Archive for the ‘iPhone astrophotography’ Category

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The lunar ‘Cheshire Cat’ revisited, and problems of contrast

November 3, 2014

2014-11-02 London moonwatching

Just a quick post before I head off to work. London has the telescope bug and he has a birthday coming up, so we were looking at various scopes on Amazon and other places. He wasn’t clear on the distinction between the three main telescope designs, so we hauled out the DK Universe book and looked at the ray diagrams the three kinds (refracting, reflecting, and catadioptric). He was familiar with refractors, like his 60mm Meade, and reflectors, like his Astroscan, but was less familiar with catadioptric scopes, which is not super-surprising since I’ve used my Maks hardly at all in the last year and a half, other than last year’s All-Arizona Star Party. The sky was clear and the moon was high, so we popped outside and set up my 90mm SkyWatcher Mak for a quick look at the moon. Astonishingly, I had not had this scope out in more than two years, since July of 2012.

2014-11-02 waxing gibbous moon - snapseed

Here’s my best iPhone photo of the moon from last night. Up near the top of the terminator you can see two glowing dots like eyes peering over the limb of the moon. If you click through to the full-size version, you’ll see that the eyes have a wide mouth below them and that one nostril is showing. Yep, that’s the lunar “Cheshire Cat”, which I first identified back in November, 2010. It was nice to see it again.

While I was processing that photo I noticed something alarming: a circle of glare around the moon that was bright enough to make the eyepiece field stop visible. It’s more apparent in this over-brightened version:

2014-11-02 waxing gibbous moon - light scatter

I was shooting through the Celestron 8-24mm zoom, just like Saturday night. Since I had a comparable shot with the same eyepiece through the C80ED from that evening, I dug out the raw photo and tried brightening it up to see how much glare would appear.

2014-11-01 waxing gibbous moon - light scatter

The answer is “almost none”. I used the same tool in GIMP (‘Curves’), and I brightened the image way beyond what I did with last night’s shot through the Mak, and the space around the moon is still pretty black in the C80ED shot. Not grey, as in the Mak shot. And this was only with tweaking the brights up, and not moving the darks down, which would be cheating since it would mask the problem.

It’s tempting to read this as a refractor-vs-Mak thing, but it might not be so straightforward. In this case the refractor has very good optics and coatings, so it’s near the upper end of what refractors are capable of in terms of control of stray light. But the Mak does not have fully multi-coated optics–this SkyWatcher version only has ‘coated’ optics, which means possibly as little as one coat of MgF2 on only the outer surface of the corrector. I have heard from someone (Doug or Terry, maybe?) that this particular model of SkyWatcher 90mm Mak has poorer contrast than the comparable but fully multi-coated Celestron C90–irritatingly I cannot find that post or comment at the moment, but I’ll post it if it turns up. Also, the C80ED has a long-ish dew shield which helps control stray light entering the objective, whereas the Mak does not; you can buy or fashion such things for Maks, but I haven’t taken either of those steps. Finally, I’ve seen some threads on CN about glare from the baffle tube in Maks and SCTs, so that’s another possible culprit here.

An informative test would be to pit the C102 against the Apex 127 on the moon, with a homemade foam or cardboard dew shield on the Apex to eliminate that variable. If I get time this evening or next, I may just try that.

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A visit to the Griffith Observatory

November 2, 2014

2014-11-01 Griffith Park 1 - the observatory

Yesterday London and I went to the Griffith Observatory for the first time in a while. We used to go fairly frequently between 2010 and 2012, but this was our first visit in 2014 and might have been our first visit since 2012.

2014-11-01 Griffith Park 3 - moon over the observatory

It had been rainy earlier in the day but the skies opened up nicely in the late afternoon, and the waxing gibbous moon was bright overhead.

2014-11-01 Griffith Park sunset 2 - snapseed

Happily there were still some clouds to the west and north, which made for a fantastic sunset.

2014-11-01 Griffith Park 5 - London with Galileo telescope

One of my favorite displays is the replica of Galileo’s ‘Old Discoverer’ in the Hall of the Eye. It is still amazing to me that Galileo saw and learned so much with a 1-inch objective mounted in a paper tube. London is blocking the objective end here, but I had to pose him there for a reason.

2009-11-01 Griffith Park - London with Galileo telescope

Here’s the same shot, five years earlier. London was not quite five years old, and he was very excited about being at the “Griffick Ugzerbatory”. I didn’t realize until I checked the dates on the photos that that first visit was not approximately five years earlier, it was exactly five years earlier, on the first of November, 2009.

2014-11-01 Griffith Park 4 - observatory model

Farther down the same hall, there is this miniature architectural model of the observatory. The three domes all house different things. The one on the west end of the roof, nearest the camera in this shot, holds the triple beam coelostat for live viewing of the sun. The huge middle dome holds the planetarium, and the dome on the east end holds the big 12-inch Zeiss refractor–you can even see a translucent miniature version of the scope in the model dome.

2014-11-01 Griffith Park Zeiss refractor 1

And here’s the 12-inch Zeiss itself. There are actually five scopes on the mount currently: the big Zeiss, f/16.7, at the center, a 9.5-inch f/14.8 Zeiss refractor piggybacked on top, a 2- or 3-inch finderscope on the lower left, and two 8-inch Celestron SCTs on either side.

Originally the mount only held the 12-inch and the finderscope–you can see photos of it mounted that way on this page, which has a very interesting history of the scope and its uses. The 9.5-inch was added in 1955. The double refractors allowed one scope to be used for visual observation while the image from the second was sent to a closed-circuit TV. That job is now farmed out to one of the Celestron SCTs, which are much more recent additions.

2014-11-01 Griffith Park Zeiss refractor 2

Here’s another view. The total moving mass of the rig is 9000 lbs, or 4.5 tons.

We have gotten to look through the big Zeiss a couple of times, but we didn’t do so last night. There is usually a line with a wait time of about an hour. The way to beat the system is to be on the roof and near the east end at the moment that they open the dome and the line first forms–we have been lucky to be in that position once before. But last night we were in the middle of a planetarium show when the dome opened at 7:00, and we didn’t fancy standing around in the cold for an hour. Especially because there was a public star party on the lawn in front of the observatory, with about half a dozen scopes set up. There were long lines for the big scopes, but one guy had a 90mm Mak on an EQ mount that everyone seemed to be ignoring. London and I got razor-sharp views of the moon through that little scope with no waiting at all.

2014-11-01 waxing gibbous moon - raw

At home I hauled out the C80ED and the 8-24x zoom eyepiece for a quick look myself, and to make another photographic attempt with the iPhone. The two biggest challenges are getting the camera the right distance from the eyepiece, and getting the sensor fully illuminated without being overexposed. Through much trial and error I found that if I left the eyepiece cup up and stripped off only the outer layer of my 3-layer Otterbox phone case, I could rest the second layer of the phone case on the rubber eyepiece cup and have the phone at just the right distance. But that only worked perfectly with the zoom set to 18mm (33x), which is how I took this shot. There is some CA, but I’m pretty sure that was mostly from the eyepiece. If it’s clear tonight, I’ll try again with the ES eyepieces to see if I can isolate the cause of the CA.

One thing I desperately need to do is get one of the iPhone apps that lets you control the ISO and shutter speed of the camera. As it is, I’m just using the camera as-is, so I’m constantly fighting with its auto-exposure and auto-shutter. There’s a delicate balance–if I don’t magnify the moon (or the filtered sun) enough, all I get is a featureless white spot. But if I magnify the subject enough to spread out the light and give the camera’s internal processes some detail to bite on, then it’s hard to get the object fully illuminated–I get vignetting, or kidney-bean blackouts, or both at once. Eric Teske has a nice list of iPhone astro apps on his (ridiculously entertaining) blog–past time I started using them.

2014-11-01 waxing gibbous moon - snapseed

Still, for a shot through an 80mm refractor with the came-bundled camera driver on my phone, I’m pretty happy. One thing I really like about the iPhone is the number and utility of photo-editing apps. The first moon image here is the raw shot, only reversed left-to-right to match the moon’s orientation in the sky. The one immediately above I processed in Snapseed: sharpened, contrast enhanced (using the ‘Ambiance’ tool), and desaturated to take out the CA. Given the relatively small number of GIMP features that I actually use, Snapseed is a fast and easy alternative. I’m going to keep messing with it and see how far I can go.

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The Oct. 8, 2014, full moon

October 13, 2014

Full moon 2014-10-08

A big motivator for me in digiscoping is to see how good of a result I can get from very modest equipment. This image of the full moon from just a couple of hours before the recent lunar eclipse is a good example. It’s my sharpest iPhone moon photo. When I imported it into GIMP to touch it up, I noticed that it wasn’t left-right reversed like all of the other photos I’d taken through my 4-inch refractor. I couldn’t figure out why that would be. Then I remembered that I had started the evening using the GalileoScope, which David DeLano had equipped with a diagonal and a helical focuser. The diagonal is a prism, which can be ghastly, but this one is from StellarVue and it holds its own against a mirror diagonal. So I actually got this photo with the camera on my phone, shooting through a 2-inch plastic telescope that initially sold for $15. I like that.

Here are my previous full moon photos:

Evidently photographing the perfectly full moon is roughly annual obsession of mine. I haven’t put the new photo against the older ones to see how it stacks up in terms of libration and limb features, but I’m sure I’ll get around to that sooner or later.

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Observing Report: Total lunar eclipse on Oct. 8, 2014

October 11, 2014

London watching the Oct 2014 lunar eclipse 1

It had been six and a half years since I had actually watched a lunar eclipse (back in February, 2008), so I stayed up late Tuesday night to watch the lunar eclipse early Wednesday morning.

London watching the Oct 2014 lunar eclipse 2

London didn’t stay up that late, he went to bed at the usual time and I got him up about 3:00 AM. We spent about half an hour together, watching the Earth’s shadow gradually overtake the moon until the moon was entirely eclipsed. I had set up my pimped-out GalileoScope (it’s on the second tripod in the background), but after a lot of scope- and eyepiece-swapping, the setup I settled on was my 4-inch refractor and 8-24mm zoom eyepiece.

Oct 2014 lunar eclipse composite

My Nikon Coolpix 4500 gave up the ghost last year. I got a Canon S100 to replace it, but I dropped it out in the field this spring, and I haven’t gotten it repaired yet. So other than the DSLR Vicki uses at work, my only camera is the one in my iPhone 5C, and I used it for all of my eclipse photos. For most point-and-shoot photo purposes, it’s all I need. As an astronomical camera, it leaves much to be desired. There are apps out there that let you control the exposure and shutter speed, but I haven’t investigated them much yet.

When you first hold the iPhone camera up to the eyepiece, all you will see of the moon is an undetailed spotlight. The trick is to tap and hold on the moon for a few seconds to kick in the exposure lock and focus lock. That will gear down the exposure to the point that you can start getting decent photos. The focus lock is a little squirrelly – sometimes one part of the image is better-focused than another (for example, because the camera is not perfectly parallel to the light beam coming through the scope), and the camera will seize on the out-of-focus portion for the focus lock. In that case, you can usually get things back to good by tweaking the focus of the telescope, using the image on the camera screen to tell when the image is best focused.

All of the photos above have been rotated, resized, and lightly sharpened in GIMP. Max eclipse was at 3:55. The moon was entirely in Earth’s umbra, but it wasn’t centered in the umbra, so the northern limb was definitely brighter than the southern one, and the photos record that.

In sum, it was a lot of fun. Even the fussing about with the camera was rewarding – the iPhone may not be a replacement for a decent point-and-shoot camera when it comes to digiscoping, but it was still satisfying to learn how to use it for that purpose.

Now we’re getting prepared for the solar eclipse in a couple of weeks, and keeping our fingers firmly crossed for clear skies. Stay tuned.