Testing Kevin’s Camera

I was speaking with Kevin some months ago about astrophotography, and he mentioned that his camera, an Orion StarShoot Solar System IV, did not seem to be working correctly. The two problems that he mentioned were that he could not see the entirety of the moon in a single frame, and that he could not see Jupiter at all. I surmised that there was nothing wrong with the camera, and I explained that video-type planetary imagers like this had notoriously small sensors that have a very narrow field of view. Precisely aligning the telescope to cause Jupiter’s image to fall on the small sensor would therefore be difficult, and this would explain why only a small portion of the moon would be visible. He loaned me his camera for testing, and I have concluded that I was correct about the nature of his problem.

I tested the camera on my C11, and obtained the results seen in the images below, which seem so confirm that the camera was working. As I began to write up the test, I learned that the camera’s sensor format was 1/3-inch. To put this fact into perspective, 6 of these sensors arrayed side by side would be required to span the entire width of the 2-inch rear aperture of the telescope. In other words, the sensor is only 16% as wide as the aperture, which explains why Kevin would not have seen Jupiter in the sensor frame unless its image was almost perfectly centered in the rear aperture.

The focal length of the C11 with a 2x PowerMate is 5600mm. This results in a .14 x .14 arcsecond field of view on each of the camera’s 3.6 x 3.6 micron pixels. I set the camera to a 640 x 480 mode to improve the frame rate, thinking that I was selecting a smaller region of interest, or a subset of the sensor’s pixels at the center of the sensor. What I did not realize is that this binned the output into 2×2 arrays of pixels that would be output as a single 7.2 x 7.2 micron pixel.  This had the effect of halving the resolution that the camera was capable of producing. I would like to be able to repeat this test in the 1280 x 960 mode.

Before mounting the camera on the telescope, I wanted to make certain that Jupiter would be visible on the camera sensor by using eyepieces to center up Jupiter in the field of view. I started with a Panoptic 35mm, which I found very difficult to determine if Jupiter is centered. This is because this eyepiece’s exceptionally wide field of view made it impossible for me to see two opposite sides of the visible field at the same time, so centering at this stage was very much an estimation. Next, I replaced the 35mm with a 17mm Nagler, and repeated the process. It was better, but it was still somewhat difficult to determine if Jupiter was centered with this eyepiece. Next I put in a 12mm crosshair eyepiece, and was able to center up perfectly. When I replaced this eyepiece with the camera, and brought it up on the laptop screen, Jupiter was centered perfectly. Finally, I put the 2x PowerMate in the optical path, and only needed to adjust slightly.

On March 30, 2015, the night of the test, Jupiter’s apparent diameter was 41.6 arcseconds. I used PhotoShop to measure the diameter of the image produced by the combination of the optical configuration and the sensor. The measurement across the center of Jupiter’s white equatorial band was 142 pixels. This compared favorably to the computed prediction of 154 pixels. I attributed the difference between the computed and actual values to me being less than completely careful in taking the measurement and the limb darkening on the disk of Jupiter. Being able to predict the image size  increased my confidence that Kevin’s camera is working just fine.

Just a few words about the clarity of the images as I wrap this up. As I previously mentioned, the resolution of the images could have been improved by using a 1280 x 960 setting instead of 640 x 480, thus doubling the resolution. The night was fairly still and dry, so the “seeing” probably did not affect the image quality very much. Focus seems to be very difficult to achieve while watching video stream off of the sensor, so that could have been a factor affecting image clarity. I made no attempt to rule out focus as a problem by capturing several frame sets while slightly changing the focus each time. And lastly, I believe that the telescope might have some issues, which I believe can be ruled in or out by eliminating focus as an issue.

For good measure, I did a couple of captures on the moon. Clarity, seems to be an issue here too. This did demonstrate that the entirety of the moon’s surface is not likely to be visible in Kevin’s telescope. Note that in the Plato photos, adding the 2x PowerMate magnified the blurriness.

All of the images that follow were lightly processed. I stacked the frames in Registax, and then I adjusted the curve and levels in PhotoShop.

Jupiter 3-30-2015-6
Jupiter by Jim Johnson on March 30, 2014 using a Celestron C11, Televue PowerMate 2x, and an Orion StarShoot Solar System IV.
Moon 3-30-2015-7
Plato and Environs on the Moon by Jim Johnson on March 30, 2014 using a Celestron C11 at prime focus, and an Orion StarShoot Solar System IV.
Moon 3-30-2015-8
Plato and Environs on the Moon by Jim Johnson on March 30, 2014 using a Celestron C11 a PowerMate 2x, and an Orion StarShoot Solar System IV.

 

Moon 3-30-2015-9
Copernicus and Environs on the Moon by Jim Johnson on March 30, 2014 using a Celestron C11 a PowerMate 2x, and an Orion StarShoot Solar System IV.

© James R. Johnson, 2015.
jim@jrjohnson.net

April 2015

INTRODUCTION

April is a great month for casual star gazing. The weather is tolerable, it still gets dark at a reasonable hour, and the evenings are relatively bug free. Be sure to take every opportunity to just go outside and look up. Star gazing is just that easy. As interest is piqued, spend some cloudy nights reading about anything that you’ve seen night sky.

About Scope Out      How to begin Observing the Night Sky

THE CONSTELLATIONS

The sky map below represents the sky as it will appear in mid-April  at the end of astronomical twilight, or the arrival of complete darkness, at 9:20pm EDT. The Scope Out monthly focus is on the constellations that are  just to either side of the meridian, which is near the 10th hour (10h) of right ascension line in the April sky map. For a primer on how to use this sky map, please read How to begin Observing the Night Sky.

Scope Out divides the celestial sphere into three zones to aid in finding constellations:

1. Circumpolar Constellations: Find Lynx, Perseus, Camelopardalis, and Ursa Major in the northern sky above Polaris.

2. Northern Constellations:  Orion continues to move toward the western horizon. Although it no longer dominates the sky, it does catch they eye before setting below the western horizon about an hour or so after sunset. Find April’s remaining northern constellations, Cancer, Leo, and Leo Minor near the zenith.

3. Southern Constellations: The best-placed constellations in April are Hydra, Sextans, Pyxis and Crater. Some of these, Pyxis and Crater, for example, never rise very far above the horizon because of their deep southern declination.

April
The April Sky at Nightfall. Jim Johnson, December 2014.

 

THE PLANETS
planets
Colors of the planets. This picture is not to scale. Image from NASA’s Planetary Photogrounal at http://photojournal.jpl.nasa.gov/

Mercury begins its best evening apparition during the last week April. Look for it low on the western horizon about 45 minutes after sunset. It reaches its best viewing opportunity during the first week of May. Mercury and Mars put on a planetary show on the western horizon around April 20th, and can be seen with binoculars soon after sunset. Venus’ brightness will dominate the western sky after sunset, and will continue to appear a little higher above the horizon each evening.  It will move through Taurus passing near the Pleiades and Hyades around mid-month. Jupiter is in its best nightfall view position near the meridian at sunset. Saturn continues to rise earlier each evening, appearing above the eastern horizon about two hours after sunset. Uranus is lost in the Sun’s glare and will soon reappear as a morning object. Neptune is now a morning object, but it is still too close to the Sun’s glare to be seen before dawn.

THE LUNAR CALENDAR

moon_phases_small_full April 4
Full Moon
April 8
Conjunction with Saturn
moon_phases_small_lastqtr April 12
Last Quarter
moon_phases_small_new April 18
New Moon
April 19
Conjunction with Mars and Mercury
April 20 and 21
Conjunction with Venus
moon_phases_small_firstqtr April 25
First Quarter
April 25 and 26
Conjunction with Jupiter

EVENTS

Lunar Eclipse – April 4th

The eclipse begins when the moon enters the prenumbra of the Earth’s shadow at 5:35am EDT. The partial eclipse begins when the moon enters the umbra of the Earth’s shadow at 6:15am, and totality begins when the moon is completely enveloped within the umbra at 7:54am. The moon, when viewed from Maryland, sets a few minutes later, which is before the moon emerges from the umbra.

Venus Passes near Pleiades – April 11th

Venus passes within 3° of the Pleiades. Not many lay observers have actually witnessed a planet’s wanderings among the fixed stars, so watch Venus all month to view its progression between the Pleiades and Hyades. The moon will pass through this area of the night sky on the 20th and 21st of April.

Conjunction with Mercury and Mars – April 19th

This will be a very difficult conjunction to view, because the very young moon and the two planets will lie very close to the Sun at sunset. The view will be spectacular for those lucky enough to have a clear view all the way down to the horizon on this evening.

Lyrid Meteor Shower – April 22nd

This is not one of the better meteor showers of the year, but meteor enthusiasts will not want to miss it. The best time to observe is from 11pm until dawn.

 

© James R. Johnson, 2015
jim@jrjohnson.net