Astrophotography

People today are spoiled by images from Hubble and other professional observatories. These images can do wonders to spark an interest in astronomy, but they can also lead to unrealistic expectations that leave newcomers disappointed at the eyepiece. So, why not make your own images? The equipment today that is available to amateurs is light years ahead of what was available to professional astronomers only decades ago.

It’s been said that astrophotography and astronomy are two completely separate hobbies. I’ve found myself more attracted to astrophotography rather than visual observing lately for a couple of reasons. The first is the “Hubble effect”, meaning that the images I take are much more pleasing than looking through an eyepiece. The second is that I can share my photos. The last reason is that astrophotography is a blend of two of my interests; astronomy and computers/electronics.

After a couple of years in the hobby, I have finally amassed a decent imaging setup. The Astro-Tech AT90EDT APO refractor is an ideal scope for a beginner. The relatively short focal length of 600mm is much more forgiving of tracking errors than an SCT and it doesn’t require a mammoth mount to carry it (though it is built like a tank). I acquired a used Losmandy G-11 over the summer which can carry the AT90EDT without breaking a sweat (plenty of upgrade room :). The mount came with the 492 digital drive, so it doesn’t do GoTo’s. It also came equiped with the Celestron Advanced Astromaster DSC system. I found this to be very hit or miss. Sometimes it would put objects right in the middle of the FoV and other times it would be more than 90 degrees off. Another benefit of a short focal length refractor is finding targets since it’s a manual process at this point. I have been using a Canon Rebel T1i DSLR as my imaging camera.

I was able to capture several targets over the summer and fall but I think I’m done imaging until spring since winter in Minnesota is pretty harsh. My processing skills have improved, but they still have a long way to go. I’m also limited by my refusal to use any OS other than Linux. Anyhow, here are some of my best images from this past season.

My imaging rig. A Losmandy G-11 carrying an AT90EDT with a Canon Rebel T1i DSLR.

The Double Cluster

I think this is my best image to date, despite the red gradient. NGC6992.

The moon

M101

My very first astrophoto. Strange star shapes were caused by several problems in the G-11 including a gunky gear box in the RA motor, seized needle/thrust bearings in the RA axis, and jumpy worm bearings.

My first M31

A Year Later

Well, it’s been a year since my last update but not because I’ve forgotten about this. A lot has happened since then.

I’ve started working on a 6″ mirror. It’s just about polished out and ready for figuring. I now understand why so many ATM’ers have unfinished mirrors from ~20 years ago…

I acquired a (Celestron branded) Losmandy G-11 and become addicted to Astrophotography. My primary scope for photography has been the Astro-Tech AT90EDT APO. At 600mm, it’s been very forgiving of alignment and tracking errors which is necessary since I currently lack an autoguider. This brings me to the next project I’ve been working on; using the Raspberry Pi to facilitate astrophotography.

I picked up a Point Grey FireFly MV on ebay for cheap. I have written INDI drivers (https://github.com/bgilsrud/indi-ffmv) to allow it to be used as a guide camera and am in the process of writing an INDI driver to use the Raspberry Pi’s GPIO’s as a guide interface. The Pi could also use the gphoto INDI driver to control my (unmodified) Rebel T1i and maybe in the future the encoders from the Celestron Advanced AstroMaster computer.

My new years resolution will be to update the blog more frequently rather than waiting until next year ;).

Foucault tester

As I mentioned in my last post, I was less than impressed with the views through my recently built telescope. I was blown away by the views of DSO’s (Deep Sky Objects) such as nebulas and galaxies, but when I pointed the scope at Jupiter I saw nothing more than a mushy blob with two lines through it (the northern and southern equatorial bands). Jupiter looks much sharper in my 90mm APO (Astro-tech AT90EDT) than it does in my 333mm newtonian! A star test revealed that the mirror was overcorrected. This is not surprising since Coulter wasn’t exactly known for their precision optics.

So what am I going to do about this? Fix it, of course! But before I take a pitch lap to the 13.1″ f/4.5 mirror I should probably know what I’m doing. All of the advice points that starting one’s mirror making career is best done with a 6″ f/8 or 8″ f/6. Since I have 2 6″ pyrex blanks, I’ll start with a 6″ f/8. I have been accumulating all of the books, mirror blanks, and mirror test equipment that I could get my hands on over the past few months. I recently built myself a foucault tester based on the Stellafane design. I used it for the first time last night to test my 13.1″ mirror (as well as a 10″ meade DS-10 mirror that I found on craigslist for $75!).

You might notice that the wood used to build the tester looks very similar to the wood of the original 13.1″ scope. The circle of life…

I followed the Stellafane blueprints pretty closely. I made the base slightly larger to hold the dial indicator better.

Ronchrigram of the 13.1″ using a 90 LPI ronchi screen confirms the mirror is indeed overcorrected.

Spherometer built using Gordon Waite’s instructions.

All that’s left for me to do is to build a grinding stand, which I’m planning on doing today. I hope to update the blog more frequently rather than I have been.

Rebuild complete!

I originally made this blog to track the progress of my rebuild, but obviously that didn’t happen. I have completed the telescope and have been using it for several months. Mechanically, I am very pleased with it; optically, not so much. I’ll follow up on the optical side of things in another post.

Side shot.

Looking down the barrel.

Dovetail spline jig

The caboose.

18-point mirror cell for 13.1″ mirror.

I used THE K&B Dobsonian book as a guide for this build. I think this is probably my first attempt at a woodworking project so I am more than happy with the results. I found that I enjoyed the woodworking aspect of this project as much as I enjoy actually looking through the telescope. During the project, I acquired many woodworking tools including a table saw, drill press, and router. I was able to justify buying these as the costs will be amortized over many projects and they will also be used for remodeling projects (my wife liked this reason more :).

The UTA (upper telescope assembly) and mirror box of the telescope are made of 1/2″ (12mm?) baltic birch plywood that I bought from my local Rockler. I would estimate that I used 5 2’x4′ sheets which cost ~$25 each. I noticed that the quality of these sheets varied considerably and that it is very worthwhile to take the time to find a good sheet (pretty face and not warped). You can see the jig that I made for dovetail splines above (based on the plans I found here). I am very happy with the way the dovetails turned out. The dark wood highlights are walnut.

The altitude bearings were made by gluing two sheets of 1/2″ BB together than using a circle jig to route out a circle. Next, the cutouts were routed. This was very tedious, but I learned quite a bit while doing it. I should’ve used the drill press to drill each of the end points rather than trying to route to edges.

I modeled the UTA truss clamps after the Astrosystem’s Telekit. The lower truss clamps were based off of Fiske Miles design. These work very well! The trusses themselves are from moonlite.

The mirror cell is made of aluminum box channel that has been bolted together. This worked just fine but I think on future projects I will have these TIG welded. The cell is an 18-point design that was PLOP optimized. I used a stainless steel wire with turnbuckles for the sling.

The focuser is a 2-speed moonlite CR2 and works like a dream.

From the Top

I started this blog to document my rebuild of a 13.1″ dobsonian telescope. It all started in August of 2011. Below is the scope as it was on the day I bought it (for $400!). It would’ve been a great deal for the optics alone.

Original form