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January 13 First Quarter Moon with a 72mm Refractor

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#1 james7ca

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Posted 15 January 2019 - 08:19 AM

Continuing with my series on the moon using small refractors and a ZWO ASI183MM Pro camera.

 

Previous links in the series:

 

SV50ED
  https://www.cloudyni...m/#entry9037374

 

AT72ED

  https://www.cloudyni...r/#entry9071789

 

  https://www.cloudyni...m/#entry9042580

 

 

My most recent image (below) taken on the evening of January 13, 2019 using a Astro-Tech AT72ED (72mm aperture, f/6), a Tele Vue 2X Powermate, a ZWO ASI183MM Pro camera, and a stacked set of Baader Semi-APO and 610nm Long Pass filters (to limit the bandwidth to between 610nm and 675nm). Best 10% of 3000 frames using Autostakkert!, sharpening and histogram adjustments in PixInsight with final tweaks with Photoshop CC2017. Tracking was done on a Celestron AVX.

Attached Thumbnails

  • First Quarter Moon with AT72ED (Small).jpg

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#2 james7ca

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Posted 15 January 2019 - 08:54 AM

And here is a crop to better show the detail captured with this small refractor.

 

I'm still waiting for a night with good seeing to test how much detail this setup can actually reveal. When (or if) that ever happens I'll probably switch to a green filter to improve the resolution potential.

Attached Thumbnails

  • First Quarter Moon with AT72ED (Crop).jpg

Edited by james7ca, 16 January 2019 - 02:52 AM.

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#3 Jon Rista

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Posted 15 January 2019 - 01:29 PM

I was pretty surprised at how much more detailed RGB-only data was vs. L. The L data, even at the sharpest I could get it, was visibly softer than the RGB only data. Seeing was average, not great, but not particularly bad. So I wouldn't be surprised if the green filter or any of the color filters, helped improve your detail.



#4 james7ca

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Posted 15 January 2019 - 07:01 PM

I was pretty surprised at how much more detailed RGB-only data was vs. L. The L data, even at the sharpest I could get it, was visibly softer than the RGB only data. Seeing was average, not great, but not particularly bad. So I wouldn't be surprised if the green filter or any of the color filters, helped improve your detail.

Yes, green would help but I'm using a fairly inexpensive ED scope (not an APO) so I have to be careful, I think, about using too broad of a bandpass. The above image was taken in a bandpass between 610-675nm (red/near IR) so Dawes' limit would probably be about 2 1.8 arc seconds and I think I can just barely detect Aldrin [Collins] crater which is just under 2 arc seconds in diameter. I definitely resolved Armstrong crater and that's 2.4 arc seconds. There are a few small craters around Rima Hyginus that are clearly resolved, but I suspect those are all over 2 arc seconds but I'm going to check using LRO images.

 

I have a Baader Solar Continuum filter that has a 10nm bandpass at 540nm which should be near to the best corrected wavelength on this objective, but it cuts a lot of light so my exposures will have to be longer and so I'll need some really steady seeing to make use of this filter. I already know from my work on close double stars that imaging in the blue can make a big improvement in resolution, so I think given good conditions that this little 72mm refractor can do even better.

 

[UPDATE]

I can't really see Aldrin Collins in this shot, although I think I did detect it one day earlier when the lighting wasn't as flat.

[/UPDATE]


Edited by james7ca, 16 January 2019 - 06:56 AM.


#5 james7ca

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Posted 15 January 2019 - 07:54 PM

So, I was able to measure a few craters around Rima Hyginus and I clearly resolved one or two that were about 2.3 arc seconds in diameter and I detected one that was just over 2 arc seconds.

 

Below is the image from LRO at a 125m per pixel scale. I actually measured the craters at a 16m per pixel scale which means that they appeared over 200 pixels in diameter (for the measurement). The crater that I considered "detected" is a little out of round, so I average the largest and smallest diameters which gave about a +/- 0.1 arc second range.

 

Obviously, the lighting and perceived positions of the rim create some uncertainty in these measurements, but I suspect that my LRO measurements are correct to about that same +/- 0.1 arc second range (that's near to +/- 200m).

 

Unfortunately, the USGS moon survey site is offline because of the government shutdown, so I can't identify (name) any of these craters. Wikipedia shows that the craters Hyginus C through H are all in the range of 4 to 5 kilometers, so some of those could be candidates. The crater resolved at 2.25 arc seconds is 4.26km, the crater that I labeled as detected is 3.84km.

Attached Thumbnails

  • LRO Image at 125m Per Pixel.jpg

Edited by james7ca, 15 January 2019 - 08:19 PM.

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#6 Jon Rista

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Posted 15 January 2019 - 08:03 PM

Yes, green would help but I'm using a fairly inexpensive ED scope (not an APO) so I have to be careful, I think, about using too broad of a bandpass. The above image was taken in a bandpass between 610-675nm (red/near IR) so Dawes' limit would probably be about 2 arc seconds and I think I can just barely detect Aldrin crater which is just under 2 arc seconds in diameter. I definitely resolved Armstrong crater and that's 2.4 arc seconds. There are a few small craters around Rima Hyginus that are clearly resolved, but I suspect those are all over 2 arc seconds but I'm going to check using LRO images.

 

I have a Baader Solar Continuum filter that has a 10nm bandpass at 540nm which should be near to the best corrected wavelength on this objective, but it cuts a lot of light so my exposures will have to be longer and so I'll need some really steady seeing to make use of this filter. I already know from my work on close double stars that imaging in the blue can make a big improvement in resolution, so I think given good conditions that this little 72mm refractor can do even better.

Yeah, I was noticing a similar thing, that bluer wavelengths seemed to resolve more detail. I was kind of surprised by that, as everyone talks about seeing being better in red than blue...but, maybe my seeing wasn't bad enough in general for that to matter.



#7 Tom Glenn

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Posted 15 January 2019 - 11:29 PM

Nice result James.  On the LRO Quickmap site, if you click on the "Layers" tab on the left, and then click on "Nomenclature", you can get all of the named features.  If you then click on the name, you can get data for any crater that has a name.  See below.  Your resolved crater is Hyginus C, diameter 4.15km, and the detected crater is Hyginus F, diameter 3.81km.  I'd be interested to see the full size crop of this region from your image, to see what kind of resolution you are talking about.  Your presentation scale above looks excellent, but has been reduced too much to see this region clearly.  

 

LRO_Hyginus.jpg

HyginusC.jpg

HyginusF.jpg


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#8 Tom Glenn

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Posted 15 January 2019 - 11:33 PM

Actually, now that I'm looking closely at your image, I can see those craters, so maybe this is the full sized (or close enough) image in the crop above.  



#9 james7ca

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Posted 16 January 2019 - 12:22 AM

Tom, thanks for that tip on using the LRO website (but, to get to that specific option you have to select "Layers" -> "Overlays" ->  "Nomenclature (New)"). It looks like the "Nomenclature" values are very close to what I determined, but I measured at least two different axes and in the case of Hyginus F the crater isn't completely round so I took an average to arrive at my 3.84km (a difference of just 0.03km).

 

Interestingly, it looks like LRO is referencing the USGS website to get the names, but as I mentioned earlier that site seems to be down.

 

The crop in post #2 isn't full scale, but it is close enough that you can see all of the detail (if you look carefully). The original scale is just larger, but it doesn't look as sharp.


Edited by james7ca, 16 January 2019 - 02:55 AM.


#10 james7ca

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Posted 16 January 2019 - 01:40 AM

Here are some additional data points (the Rayleigh Criterion or resolution limit for a 72mm aperture at various wavelengths of light). The calculation is done by:

 

 theta (radians) = 1.22 wavelength/ D

 

-- and --

 

arc seconds = (radians x (360 / 2pi)) x 60 arc minutes per degree x 60 arc seconds per arc minute

 

-- and where --

"D" will be 72mm = 72,000,000nm

 

Or, simplifying all of the above (constants and unit conversions):

 

arc seconds = 0.252 wavelength(nm) / D(mm)

 

Rayleigh Criterion for 72mm aperture (0.252 wavelength(nm) / 72mm))

===============================================

635nm (center of the 610-675nm bandpass): 2.22 arc seconds

562nm (typically used for Dawes' Limit): 1.96 arc seconds

540nm (Baader Solar Continuum filter): 1.89 arc seconds

495nm (Baader H-beta): 1.73 arc seconds

455nm (center line Baader Blue CCD filter): 1.59 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

 

Note that Dawes' Limit is a slightly more stringent test than the Rayleigh Criterion and is usually represented as:

 

Dawes' Limit = wavelength / D

 

Thus, the conversion factor is just (Rayleigh Criterion / 1.22) and going back to the earlier simplification of the Rayleigh Criterion we have:

 

Dawes' Limit = 0.206 wavelength(nm) / D(mm)

 

Dawes' Limit (as "R") is also commonly written as (when converting diameter, D, from millimeters to centimeters):

 

R = 11.6 / D

 

where D is in centimeters and thus for the 7.2cm AT72ED that comes to:

 

R = 11.6 / 7.2 = 1.61 arc seconds

 

Which is equal to the earlier Rayleigh criterion at 562nm when divided by 1.22, thus:

 

1.96 arc seconds / 1.22 ≈ 1.61

 

So, given the earlier Rayleigh Criterion calculations a table for Dawes' Limit at various wavelengths is as follows.

 

Dawes' Limit for 72mm aperture (0.206 wavelength(nm) / 72mm)

===============================================
635nm (center of the 610-675nm bandpass): 1.82 arc seconds
562nm (typically used for Dawes' Limit): 1.61 arc seconds

540nm (Baader Solar Continuum filter): 1.55 arc seconds
495nm (Baader H-beta): 1.42 arc seconds
455nm (center line Baader Blue CCD filter): 1.30 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

 

Of course, both the Rayleigh Criterion and Dawes' Limit are based upon resolving two equal magnitude point sources (i.e. stars) and a crater on the moon isn't that. Furthermore, it's easier to see a line-type feature (fault line or rille) than it is a crater or point source so all of these limits need to be taken with some latitude.

 

In terms of going from 635nm to 455nm, that's equivalent in terms of resolution to switching between a 72mm aperture and a 100mm (approximately 3 inch to 4 inch refractors).

 

Now, how about my wished for 4" f/8 Newtonian?

 

Dawes' Limit for 101mm aperture (0.206 wavelength(nm) / 101mm)

===============================================
635nm (center of the 610-675nm bandpass): 1.30 arc seconds
562nm (typically used for Dawes' Limit): 1.15 arc seconds
540nm (Baader Solar Continuum filter): 1.10 arc seconds
495nm (Baader H-beta): 1.01 arc seconds
455nm (center line Baader Blue CCD filter): 0.93 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

 

This MIGHT get me down to about 1 arc second in blue light on a night with nearly perfect seeing conditions and that would represent a crater of about 1.9km (significantly better than the 4.26km that I did with the 72mm refractor in red light). More realistically and given less than perfect seeing I might expect something around 1.3 arc seconds or 2.5km (resolved). In any case, I think that could exploit the full resolution and scale of my IMX183 sensor (given my desire to capture the entire disk of the moon in a single framing).

 

Note, I've clearly split an approximately 0.92 arc second double star with my Tele Vue NP127is (5") refractor using a Baader Blue CCD filter. However, with the red CCD filter the star was just an elongated barbell shape (below, my image of the double star SAO 46152, more on the latter).

Attached Thumbnails

  • RGB Resolution Example.jpg

Edited by james7ca, 16 January 2019 - 06:29 AM.


#11 james7ca

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Posted 16 January 2019 - 03:21 AM

As another point of reference, back in September 2017 I image the area around Ptolemaeus with a 9.25" EdgeHD and an ASI178MM camera with a Baader 610nm Long Pass filter and I detected/resolved craters down to about 0.9km which would have been 0.5 arc seconds. Using the calculations above for the 235mm aperture I get the following for red/near IR:

 

Dawes' Limit for 235mm aperture (0.206 wavelength(nm) / 235mm)

===============================================

635nm (center of the 610-675nm bandpass): 0.56 arc seconds

 

So, on that night when I had good seeing I was within 10% (a little better) of Dawes' Limit.

 

Here is a link to that image Three Amigos with an EdgeHD and an ASI178MM



#12 lynnelkriver

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Posted 16 January 2019 - 09:58 AM

Love the images.  I can see a small refractor in my future!  Scott



#13 Alnitak2009

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Posted 16 January 2019 - 06:28 PM

Nice image. Very sharp and contrasty! The large flat maria area looks like a youth ballplayer to me:)

 

Don



#14 james7ca

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Posted 16 January 2019 - 07:15 PM

lynnelkriver and Don (Alnitak2009), thanks for the notice.

 

The only shape I've seen invariably in the maria is the so-called "Lady in the Moon" (profile). You can see the back of her head and hair and her eye in this lunar phase but "she" is best seen around the full moon.




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