Comet 46P/Wirtanen 12-Dec-2018

33 - 60 sec exposures, with a 30 sec interval, at ISO 1600.
16 bias frames applied.
Temp 45°F - 49°F.
Taken between 9:48pm and 10:42pm MST.
Nikon D7500; Sky-Watcher Esprit 120mm, f/7; Orion HDX110 mount.
Optolong L-Pro filter.

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The Eskimo Nebula and Comet 38P/Stephan-Oterma 08-Nov-2018

While imaging the Eskimo Nebula (NGC2392), I noticed a blurry object to the lower right. Turned out to be periodic comet 38P/Stephan-Oterma currently in the constellation Gemini. Magnitude 9.27. 

Collected 16 frames, ISO800. Seeing was excellent at ~2 arc-seconds when starting at ~ 03:03am.

Coordinates (J2000) 07h 27m 33.4113s  +20° 48' 34.070" 

Sky-Watcher Esprit 120 (840mm) and Nikon D7500.

Meteor strike in next to last frame at 04h43m55s.

Also, below and to the left of the planetary nebula is magnitude 18.46 asteroid (45234) 1999 XA228.

20181108_NGC2392-Comet_38P-25pct.gif

(Click on the image for a larger version.)

And here is a single frame (cropped) to highlight the nebula itself.

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Comet 21P/Giacobini-Zinner and Companion Clusters 15-Sep-2018

In the early morning of September 15, Comet 21P/Giacobini-Zinner could be seen cruising in front of open star cluster M35 in Gemini. To its lower right is another fine cluster, NGC 2158.

Here is one of the frames taken for the video.












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Atmospheric Dispersion Correction in Image Post Processing

Atmospheric Dispersion Correction in Image Post Processing

The Earth's atmosphere refracts light of different wavelengths to a different degree, depending on an object's altitude above the horizon, hence declination dependent. I.e., the Earth's atmosphere acts as a prism.

Consequently, objects at the zenith will not have this effect, since the light from that position are perpendicular to the Earth's surface (and atmosphere) at that point.

This image shows the effect of atmospheric dispersion by the lower edge displaying a red tinge and the upper edge a bluish tinge.

The next 3 images show the individual frames after separating the channels using Michael Unsold's venerable ImagesPlus ( http://www.mlunsold.com/ ).

Red channel

Green channel

Blue channel

The image below is a GIF showing the vertical displacement between the individual RGB frames.

3 frame GIF

Finally, the result of aligning then recombining the individual frames, again, with ImagesPlus.

Result of colors split, aligned and recombined 

With compliments to Mike.

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M57 - The Ring Nebula 04-Jul-2018

Seeing was reasonably good last night at ~2.47 arcseconds (as measured by image link).

Ambient temp 85°F. I believe it will take a cooled camera to eliminate the red streaking.

The images were taken when M57 was just about at the zenith - clearer, darker and no atmospheric color dispersion, as is encountered when imaging planets.

Tracking was excellent, between 0.44 and 0.50 arc-seconds.

Bill Shaheen

Superstition Mountain Astronomical League®

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Elements of Astrophotography - Calculating Image Scale/Field of View

Elements of Astrophotography - Calculating Image Scale/Field of View

The general formula for determining the Field of View (FOV) for a light collecting area, whether at the pixel level or for the overall imaging area is:
206,265 x pixel (or sensor) size / focal length of the telescope (or lens).
(See Note 1 at bottom.)

For the purposes of our calculations, it is simpler to use 206.265 with the pixel size expressed in uM. For example, my Sky-Watcher Esprit 120 refractor has a focal length of 840mm and my Nikon D7500 has a pixel size of 4.2 uM.(See below for determining your camera's pixel size.) 

Hence, the per pixel FOV (aka image scale) is 206.265 x 4.2) / 840mm = 1.03 arc-seconds, sometimes expressed as as/p (arc-seconds per pixel). 
The above assumes square pixels, of course, which is usually the case these days.

At the chip level (or for non-square pixels), the horizontal (width) and vertical (height) fields of view need to be determined separately.
I.e., (206,265 x sensor array in mm (width or height) / FL) / 60.
E.g., the Nikon D7500 sensor array is 23.5mm (w) x 15.7mm (h).
(You may frequently see these terms interchanged - h x w.)

So, Horizontal FOV = ((206,265 x 23.5) / 840) / 60 = 96.175 arc-minutes.
(If you encounter an intermediate overflow error, use 206.265 and multiply the result by 1000, or get a new calculator.)

As an alternative, simply multiply the per-pixel FOV by the array size on each axis.

Solving for either FOV or image scale 

Now, the question may be posed, what camera or telescope should I choose to achieve an image scale of, say, 1 arc-second? And, it's a great question, since you may want to design your imaging system to provide a particular per pixel image scale or an overall field of view.

In that case, you simply want to substitute your desired FOV (or image scale) in the above equations and solve for either pixel size (or sensor size) or image scale.

You need to choose one or the other. For example: Since as/p = (206,265 x uM) / FL, then (206,265 x uM) / image scale = FL.

Or, you may already, and typically do, have a particular telescope that you already own and want to identify a camera based on its pixel size, or close to it, in which case, (image scale x FL) / 206265 = uM.

In my case, I'll target an image scale of 0.8 as/p (as opposed to the above 1.03as/p). So, (0.8 as/p x 840) / 206.265 = 3.25um (which is a commonly available image scale).

Now let's look at arriving at an overall FOV (horizontally).
Horizontal FOV is ((206,265 x sensor width in mm) / FL) / 60 = Width FOV in arc-minutes. Note that this calculation is independent of the individual pixel size. The entire sensor is the image collecting area. 
In other words, image scale in arc-seconds per pixel determines resolution, where as chip size determines overall field of view.

Again, we assume the FOV and solve for FL. 
So, to repeat  ((206,265 x sensor width in mm) / FL) / 60 = Width FOV.
sensor width in mm =  (FOV (in arc-minutes) x FL x 60) / 206,265.

Example: (96 arc-min X 840FL X 60) / 206,265 = 23.45mm on the chip's horizontal side.

Calculating Pixel Size from camera specs

In some cases you may not know the camera's pixel scale, although that data can be had from the manufacturer's specifications, or from third party resources. But, just to illustrate, simply divide the chip's width in mm by the number of pixels along that axis. The D7500 has an array of 23.5mm x  15.7mm and the pixel count is 5600 x 3728. (In the days before square pixels, you had to make two calculations. Now, one is sufficient.)

So, 23.5mm / 5600 = 0.004196 mm per pixel, or, 4.196 uM per pixel.

Or, use a calculator

Of course, there are a number of calculators on the web that can do this for you. One such calculator that you can download is Ron Wodaski's CCD Calculator

http://www.newastro.com/book_new/camera_app.html .)

Another, on-line calculator is http://www.12dstring.me.uk/fovcalc.php .

Note 1
206265 is the number of arc-seconds in a radian. See: https://en.wikipedia.org/wiki/Minute_and_second_of_arc

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Elements of Astrophotography - Chip Size, Pixel Size, Dawes' Limit and the Nyquist Theorem - Obey

Following a recent Friday morning meeting of the Superstition Mountain Astronomical League, a few of us convened for an impromptu astrophotography mini-clinic.

To recap, we demonstrated that chip size determines overall field of view (FOV) whereas pixel size determines resolution, in arc-seconds per pixel (as/p). We then showed how they are derived for various telescope/camera combinations.

Clarification

Following the meeting, I found I needed to make a distinction between the source signal resolution and the image capture resolution.

For example, my Sky-Watcher Esprit 120, with an aperture of 4.73 inches, produces, according to Dawes' limit, a maximum resolution of 4.56/4.73 = 0.96 arc-seconds per pixel (as/p).

What that 0.96 as/p represents is the maximum resolution produced by the optical system (again, according to Dawes) and hence becomes the maximum resolution that is available to the recording system (i.e., the chip) and projected onto the image sensor.

Now, shown below is the calculated image scale for the Nikon D5300 when coupled to the Sky-Watcher Esprit 120.

(Screen above produced from Ron Wodaski's CCD Calculator

http://www.newastro.com/book_new/camera_app.html .)

Now, you may notice that the calculated image scale is 0.94 as/p. Note also that Ron Wodaski's CCD Calculator, excellent as it may be, does not show Dawes' limit for the proposed telescope - just the final calculated image scale telescope/camera combination in question.

And, here is a sample photo taken with this configuration using the Nikon D5300 (IR filter removed).

In this photo, the image scale of 0.94 as/p is very close to the 0.96 as/p Dawes' limit for the telescope, the Esprit 120. In fact, when I upgraded from the Nikon D5300 to the D7500, I was pleased that the image scale (1.03 as/p due to slightly larger pixels) still matched so nicely. And, with our typical seeing of around 2 arc-seconds (on a good night), I should be happy.

But, here's the rub ....

Complication

While driving to the post office after the meeting, it occurred to me that another, important consideration needs to be taken into account when determining the components for your target image scale - the Nyquist sampling theorem, or simply the Nyquist theorem. Basically, the theorem states that to adequately convert an analog signal to digital (sampling), you need to sample at a frequency of at least twice the frequency of the source signal.

This is more commonly understood in the context of audio recordings but applies to other forms of signal as well. 

So, either the 0.94 as/p for the D5300 (or the 1.03 as/p for the D7500, which we will see) should be sufficient to capture images when the seeing is, say, 2 arc-seconds, which is infrequent and why those scales work under most circumstances.

But, my aim has been to record at roughly 0.5 arc-seconds/pixel in order to take advantage of those rare times when the seeing is close 1.0 arc-seconds. Ok, probably won't happen but we may very occasionally experience 1.5 arc-second seeing.

And, by imaging at the tighter scale of 0.5 as/p, I would in effect be sampling at 4x the source signal, when we have 2 arc-second seeing.

Now, that 4x may amount to what's called oversampling and I have seen a recommendation of 3x in various astrophotography forums.

Enter the 8 inch

Now, lately I've been testing the Celestron NexStar 8SE, which yields a 0.43 as/p image scale (with my current camera, the D7500).

And that more than satisfies the Nyquist sampling requirement for 2 arc-second seeing. The trick is to autoguide the combination without having to resort an in-line off-axis-guider (OAG) and instead use a separate guide scope mounted piggy-back on the OTA.

I'm still working out the wrinkles in that configuration and am strongly considering the EdgeHD version with a 0.7x focal reducer which yields a more comfortable 0.61 as/p, as shown below, which would meet the Nyquist requirement at 1/3x 1.8 arc-second seeing. Close enough.

(Screen above produced from Ron Wodaski's CCD Calculator

http://www.newastro.com/book_new/camera_app.html .)

Now, I've also been considering the Celestron 9.25inch EdgeHD, along with 0.7x focal reducer. Coupled with the Nikon D7500's 4.22um pixel size, that combination would yield an image scale of 0.53 as/p.  And Parijat would be so pleased. But, I'm also considering a future camera having smaller pixels and so will probably end up with the much less expensive 8 inch EdgeHD (and less expensive focal reducer).

For you own experimentation, here is a link to Ron Wodaski's CCD Calculator.

Stay tuned for a prequel on how to calculate image scale at the pixel or chip level.

William Shaheen

Superstition Mountain Astronomical League®
Gold Canyon, Arizona

http://supermtnastro.com/

June, 2018

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Jupiter with GRS 24-May-2018

Jupiter - GRS Transiting at 9:23pm MST. (SkySafari)
North is up; East to the left.
Image capture at 9:27pm MST.
Temp 75°F at 10:45pm.

Celestron AVX mount; Celestron NexStar 8SE; Baader 2" Clicklock SCT adapter; Televue 2" ext tube;
No Barlow; Celestron NexImage 5 Planetary Imaging Camera. iCap 2.4 image capture, 640x480 unbinned capture; Capture image scale 0.23 arc-seconds/pixel.

Processed in RegiStax 6.1; Adjustments in ThumbsPlus 10; Resized to 50%.

Original video available on YouTube at https://youtu.be/yDx6WVlF9gY

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