Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

January 12 Crescent Moon with a 72mm Refractor

  • Please log in to reply
15 replies to this topic

#1 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 13 January 2019 - 03:58 AM

My Astro-Tech AT72ED filtered using a stacked set of Baader 610nm Long Pass and Semi-APO filters with a Tele Vue 2X Powermate and a ZWO ASI183MM Pro camera (ending bandpass 610nm to 675nm). Tracking was done on a Celestron AVX.

 

Best 10% of 2000 frames using AutoStakkert!, sharpening and histogram adjustments in PixInsight with final tweaks with Photoshop CC2017.

 

This is a fairly good time of the year to capture the waxing crescent moon since at 6-days of age it is now just past the position of the spring equinox (and moonset was almost due west). The moon's altitude was just over 48 degrees when this sequence was captured. On Sunday night the first quarter moon will be about 60 degrees above the horizon shortly after sunset (for 33 degrees north latitude, local transit is at 5:41PM PST). In April the situation will be even better since the first quarter moon will be almost 78 degrees above the horizon at sunset.

Attached Thumbnails

  • Crescent Moon with AT72ED and ASI183MM (Small).jpg

Edited by james7ca, 13 January 2019 - 05:03 AM.

  • Carol L, siriusandthepup, Achernar and 12 others like this

#2 Gary Z

Gary Z

    Apollo

  • *****
  • Posts: 1392
  • Joined: 26 Jan 2012
  • Loc: New Mexico

Posted 13 January 2019 - 04:58 AM

Fantastic!!!

 

Gary



#3 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 13 January 2019 - 04:59 AM

I found an image that I took 7 years ago using this same scope but with a Sony NEX-5N camera using eyepiece projection. At that time I didn't even have a mount so this was taken on a fixed stand with the AT72ED and camera mounted on a barely movable steel rail using bolts. Taken on February 28, 2012 when the moon was nearly in the same phase as last night. Processing was done in 2012 with PixInsight and Apple's Preview application (from a single still image).

 

The newer image is sharper which is kind of expected given the change in the camera and the use of lucky imaging techniques. In the 2019 image you can clearly see Armstrong crater which has a diameter of 4.6km and that suggests a resolution of better than 2.4 arc seconds (Dawes' limit for AT72ED is about 1.6 arc seconds, but this image was taken in red light/near IR so the limit would likely be closer to 2 arc seconds, at least on an equal magnitude double star and lunar craters aren't really the same). In the original I'm almost sure that I detected Aldrin crater at 3.8km, which would be just under 2 arc seconds.

 

Here are my original comments from Flickr (as posted back in 2012):

 

 

The waxing crescent moon as photographed at 7:45PM PST on February 28, 2012.  Captured with an Astro-Tech AT72ED telescope (2.8"/72mm aperture, 430mm prime focal length, f/6) coupled to a Sony NEX-5N digital camera (ISO 100, 1/20 second, afocal projection, 10mm eyepiece, Minolta Rokkor-X 24mm 1:2.8 lens).

....

[links to Flickr, removed]

....

RAW processing, histogram adjustments, deconvolution (sharpening), and image resizing done in PixInsight v01.07.05.0779 with additional tweaks using Mac OS X's Preview application.

 

All rights reserved.

Attached Thumbnails

  • Crescent Moon from Feb 2012.jpg

Edited by james7ca, 13 January 2019 - 05:59 AM.


#4 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 13 January 2019 - 06:00 PM

Gary, thanks.


  • Gary Z likes this

#5 Tom Glenn

Tom Glenn

    Surveyor 1

  • -----
  • Posts: 1902
  • Joined: 07 Feb 2018
  • Loc: San Diego, CA

Posted 14 January 2019 - 12:43 AM

Nice image James, and interesting comparison to prior results.  In some ways, 7 years doesn't seem like that long ago, but with regard to available cameras and processing techniques, quite a lot has happened in the interim.  



#6 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 23591
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 14 January 2019 - 12:50 AM

Nice work, James! The 183 really brings out some amazing detail even with more modest scopes.

 

The NEX did a pretty nice job 7 years ago, too!



#7 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 14 January 2019 - 01:44 AM

Tom and Jon, thanks for the notice.

 

The IMX183 really isn't that much different than my IMX178 cameras (which I've used extensively for lunar and planetary imaging), but the IMX183 is large enough that I can capture the full disk of the moon at a focal length of about 830mm (with a little bit to spare during the super moons and for some margin of error for tracking). It also happens that with the AT72ED I get critical sampling (lum/green) at about that same focal length (at f/12 and 72mm x 12 = 864mm).

 

So, whenever or if ever I get some good seeing conditions it will be interesting to see what kind of results I get within a single frame. What I'd really like to have is a 4" f/8 Newtonian since that would give me 102mm x 8 = 816mm of focal length with fully color-corrected optics (all reflective). I know that Tom has a small reflector that he uses for lunar imaging, but they seem to be hard to find and if I get really ambitious I may try to make my own.


  • Jon Rista likes this

#8 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 23591
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 14 January 2019 - 03:04 AM

 

So, whenever or if ever I get some good seeing conditions it will be interesting to see what kind of results I get within a single frame. What I'd really like to have is a 4" f/8 Newtonian since that would give me 102mm x 8 = 816mm of focal length with fully color-corrected optics (all reflective). I know that Tom has a small reflector that he uses for lunar imaging, but they seem to be hard to find and if I get really ambitious I may try to make my own.

Yeah, I would love to have an 8" f/4 newt as well. I was debating which scope to get, FSQ106 or 8" ONTC, and ended up ordering the FSQ this time around. Newt is the next thing.

 

I am curious, though...could an 8" newt be used without a coma corrector to image the moon? 



#9 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 14 January 2019 - 03:22 AM

I don't think an f/4 Newtonian would be a good choice for photographing the moon. In fact, f/8 might be a stretch in terms of covering a sensor as large as the IMX183 (for the diagonal, which is approximately 15mm, but I'll need just the center portion of the full frame -- see below).

 

According to a post by Jon Isaacs the coma free field (diameter) for Newtonian reflector can be computed as follows:

 


CFR = 0.022 x F^3 mm where F is the focal ratio of the scope...

However, I don't know what his criteria is for "coma free."

 

So, at f/4 we have: 0.022 x 4^3 ≈ 1.4mm

 

And at f/8 we have: 0.022 x 8^3 ≈ 11.2mm

 

This compares to the height of the IMX183 of 8.8mm and thus f/8 should be "safe" as long as the moon remains centered in the field.

 

I'm not sure whether my 6" f/6 Newtonian can cover the entire field of the IMX178 and the calculations suggest:

 

0.022 x 6^3 ≈ 4.7mm

 

And the size of the IMX178 is 7.4mm x 5mm with a diagonal of 8.9mm.


Edited by james7ca, 14 January 2019 - 03:30 AM.

  • Jon Rista likes this

#10 Tom Glenn

Tom Glenn

    Surveyor 1

  • -----
  • Posts: 1902
  • Joined: 07 Feb 2018
  • Loc: San Diego, CA

Posted 14 January 2019 - 03:48 AM

I don't think an f/4 Newtonian would be a good choice for photographing the moon. In fact, f/8 might be a stretch in terms of covering a sensor as large as the IMX183 (for the diagonal, which is approximately 15mm, but I'll need just the center portion of the full frame -- see below).

 

According to a post by Jon Isaacs the coma free field (diameter) for Newtonian reflector can be computed as follows:

 

However, I don't know what his criteria is for "coma free."

 

So, at f/4 we have: 0.022 x 4^3 ≈ 1.4mm

 

And at f/8 we have: 0.022 x 8^3 ≈ 11.2mm

 

This compares to the height of the IMX183 of 8.8mm and thus f/8 should be "safe" as long as the moon remains centered in the field.

 

I'm not sure whether my 6" f/6 Newtonian can cover the entire field of the IMX178 and the calculations suggest:

 

0.022 x 6^3 ≈ 4.7mm

 

And the size of the IMX178 is 7.4mm x 5mm with a diagonal of 8.9mm.

As far as I can tell, the criteria used for "coma free" in these posts is referring to visual observation and means that the size of the coma is less than the size of the Airy Disk, such that it would be undetectable in visual observation.  However, I would think that the criterion would be even stricter for imaging, because we are typically sampling several pixels across the FWHM of an Airy Disk, and the full size of the Airy Disk is much larger than this.  Also, I've come across other formulas on CN that have stricter values than the 0.022 multiplier.  But still this is geared towards visual observation.  

 

 I also came across a formula in one of Thierry Legault's books that gives the length of the coma tail as the following:

 

3d/(16F^2)

 

Where "d" is the distance off-axis, and F is the focal ratio.  I haven't been able to confirm this formula elsewhere however, and I'm not sure why the focal ratio is squared in this case, yet in other posts on CN there is always mention of how coma increases as the cube of the F ratio.  

 

In any event, if we use this formula and take the off axis value to be 4.4mm (half that height of the IMX183 sensor), then the length of the coma in an f/4 Newt should be 52um, which means it would be spread across 21 pixels!  In an f/8 Newt, this value drops to 13um, which would mean it's spread across 5 pixels.  This still seems like quite a lot to me, although for an f/8 scope, the spot size of the Airy Disk is only ~11um, so it's not that much larger than the Airy Disk, which kind of matches up with those posts you are referring to.  Although the spot size of the FWHM is only ~5um, and since we often sample for this value, an aberration of 13um would certainly have a negative influence. 

 

Of course, a coma corrector can be used......but this locks you into the prime focal length, as it's difficult, if not impossible, to combine a coma corrector with barlows.  



#11 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 14 January 2019 - 06:03 AM

I looked at Amateur Telescope Optics and there is a graph that contains a formula for coma in Newtonian telescopes:

 

radius (mm) for diffraction-limited performance given only coma = f^3 / 90 ≈ 0.0111 x f^3

 

Which means that the diameter of the coma-free field would be 2 x 0.0111 x f^3 = 0.0222 x f^3

 

So, this agrees with the formulae given by Jon Isaacs.

 

However, this same reference (Amateur Telescope Optics) says that while coma varies by the cube when measured on field height it varies by the square over a quality field size. That kind of makes sense and maybe that is why some references use the square.

 

That said, there is a lot more in Amateur Telescope Optics about coma and off-axis performance so I'm not convinced that these simple formulae are really that accurate when applied to imaging.



#12 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 23591
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 14 January 2019 - 01:42 PM

I don't think an f/4 Newtonian would be a good choice for photographing the moon. In fact, f/8 might be a stretch in terms of covering a sensor as large as the IMX183 (for the diagonal, which is approximately 15mm, but I'll need just the center portion of the full frame -- see below).

 

According to a post by Jon Isaacs the coma free field (diameter) for Newtonian reflector can be computed as follows:

 

However, I don't know what his criteria is for "coma free."

 

So, at f/4 we have: 0.022 x 4^3 ≈ 1.4mm

 

And at f/8 we have: 0.022 x 8^3 ≈ 11.2mm

 

This compares to the height of the IMX183 of 8.8mm and thus f/8 should be "safe" as long as the moon remains centered in the field.

 

I'm not sure whether my 6" f/6 Newtonian can cover the entire field of the IMX178 and the calculations suggest:

 

0.022 x 6^3 ≈ 4.7mm

 

And the size of the IMX178 is 7.4mm x 5mm with a diagonal of 8.9mm.

I assumed I'd be using a 2x barlow, which should give you the needed f/8, right?



#13 james7ca

james7ca

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7338
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 14 January 2019 - 04:00 PM

I assumed I'd be using a 2x barlow, which should give you the needed f/8, right?

 

Yes, you could use a barlow but that won't correct for coma and in any case the point of getting my 4" f/8 Newtonian is that I could cover the entire disk of the moon with a single framing of the IMX183. An 8" f/4 with a 2X barlow would produce too large of an image to capture the entire moon with the IMX183. So, you could do that but you'd have to do a mosaic to cover the full moon which is something I want to avoid.

 

That said, an 8" f/4 with a 3X barlow would provide critical sampling and would probably make a good planetary/lunar scope.



#14 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 23591
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 14 January 2019 - 04:08 PM

Yes, you could use a barlow but that won't correct for coma and in any case the point of getting my 4" f/8 Newtonian is that I could cover the entire disk of the moon with a single framing of the IMX183. An 8" f/4 with a 2X barlow would produce too large of an image to capture the entire moon with the IMX183. So, you could do that but you'd have to do a mosaic to cover the full moon which is something I want to avoid.

 

That said, an 8" f/4 with a 3X barlow would provide critical sampling and would probably make a good planetary/lunar scope.

Ah, gocha now. Does anyone make high f-ratio newts like that? Or is that something you would have to build yourself? I suspect you would at least have to grind your own mirror...a while back Chris was looking to get a 6" ONTC newt with their higher quality mirrors, and no one wanted to make the mirrors.


Edited by Jon Rista, 14 January 2019 - 04:09 PM.


#15 Tom Glenn

Tom Glenn

    Surveyor 1

  • -----
  • Posts: 1902
  • Joined: 07 Feb 2018
  • Loc: San Diego, CA

Posted 14 January 2019 - 04:55 PM

Jon, I have a 4.5 inch f/8 Newtonian, but it's an entry level scope that I picked up from a friend of mine for $50, and I would hardly recommend it for anything other than a complete beginner that wants a cheap scope purely for visual use.  It has a spherical mirror, a horrible focuser, and right now my secondary mirror holder is patched up with duct tape.  The scope is not in any way designed for imaging.  That said, it has produced some pretty good results given the low cost.  

 

https://flic.kr/p/29nomR1

https://flic.kr/p/22LJB42

 

With regard to coma, a barlow will not reduce coma, so despite changing the light cone to f/8, a 2x barlow on an f/4 scope will still have the coma of an f/4 scope.  I had initially thought that perhaps by reducing the FOV, a barlow would reduce coma, but it turns out not to work this way.  While the linear size of the coma is reduced as you get closer to the central axis, you have also now magnified the image (and increased the sampling rate) with a barlow so the camera ends up recording the same amount of coma.  The reason that this isn't a problem on planets is that they are so small that they easily fit within the "coma free zone", whereas the Moon will not, unless you use a very small ROI of the IMX183 sensor, which on the Moon kind of defeats its main strength.  With an f/4 scope, even on planets, collimation is critical because if you are slightly off, then the coma-free zone will not correspond to the center of the optical axis where you are imaging, and so the entire planet may be in a region with significant coma.  Slower Newtonians are more forgiving in this respect.  



#16 Tom Glenn

Tom Glenn

    Surveyor 1

  • -----
  • Posts: 1902
  • Joined: 07 Feb 2018
  • Loc: San Diego, CA

Posted 14 January 2019 - 06:44 PM

I looked at Amateur Telescope Optics and there is a graph that contains a formula for coma in Newtonian telescopes:

 

radius (mm) for diffraction-limited performance given only coma = f^3 / 90 ≈ 0.0111 x f^3

 

Which means that the diameter of the coma-free field would be 2 x 0.0111 x f^3 = 0.0222 x f^3

 

So, this agrees with the formulae given by Jon Isaacs.

 

However, this same reference (Amateur Telescope Optics) says that while coma varies by the cube when measured on field height it varies by the square over a quality field size. That kind of makes sense and maybe that is why some references use the square.

 

That said, there is a lot more in Amateur Telescope Optics about coma and off-axis performance so I'm not convinced that these simple formulae are really that accurate when applied to imaging.

James, those pages are an interesting read.  The equations can quickly become confusing however, and in some cases the variables involved would be difficult to measure for an amateur.  By following the link to another page on coma at the same website, I was able to find a formula that agreed with Thierry's formula for the length of the coma tail.  It was termed "coma transverse aberration" and was given by the formula T=3h/16F^2, which is the same formula I provided above.  

 

But to confuse matters even further, the size of the coma and how it varies is different depending on whether you are reporting the angular diameter of the coma, or its linear component.  And it's impossible for me to say which would be more important.  You are probably correct that while these formulas make for an interesting read, they may or may not be useful as anything other than a ballpark estimate when it comes to imaging.  You can simply take an image of a star field and look at the stars to measure real performance.  

 

Most of the formulas for tolerable coma are geared towards visual observation and make assumptions about what angular diameter the human eye can detect, and put the coma into perspective with other aberrations that are present.  I came across an interesting data point that is now obsolete, in a book on optics that I have that was first printed in 1988 (Telescope Optics by Rutten and van Venrooij).  It reports a "photographic standard" of coma size at 25 microns at the image plane.  This followed from measuring what the smallest star sizes were when taken on 35mm film by "professionals" (whatever that means).  If the comatic aberration did not alter the spot size of the scope significantly beyond 25 microns in diameter, then it was deemed acceptable.  The book also made mention of how the coma tail is often so faint that when developing film, it will be less apparent than you might predict.  This is of course now obsolete, as we image with digital cameras that are extremely sensitive and have very small pixels, such that a 25 micron spot size seems huge.  




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics