to the nucleus to be at least 10 arcsecs (using this, this, this, and star charts).

 

So it now seems to me that it was probably wrong to think along the lines "Since the jet is very small (~1 arcsec?), we need a sufficiently fine image scale to see it".  Perhaps a better way to look at this might be the following:

 

[Aperture and image scale here may be red herrings:]

The M87 galaxy itself is a relatively bright source of light with an intensity distribution spread spatially.  The jet (or rather its bright point close to the galaxy) is also another light source, with another spatial intensity distribution.  The distance between the peaks of these two light distribitions is at least 10 arcsecs, many times greater than the diffraction limit of even the smallest telescopes, so aperture should be less of an issue in "splitting the two peaks".  Also, the image scale need not be as fine as I had initially thought; perhaps it is enough to be less than, say, half (Nyquist?) or one-third of the distance between the peaks.  Of course conditions/seeing will be a big factor, but suppose we have good conditions.

 

[The key factor may be the amount of dynamic range available within a certain luminosity bracket:]

A main difficulty in seeing the jet seems to be that the spread of M87's stronger-intensity spatial distribution tends to swamp that of the jet.  (This is just qualitative; to state this precisely would need quantitative/FWHM measurement data over the visible wavelengths.)  So to clearly distinguish the peaks of these two distributions it may be useful to capture using an appropriate "intensity bracket".  (EAA/AP folks will know better about how to do this; may be one needs to start with an optimal low gain setting and stop sub-frame exposure at an optimal point of time; sounds difficult.)  Also, this means that the dynamic range of the sensor can be an important factor (dynamic range = well-depth/read-noise; the ADC bit-depth can be a rough indicator of dynamic range).

 

An example AP image where the jet is caught at 3.4 arcsecs/px:

 

This Markarian's Chain image (AP, processed, not EAA) by Derek Santiago (CN member schmeah) was taken with a small 85mm f/5.3 scope, and a 7.4um pixel size camera, giving a "course" image scale of 3.4 arcsecs/px.  It is a 2048x2048 image over a 2 deg x 2 deg region of sky, where M87 lies in a small crop area.  Yet, it captures the jet remarkably well!  (I needed to adjust my image viewer's curve for luminosity-value/gamma, and then the jet stood out stunningly, pointing toward 5 o'clock.)  Note that very high quality equipment were used:  A Takahashi FSQ85 scope on an Avalon M-Uno mount, and a QSI 640 camera, which has CCD sensor and 16 bit ADC.


Edited by adg, 19 May 2019 - 05:07 PM.



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