AlenK

Regarding making contributions, I initiated three threads here when the site first opened. One of the threads got zero replies. I won’t try again. The continuing existence of DPR means that an Astrophotography forum has very little reason to exist. I don’t know about other forums here; maybe most are successful enough to warrant continued existence.

Very nice. I love the natural look.

As it happens, I don’t normally use any focal length below 762mm but higher than 300mm. For 300mm I use a f/4 telephoto lens while for longer focal lengths I most often use a 762mm focal length telescope, which is a 6-inch diameter Schmidt-Newtonian that produces fairly well-corrected images without diffraction spikes not unlike those from an APO refractor. To capture wider fields of view with that telescope, I do mosaics.

Well then, it doesn’t sound like your handle here is accurate! :D

In the spirit of sparking new discussions (this new forum is almost moribund), let's talk mosaics. Or as most people seem to know them, panoramas. Personally, I prefer "mosaic" to "panorama" because the latter was historically applied to image frames that were significantly wider than they are tall whereas the former covers image frames of any aspect ratio. (It is not for nothing that several widescreen film formats first introduced in the 1950's and the 1960's have "rama" in their names.)

For the uninitiated, a mosaic is a series of overlapping frames, typically arranged in a rectangular grid of MxN panels. For a traditional horizontal panorama, N (the vertical dimension) would be 1. Some overlap is necessary between adjacent panels to ensure that they can be properly stitched together with software.

I am a big fan of mosaics. Why? Primarily for two reasons. One, because I bought an APS-C DSLR and two, because I am naturally thrifty. Back in the day, I had been using a 35mm film SLR. Moving to an APS-C DSLR meant that I was no longer getting nearly as wide a field of view (FOV) through my telescope nor through the lenses I already owned. That was good for some deep-sky objects (the smaller ones) and not so good for others (the bigger ones). For wide-field images through lenses, it meant that an 18mm focal length, for example, was only giving me a FOV roughly equal to the FOV from a 28mm lens on a full-frame camera (which of course is what a 35mm film camera is).

This is where the thrifty part comes in. Doing a mosaic is much less expensive than buying new optics, whether telescopes or lenses, that would reclaim the wider fields of view I had with a full-frame camera and less expensive than buying a full-frame digital camera. In fact, when it comes to lenses, I have found that mosaics using an inexpensive lens give just as good an image as one taken with a more expensive lens with an equivalent FOV. That is due to image scale and aperture.

Because a mosaic uses multiple exposures, it can obviously reproduce a given FOV with higher pixel resolution than a wider-angle lens rendering that same FOV on the same camera without using a mosaic. That also means that the aberrations the lens creates when it focuses a star are rendered smaller, making them less obvious than in a single image taken with the same lens.

Even if the mosaic image is downsized to, say, match the resolution of the non-mosaic image taken with the wider-angle lens (an apples-to-apples comparison), the improvement in the rendering of stars is largely maintained. It’s as if a better (usually more expensive) lens has been used than was actually the case.

Aperture is the second reason mosaics taken with an inexpensive lens can match a non-mosaic image taken with a better, wider-angle lens. At the same f-ratio, the longer focal length lens has a larger aperture diameter. It is aperture diameter, not f-ratio per se, that determines how much light a lens can collect. And it is primarily how much light you collect that determines the quality of an astrophoto (provided everything else is in order, such as focus). Even if a slower (higher f-ratio) lens is used to do the mosaic, it could still have a larger aperture diameter than a shorter focal length lens.

So what are the downsides to a mosaic compared to a single frame image? For one, it is more difficult to take since you must re-aim the camera for each panel in such a way that you get the desired degree of overlap. This gets easier with practice and there is hardware that can help, such as an indexed panning head for your tripod.

The second downside is increased time and effort at the image processing stage. You must now process MxN frames, or MxN stacks, rather than just one frame or one stack of frames. There is really no way around this.

But I would argue that the upsides of mosaics more than justify the downsides. Supporting this view is the high quality of the results obtained by using mosaics that can be found on the internet. There are now many people posting impressive mosaics. My favorites are from Eric Benedetti out of Utah. He posts mostly nightscapes (Utah is blessed with an abundance of interesting foregrounds) but also does deep-sky images with lenses. He does mosaics almost exclusively now.

So, have you tried taking mosaics? If you have, how did they go?

This seems an opportune time on this new forum to ask such a question. I'll provide the first answer. Primarily, I do “deep sky” astrophotography. Most of that is through lenses on a DSLR, with about 30% of images taken with the same DSLR attached to one of several telescopes. I have been gradually widening my fields of view to the point where I now end up with terrestrial foregrounds in my shots as a feature rather than an accident, i.e., I do more nightscapes.

One thing I have not seriously done is planetary imaging. However, I do occasionally take images of the full disk of the moon through a telescope. That’s more a case of shooting the thing that is destroying the nice dark sky I wish was there instead of fighting it.

Because deep-sky and nightscapes do benefit greatly from dark skies, I primarily shoot from country sites that are less than an hour’s drive from the moderate-sized city in which I live in mid-latitude North America. (As close as I will come to pinning down my actual location.) The few times I have tried to do such imaging in my backyard have been excercises in frustration with little reward.

Your turn!

There are a fair number of good videos there. Thanks for the link.

As for other resources, I often check with Roger Clark’s website. He doesn’t post videos but his voluminous articles are worth reading, even if you don’t agree with all of his points (as a number of people don’t).

Oh, and congrats on being the first poster on the Astrophotography sub forum.

Cheers, all. Let’s get this party started. I am coming over from DPReview just days before the Big Freeze over there.

To introduce myself, I have been doing astrophotography on and off since the late 70’s but much more so recently. My one and only camera now for astrophotography is a Pentax K-3II. (Let’s ignore the three film SLRs I have that I likely will never use again.) I bought the K-3II expressly for that purpose mostly because of its built-in Astrotracer but also because of the great Sony sensor, the robust feature set and the tank-like build quality. It is serving me well and I use it as much as I can. Heck, I even take regular photos with it occasionally, especially when I travel.

I have mostly done deep-sky stuff through both telescopes and lenses. Lately, I have gotten more into nightscapes and ultra-wide angle images, especially using mosaics.

I would point readers to some of my images but alas, I was posting them on DPReview in a gallery, which like everything else there is slated to disappear soon. I am looking for a good no-cost home for them again, something that allows original-size JPEGs to not only be posted but linked in other posts elsewhere. (DPReview’s galleries allowed the former but not the latter.)

K-3II owner. Bought it for astrophotography mostly because of built-in Astrotracer but also for its great Sony sensor, solid feature set and tank-like build quality.