Top equipment, but hazy sky!

Last night I was in Tai Po. The main task was APO testing. The dealer imported a few APM/TMB APOs. It was a rare chance that one could have a side by side comparison of 100mm/F800mm, 130mm/F1200mm and 203mm/F1420mm APOs. The 203mm/F1420mm was owned by the dealer. The other two APOs were goods. However, it was ruined by the bad sky condition! The sky was very hazy. The seeing was very bad. The moon was covered by a yellowish Arabian mask. It would be nice if you are not stargazer. When I back home, I browsed around the Internet. It seemed that all local observation sites, like Pak Tam Chung, were disappointing. I got ready the equipment in my boot in the afternoon. It would be meaningful to know how good those APM APOs compared with Equinox. Finally I decided not to setup my equipment and just tried those APOs.
At a first glance, the tube material was robust lightweight polymer. Both 100mm/F800mm and 130mm/F1200mm can be mounted on EQ3Pro. I forgot to carry both APOs and got a feeling of how heavy they were, but they should not belong to heavy class. Everything was strongly and nicely built. The base and the bracket of the finder scope were quite unique. I would not say it is stylish, but the design concept was neither Japanese nor Chinese, but German. The Feather Touch focuser was solid. Well back to the main dish, optical performance. Both APOs delivered high contrast images. I tried another 12” Newtonian last night. The Newtonian deliver brighter images, but it contrast was far below than that of APOs at similar magnifications. We boosted the magnifications to 288X and ~500X for 100mm/F800mm and 130mm/F1200mm respectively. These mean we pushed the limit to 72X/inch (288X) and 96X/inch (500X) respectively. The 100mm/F800mm was able to handle 72X/inch. Although the Jupiter image was breakdown under 96X/inch for the 130mm/F1200mm, one could still see some color fringes of Jupiter. That was not a flat images disc of Jupiter.
Last night there were two points I especially want to mention, the chromatic aberration and the resolution of the surfaces the Jupiter’s satellites. When I used the APOs to observe the Jupiter, the north pole of the Jupiter was a bit bluish. The dealer said it was due to atmospheric chromatic aberration, not the chromatic aberration of the APO. Here comes the question! Is the dealer telling the truth? To me, this is a physics question. If the optics got no or low astigmatism, the chromatic aberration should be cylindrical symmetric. There should be false color around the star, not just appearance in certain direction. My conclusion is that the bluish color at the north pole of Jupiter was caused by atmospheric aberration. One can do some experiments to verify this. Use the same telescope to observe the same object and see if the position of the aberration changes with time. About the resolution of the surface of the Jupiter’s satellites, moon surface, we saw all four satellites, IO, Europa, Callisto and Ganymede. The dealer and I were not sure about the moon surfaces were revealed or they were just Airy discs. As the magnitude of those satellites were small, probably larger than 5. When I observed the satellites by the 100mm/F800mm APO, it seemed that I saw the 1st order diffraction ring, but I was not sure. But it was possible that it was Airy disc. When we used the 130mm/F1200mm APO, the image of the satellite improved a bit. The hazy sky kept on discourages us whole night. It seemed that the 130mm/F1200mm APO gave a more solid image. However, it was illusion or fact. We need to wait for the sky. But it is highly possible that these two APOs will be delivered to new owners. So what can I do! Ha! Ha! Use theory to justify it! As these two APOs were claimed to be diffraction limited, we can do some calculations and see if it is possible to see the moon surface.
The visual angles in arc second of the satellites are:
IO 1.2”
Europa 1”
Callisto 1.6”
Ganymede 1.7”
By using the Rayleigh Criteria, we can calculate the theoretical limit of the resolution of the telescope.
R = 1.2 L/D where R is the resolution, L is the wavelength and D is the diameter.
The resolution calculated is in radian, we can convert it back to degree easily. In order to get a feeling of how good is the optics, I assume the average wavelength of visible light to be 550nm. After some simple calculations, for diffraction limited optics, the resolution of different diameter in arc second is given below:
100mm 1.36”
130mm 1.05”
203mm 0.67”
It means that 100mm APO is not able to resolve the Jupiter’s moon surface. The image was just an Airy disc. The 130mm APO can barely resolve the Jupiter’s moon surface. The 203mm APO can resolve Jupiter’s moon surface. This is a simple discussion, meaning that if the optics is diffraction limited, it is possible to resolve it. On the other hands, the practical optic is not possible to go beyond the theoretical limit.

APM 100mm/F800mm

APM 130mm/F1200mm

Newtonian 305mm/F8.5


Equinox 120ED and DBK21

Last Friday,18th September 2009, I got a chance to use the Equinox 120ED and Imaging Source DBK21 and DMK41 in the field. The sky condition was exceptionally good. I went to Pak Tam Chung at about 10:00pm after monitoring my daughter’s homework. Since I got lot of outstanding tasks, time was limited for me. I planned to image the Jupiter and one to two deep sky objects, but I end up with the Jupiter only. The main reason was that I was inexperience in handling the Imaging Source camera. I’ve learnt the basic of how to image Jupiter from David, but lack practice. This was the first time I use the DBK to image Jupiter. I knew some crucial parameter in imaging the Jupiter. However mirror setting could ruin the photo! It takes time to learn and art cannot be rushed!
When I arrived PTC, I started the setting at once. I was kind of slow and inexperience in setting up the equipments. Since this telescope was a rare species in PTC, It attracted some stargazers. Finally when the setting was done, we viewed Jupiter. Everyone was amazed by the 120ED. Mirror God used his, prestigious telescope tester, Takahashi 3.6mm eyepiece, to test the Equinox 120ED. Here was his exact wording:
"Hmm... 幾好幾好!真係唔錯!雖然唔係一級的鏡,只是色差差少少!好少!只係輸比Astrophysics少少!想不到兩枚玉都有咁好performance!"
This was not the end of the story. When I finished the setup of DBK 21, the movie shocked everyone. It was because I was green in setting the DBK, everyone knew it when they saw my clumsy acts, but the movie was quite sharp already before stacking. It was good because the seeing & transparency were good. These unleashed the power of Equinox' optics and the power of DBK's electronics. My comment is that Equinox 120ED delivers high contrast, high color saturation and high resolution images in its own class. The color was a bit warm and yellowish in viewing Jupiter. The price is good. No complain for such performance price ratio, $13800 for a 120ED! If one wants to take serious photos, a Feather Touch focuser is needed, perhaps a high quality reducer. I am still very busy on school’s work and no time to learn how to stack the photos. The photo below is just a default click-click-click output of Registax 5.0. I will process it seriously later.

Jupiter, Equinox 120ED, TV 5X, DBK 21


The Delighting Jupiter and Moon 賞心「月木」

Starting from mid August, I was busy preparing the commencement of the academic year. Although I had a few stargazing activities, I didn’t have time to record what I have done. I did visual observation for the Jupiter opposition. I tried my friend’s Obsession 18” UC. I placed the telescope order for my school etc. I was lucky enough to have the full support from my principal that I can buy enough equipment to start astronomy in school. We bought a Sky-Watcher Equinox 120mm ED Refractor, a Sky-Watcher Dobsonian 12” Collapsible, Sky-Watcher EQ3Pro equatorial mount, Binoculars, The Imaging Source DBK 21 and DMK 41 etc. All the equipment enables me to start stargazing activities, promoting science education and enriching the NSS Physics teaching in Astronomy and Space Science.

Last night was the first stargazing activity this academic year. I made an elegant Chinese name for this activity, 賞心「月木」Delighting Jupiter and Moon. I used the Stellarium to find out the appropriate time to hold this activity. It was because my school was surrounded by a public housing estate and a hill. The field of view from the school roof is very narrow. Without careful planning, the observation would definitely be doomed! Originally I planned during the observation time, the Jupiter and the moon would appear between the gap of two buildings, finally I changed the observation site from the school’s roof to a nearby roundabout. I guessed students would be happier with an unobstructed view.

After a short introduction about the Jupiter and the Moon, we went to a nearby roundabout. The transportation was easy as we had about 20 students. However, the setting was quite exhausting! The temperature was high and I needed to setup three telescopes. Finally I gave up and ended up with two refractors. You know I need to control and discipline about 20 students in the street and at the same time I needed to setup the equipment, it was really something! A few of my students were good. They asked if they could help. But I did not have enough time to train them about the setting. I dare not to let them do it! I don’t want to risk the expensive equipment. Well! I will train them afterward. Anyway, I finished the setting in wet! When I finished setting up the first telescope, Equinox 80ED, everyone was amazed by the Moon. This was the first time they saw maria and craters. The second wave came after the setting up of second refractor. The target was Jupiter. Everyone was shocked by the Jupiter and its 4 moons, Io, Europe, Ganymede and Callisto! This was the first time in their life that they had such a close contact with Jupiter! I was the prime master of climax! I deliberately increased the magnifications gradually. Every time when I boosted up the magnification, changed to a shorter focal length eyepiece, everyone was so amazed and excited! The only defect of the activity was that students needed to wait for a long time! The queue was quite long! When it was about 9:30pm, a few students reported me that there were a few drops of rain! I was scared and packed all the telescopes at once. When we backed school, there was no rain at all! What a false alarm! Some students were not very happy because they didn’t have a chance to look at the Jupiter! I promised them to organize anther activity shortly and they will have the first priority to join!

After dismissing the students, I stated in the school and did some more observations with another Physics teacher in the school. We used the Sky-Watcher 12” Dobsonian and Equinox 120ED. Due to the light pollution, we only observed the Moon and Jupiter. Both seeing and transparency were modest. The magnification was about 200-300X. We were not able to explore the limit of these two telescopes. However, the primarily visual ability was good. A further test is needed.