In search of Schmidt-Camera-like performance

Meade Schmidt Camera Ad

In the 1980's, Meade Corporation advertisements in the astronomy magazines were mesmerizing. Their products were beautiful, but they were mostly unaffordable to people like me. I could only dream of owning one of their telescopes. Until one day, when I just had to get my hands on one, I worked out a deal to sell my pickup truck to my uncle for just enough money to buy a Meade 8-inch SCT (Schmidt-Cassegrain Telescope). And, boy, was it worth every penny! For him, that is.

Just kidding.

Honestly, I loved my Meade 2080 LX3 for 20 years, despite its poor optical quality. Since I mainly used it for low-power astrophotography at f/6.3, I wasn't really bothered by the defects. It was only when I tried to resolve tight double stars or planetary detail at high power that I was met with disappointment. Meade was really in a bind over Halley's Comet in 1986. Everybody wanted a telescope to view the comet, so Meade and other telescope manufacturers cranked up their production. Their SCT optics were made by hand, and they hand-matched corrector plates to primary and secondary mirrors. To increase throughput and maximize their profits, they lowered their standard of optical quality. Some of their customers got excellent optics, I'm sure. But others, like me, got the bottom of the barrel.

Be that as it may, the mechanical and electronic quality was very good. And, of course, the aesthetic quality made it all worthwhile. It was a beautiful scope that I loved to show off. I was proud of it. More importantly, I got good results photographically.

In those days, both Meade and Celestron offered their SCTs in the f/10 variety, which was very slow in terms of astrophotography. And slow films like Kodak 400 made them practically useless for shooting the deep sky. The workaround for us astrophotographers was to buy a focal reducer and shoot at f/5 or f/6. For some reason, an f/6.3 focal reducer was very popular. And then film sensitivity increased and we eventually got pretty good results at f/6.3 with Konica 3200-speed film. It was a grainy film, but we didn't care. At some point, Meade offered an f/6 SCT, but it required increasing the size of the secondary mirror obstruction, which became a point of contention for many consumers. Thus, it was still advantageous to purchase the f/10 models and simply buy the focal reducer.

I enjoyed shooting at f/6.3 on fast film, but there were some large targets, like the California Nebula and comets, that demanded more field of view from the telescope and better sensitivity from the film. I really wanted one of those Schmidt Cameras that I saw in the Meade ads. There were a couple of astrophotographers who regularly submitted their Schmidt-Camera photos of comets and large deep-sky objects, taken with fine-grain films like the black-and-white Tech Pan 2415. See, for example, Sky & Telescope August 1986, pages 122 and 123. I wanted to be able to take pictures like theirs.

Frankly, it wasn't so much the price of Schmidt cameras that I couldn't afford, as it was the difficulty of using them that I found unacceptable.

Schmidt cameras were almost the same optical design as my SCT. Except they had no secondary mirror. The primary mirror was f/1.8 (very fast), and if you placed the film at the prime focus, you had a 4- to 8-inch aperture telescope that was faster than most camera lenses! The difficult part was that the film also had to be shaped like the spherical secondary mirror of a Schmidt-Cassegrain telescope! So, you would need to cut your negatives into little strips and put them in a curved film holder. And after being exposed, you'd have to retrieve those strips of film in total darkness and store them in total darkness until you got a chance to develop them in a darkroom. In those days, the 1-hour photo stores weren't equipped to take your film in the form of a bunch of tiny pieces and develop it. Thus, most Schmidt-camera users had their own darkrooms and processed their own negatives. That was something I had zero interest in doing!

What I wanted was a faster telescope without the need for doing my own film processing. And without the need to chop up my film. So, Schmidt cameras were not going to work for me. Instead, I looked around for telescopes with faster f/ ratios and smaller apertures, so that I could use the same camera I had already been using.

Chicago Optical 6-in RFT piggybacked
onto my 8-inch Meade 2080 LX3 SCT.

One such telescope caught my eye in the magazine advertisements. It was the Chicago Optical 6-inch RFT (rich-field telescope). With a focal ratio of f/4, it would be fairly fast and give a wide field of view for those large targets I was interested in.

In those days, people like me living out in the middle of nowhere didn't know how to find companies who made mounting rings for telescopes. If they didn't advertise in the two main astronomy magazines, we couldn't know about them. My local hardware store was where I bought supporting gadgetry for my telescope. So, after purchasing the 6-inch f/4 telescope, I paid a visit to the hardware store and bought some large hose clamps to use as mounting rings.

The end of a long night of astrophotography.

I shot a few widefield targets, like the Pleiades and the North America Nebula, through the 6-inch but they didn't impress me. There were some flexure issues, it was hard to get the tube assemblies balanced, and the whole rig was too unwieldy to carry outside or load into the car if I wanted to drive to darker skies than what I had in my back yard. I loved how the scopes looked, though, and the field of view was pretty much exactly what I was looking for, but I had to admit defeat on this "Schmidt-Camera Alternative" endeavor. I ended up giving the 6-inch telescope to a friend as an upgrade to her 4-inch Newtonian.

The Pleiades, M 45, through the 6-inch RFT

After that failed attempt, I gave up on the wide-field targets for a while and got back to imaging at f/6.3 with the Meade SCT. It was around that time that Konica introduced its 3200-speed film and that kept me busy for many years on a multitude of targets that were a good fit for the roughly 1200 mm focal length and large format of 35 mm film.

That is until one day when I got to eyeing up my seldom-used Edmund Scientific Astroscan, gathering dust on a closet shelf. Hmmm. A 4-1/4-inch f f/4.2 Newtonian. Sounds like a pretty good little "Schmidt Camera" wannabe! The only problem was its pinch-roller friction focuser. It couldn't hold its focus when I attached my SLR camera to it. I got the great idea to use a hacksaw and cut off the focuser. I planned to replace it with a different type of focuser, but alas! After hacking off the existing focuser, I couldn't find one that would fit properly and still reach focus. Oh, well. What was done was done! I found a way to attach my camera to it, and the Astroscan looked pretty awesome on the back of my SCT!

Yours truly with Meade 8-inch SCT and Astroscan

The Astroscan as a "Schmidt-Camera Alternative" got its trial run on Comet Levy in 1990. It was pretty cool because for the first time I could guide on the comet and let the stars trail, just like a real Schmidt camera! However, after examining my pictures, I realized that my hacksaw skills were not very good, and the camera didn't sit perfectly parallel to the focal plane. What's worse is that I hadn't left enough material on the telescope's tube to fix it, so I pretty much ruined my Astroscan. What a waste! I wish I had just left it alone!

Comet Levy in 1990 through the Astroscan.

Now, today's astrophotographers may seem a bit perplexed over my widefield imaging predicament in those days. Why not just use a small refractor like we use today? And my response is, "You have no idea!"

Let's face it. Today's astrophotographers wouldn't be so plentiful without autoguiders! An autoguider is a special camera that automatically takes rapid exposures and sends commands to the telescope mount to correct for errors in tracking. The guide camera can be attached to an off-axis guider or a separate guide telescope that's mounted on the imaging rig. Either way, the autoguider does the hard work of keeping the mount tracking properly so that your images have nice round stars. So, today's astrophotographers set up their autoguided telescopes, program them for their list of targets for the night, and then these so-called astrophotographers go to bed. What??!! Yeah, they literally program their gear and then go to sleep while their telescopes and cameras work automatically out there in the back yard, collecting the images. Back in the 1980's, we didn't have autoguiders. We did the guiding by hand and eye coordination.

Guiding an exposure the old-fashioned way.

That's right! We sat outside with the telescope all night long, peering into the guiding eyepiece and pushing buttons on the hand controller to push the star back into the center each time it wandered up, down, left, or right. Not only that, but our telescopes were not "Go-To" telescopes, so we had to aim them by hand at our targets. There were no computers. We used our paper star charts to figure out where to aim our telescopes, and then we framed the shot "in the blind." That is, most of the time, we couldn't see our targets through the camera viewfinder. We had to aim the telescope using an 8x50 finder. Then we'd expose our film for an hour. When the exposure was done, we had no way to view the image until our film was developed, which could have been the next day or possibly weeks later.

It took a special person to invest so much time and effort into taking an astrophoto and then have to wait so long to see the results. Sometimes, we stood at the counter of the One-Hour Photo store a month after our shoot, looking at our pictures and finding out that our focus was terrible, or our guiding was terrible, or that our targets were at the edge of the photo because we didn't get the scope aimed properly. We may have driven a hundred miles to get to dark skies, spent all night guiding exposures, and then found out weeks later that our efforts were in vain.

My interest in coming up with a Schimdt-Camera-like imaging system in the 80's was driven by my desire for short exposures. I didn't want to guide an exposure for an hour. I wanted to guide for 5 minutes. Max! But nothing that I used compared to the f/1.8 Schmidt cameras. That is, until 2007, when I was introduced to the HyperStar.

As I said, the Schmidt-Cassegrain telescope is really nothing short of a Schmidt camera. The primary mirror of an SCT has a focal ratio of f/2 or better. It's the secondary mirror that converts the system into an f/10 focal length. But if you remove the secondary mirror and put a CCD camera there in its place, it won't be able to focus the stars properly to make a nice image with round stars all the way to the edges of the field of view.

In 2007, when I was looking to replace my Meade 8-inch SCT, I happened to show up at Starizona, a local telescope store that makes the HyperStar lens. I didn't know about the HyperStar until I visited the store. When asked for what I was looking for, I told the store owner that I wanted an SCT, but that I didn't want to guide exposures anymore, and that I didn't want to go through the tedious polar alignment procedure every night. He smiled and said, "I have exactly what you need!" He then proceeded to tell me how the HyperStar works.

Essentially, the HyperStar turns an SCT into a Schimdt camera. Unlike a Newtonian reflector, with its flat secondary mirror, the SCT's secondary mirror has a convex shape. The old Schmidt cameras had a curved film holder, so that the film surface was bent into the same shape as a secondary mirror. But you can't simply bend the sensor of your CCD camera! Hence, the creation of the HyperStar. The HyperStar lens flattens the field while preserving the fast focal ratio of the primary mirror, so that a CCD camera or DSLR camera can be placed there at the prime focus.

Imaging with the HyperStar lens

Finally! After decades of trying to cobble together something to imitate a Schmidt camera, I now have what I wanted! An 11-inch aperture f/2 telescope. The telescope tube assembly is on an alt-azimuth mount, with no need for polar alignment. The exposures need be no longer than 15 seconds. If I want more detail, I stack as many 15-second exposures as I want. The telescope tracks accurately enough in 15 seconds that I get round stars without an autoguider. I shoot from my back yard in the suburbs of Tucson, with no light-pollution filters.