Setting up an 8″ Reflector Telescope for Astrophotography

Dave Anderson

I received an awesome birthday gift from my wife last fall, an Orion XT8 Dobsonian reflector. I’ve had a bit of fun with it just stargazing, but eventually that wasn’t enough and I wanted to try photographing with it. I had some minor success with eyepiece projection photography as in the Moon and Jupiter shots below. The thing is, the specs listed on the label are just too interesting to pass up. As a prime focus lens, it’s a 1200mm f/5.9 optic. Who could resist trying to see what they could make of that?

Initial experiments with eyepiece projection on an Orion XT8 Dobsonian telescope with a 25mm Plossl eyepiece and a 2x barlow, magnification 96x projected onto 35mm full-frame sensor.

Initial experiments with eyepiece projection on an Orion XT8 Dobsonian telescope with a 25mm Plossl eyepiece and a 2x barlow, magnification 96x projected onto 35mm full-frame sensor. Subject is Jupiter.

The biggest problem that I had initially was that the camera overbalanced the telescope, meaning that if I let go of the scope it would tilt forward under the weight of the camera. Because of this, it was hard to get sharp images. The above shots were essentially handheld in that some shake was introduced by my holding onto the scope tube. Later I obtained a 5lb. sandbag and mounted it with a bungee to the scope which temporarily fixed the balance problem. I had ideas for improving the method of counterbalancing the scope but that would have to wait until I proved the feasibility of the whole setup.

Test Counterweight

Eyepiece projection can be a bit fiddly as the collection of pieces below(or equivalent) must be assembled to the focuser and it’s very difficult to fine-adjust the camera-to-eyepiece distance for focus. This is the Orion P/N 05127 Tele-extender shown with a 25mm eyepiece between the two halves of the tele-extender.

Orion P/N 05127 Tele-extender

The eyepiece is inserted into the left-side piece of the extender and secured with the thumbscrew pictured. This is then inserted into the focuser on the scope and focused on the object to be photographed. The right-side piece of the extender is threaded onto a T-mount adapter on the camera then slipped over the other piece of the extender which is mounted on the scope, and the screws lined up with the slots. The camera is then moved closer to or further from the eyepiece to adjust focus then eventually locked down when the correct focus is achieved. The first two pictures in this blog post were captured with the setup shown.

For the most part balancing the scope properly resolved a lot of the motion blur — I think — but it was so hard to achieve focus that I never saw any significant improvement over my initial attempts. Motion blur seemed to be replaced by chronic focus errors — it seems I lucked out on my first attempt. What I need to do for eyepiece projection attempts in the future is come up with a way to fine-adjust the camera-eyepiece distance. The thumbscrew setup pictured just doesn’t cut it IMO. Additionally, I don’t find the pictures to be particularly sharp at the edges; it might take more than I’m willing to invest in eyepieces to surmount this issue.

Next I decided to investigate using the telescope at prime focus. That is, use the telescope as a camera lens with no eyepiece. One big advantage of this approach is that there are no refracting surfaces(lenses) in the scope, only reflecting surfaces(mirrors) so there should be no chromatic aberrations. Another advantage is that there is only one focus adjustment, where with eyepiece projection one must focus the eyepiece and the camera separately. A disadvantage is I will end up with less magnification, but the picture quality might improve enough to make up for this, at least to some degree. Nothing ventured, nothing gained, eh? :) Besides, I can always drop in a 2x or 3x apochromatic barlow.

The first thing I needed was a prime focus adapter, Orion P/N 05270 which is nothing but a standard T-mount thread on a short barrel designed to mount in place of a 2″ eyepiece, with a standard filter thread as used on 2″ eyepieces on the end opposite the T-mount. This piece needs to be as short as possible to bring the camera close enough to the scope to allow infinity focus:

Orion P/N 05270 Prime Focus Adapter

Obtaining this cheap, seemingly inconsequential piece was a source of great annoyance. I ordered from the counter at the local Orion store, paid in advance, and for a couple of months the story every time I called was that it would arrive in a week or two. The expected arrival would come and pass, I would call again and the story was again “in a week or two”. I finally contacted an Orion Distributor who had the piece on the shelf and had it shipped to me within a couple of days. I suspect that they divert shipments from their stores to their distributors or to online sales from their own website. If I hadn’t been so busy in other aspects of my life I would have done this much sooner, but with the added disincentive of poor weather during the winter months I let it slide. The Orion store refunded my money without issue.

Since I picked this up it has disappeared from their catalog. :facepalm:

I attached this adapter to the only T-mount adapter that I had, and could not focus anywhere near infinity.

The adapter is actually a Minota MD-T adapter, attached to an Alpha-MD adapter. I had cut out the extra element on the A-MD adapter and opened up the mount as far as possible for use on an M42 bellows setup.Stacked Adapters

I tried just holding the bare camera mount over the fully retracted focuser and it quickly became obvious that the original focuser that came with the scope was much too long. I could not bring distant objects into focus. What was needed was a focuser that could retract further toward the main tube of the scope.

Here is the original focuser that came with the scope:

Original Orion Focuser

Here you can see the minimum height of the original focuser is 2-3/4″ from the main tube of the scope:

Original Focuser Height

I decided to try replacing the focuser with a Orion 2″ Low-Profile Dual-Speed Crayford Focuser, P/N 13030. It’s the lowest-profile Crayford I could find, and has a nice roller-bearing movement and an 11:1 fine adjust. Unfortunately, the mounting scheme left something to be desired. The focuser does mount right up to the scope, but the way the mounting plate is designed it holds the focuser quite some distance from the tube:

Crayford focuser installed per instructions

A quick measurement indicated I had only picked up about 1/4″ and a trial with the camera indicated that this was nowhere near enough:

Crayford focuser installed per instructions

Not one to give up easily, I decided to look into alternatives. You may have noticed in the pic above, there is quite a bit of room to move the focuser inward and still have the focus knobs clear the tube. Next step, take it apart to see if it can be moved inward far enough to focus at infinity. 😀

Loosening set screw

There are four set screws in the mounting plate. Once they are all loose, the mounting plate slips off:

Plate removal

Here is the bare focuser, if necessary I was prepared to start with just this and toss the bracket:

Bare Focuser

I mounted it temporarily by attaching the mounting plate inside the tube then inserting the focuser back into the plate. Even though I couldn’t mount the plate properly due to misaligned mounting holes, I could see that this brought the focuser over an inch closer to the tube than with the original focuser:

Focuser trial fitting

It appeared that there would be enough clearance to turn the focus knobs, though with the mounting plate misaligned it looked a bit ugly at this point:

Focuser knob clearance

Finally, I took the scope outside and confirmed that I could focus on the moon, just not with my stacked adapter — though I could do so with the lens mount held against the focuser(no adapter). I couldn’t focus properly as the focuser was askew and it was difficult to hold the camera steady with no mechanical connection. The good news was I could tell that I finally had enough range in front of and behind the proper focus point that it was going to work, though I would need a shorter adapter. :cheesy:

The next step was to void the warrany — er… — to machine down the backside of the plate a bit. :roll: The holes were drilled in such a way as to make for a very poor place to put a nut and washer; it would have been on the side of the curved profile. I cut this away just enough so that I could take up any uneven surface with a rubber washer or similar:

Modified mounting plate

Then I painted the exposed aluminum black to minimize reflections:

Modified mounting plate, painted

Here is the hardware that I used. The “bonded sealing washers” are cupped washers with about a 1/8″ thick layer of rubber vulcanized to the concave side. I figured that these would help compensate for irregularities in the mounting surfaces.

Mounting hardware

This photo shows the new hardware(top) and the original hardware(bottom). I wanted zero chance of hardware dropping on the mirror, so I used nylock nuts.

Mounting hardware

I used a hole punch to elongate the holes for mounting the focuser mounting plate. Anything that cuts metal will work here, but I chose the punch since it would not get shavings inside the scope.

Widening mounting holes

As you can see here, I only had to elongate the holes a little bit:

Widened mounting holes

Next I mounted the plate firmly in position, tightening it snugly but not so tight as to deform the tube:

Focuser mounting plate installed

Here is an inside view of the plate mounted. I didn’t get a good pic of the original holes in the backside of the plate but hopefully from this pic you can visualize the problem I would have faced if I had not machined the half-round profile off of the ends of the plate.

Focuser mounting plate installed(inside)

Next I installed the focuser on the plate. It was a bit fiddly getting to all four of the setscrews and I was anxious to finish and try it out. It may look like it was simple getting to this point but this was actually a few weeks from when I first tried to fit the new focuser due to other priorities.

Installing focuser on plate

Focuser installed:

Focuser installed

You can see how close the focus knobs are to the main tube:

Focus knob clearance

The focuser is almost an inch closer to the tube than the original:

Focuser to tube measurement

This still wasn’t enough though. While I could get infinity focus holding the bare lens mount up against the focuser, my stacked adapters added too much to the back focus distance. The extra length was never an issue when used on my bellows, only a benefit. Here is the stacked adapter setup to compare with the pics that follow: :

Stacked Adapters

What I ended up doing was having Pete Ganzel modify a chipped A-M42 adapter by installing a T-thread insert. With this setup I am adding only a small fraction of an inch to the backfocus distance:

Modified M42 Adapter

Modified M42 Adapter

I use a 3rd-party 2″ eyepiece cover as a dust cover for the adapter:

Dust Cover

Here is the camera mounted to the focuser with the short adapter. Note that the focuser is extended about 1/4″. This is about the extension needed for infinity focus. Good thing I didn’t need any more room! 😀 Once I am 100% sure I will not need to remove the mounting plate I will carefully cover the inner nuts/washers with black paint.

Camera mounted to focuser

This is an earlier pic, shown with the stacked adapter but it shows the whole setup well enough that I didn’t bother to re-take the pic with the new adapter.

Orion XT8 Telescope modified with custom-mount low-profile focuser to enable use with DSLR. Effective 1200mm f/5.9 rigged as shown.

Here are some pics of the moon and sun taken using the final setup. These are half-size crops and while the downsized versions exhibit some artifacts along the bright edges, the originals are very sharp and clean. As always, you can click these or any other images in this blog post for larger sizes. Overall I’m pleased, though in the future I hope to use tethered shooting to help make sure I have sharp focus.

Moon shot with Orion XT8 Dobsonian reflector at prime focus(1200mm f/5.9) - Cropped to about 50% size
Sun Photographed with Orion XT8 Dobsonian reflector telescope at prime focus(1200mm f/5.9) - Cropped to about 50% size. Solar filter is polished glass triple-coated with nickel-chromium stainless steel and covers whole primary aperture of telescope. This coating blocks 99.999% of the light -- approx. equivalent to a ~16.5-stop ND filter.
Moon shot with Orion XT8 Dobsonian reflector at prime focus(1200mm f/5.9) - Cropped to about 50% size


18 Responses to “Setting up an 8″ Reflector Telescope for Astrophotography”

  • best telescope Says:

    I want to comment with the pictures!

    I was really amazed with your photography. Especially, the moon. Can I use your moon photography on my project for my moonlight sonata Music Class? Thank you so much! I would be happy to hear from you then.

  • Telescopes Says:

    Fantastic photography.I really impressed.I love astronomy.I have telescope and i like to see stars, planets,etc with the use of it.

  • FineArt Says:

    Thanks for the link to this, it’s a great project. I was thinking of going with an 8″ dob myself before someone with impressive photos talked me into going with a refractor. I would not have modified a brand new scope myself so its a good thing i am not going dob.
    The amount of light is spectacular. I love your moon shot. You have a huge benefit over other scopes, with the lower ISOs available for regular photography, such as birding. Have you tried anything like that?

    I think removing the right angle mirror on mine will give me the space I need without modification. I just made the order so it may take some time. See you on Dyxum.

  • What scope? Says:

    […] the telescope's eyepiece. You'll want a 50mm or so lens on the camera for this. Quote from: Setting up an 8″ Reflector Telescope for Astrophotography | Dave Anderson's Photo Blog The biggest problem that I had initially was that the camera overbalanced the telescope, meaning […]

  • Josha Wetenkamp Says:

    I have a 10″ Orion Dob and it’s been driving me crazy that I can’t focus at infinity. Gonna take some of the things you’ve learned, add some thoughts of my own, and see if I can achieve sharp focus on the moon tonight. If I am successful I’ll post a “how to” on my blog too.

    Question: Where exactly did you take that photo of the moon rising over Lick observatory? I measured 20 miles away on Google Earth in the direction I thought it looked like you took the picture and I basically landed on my apartment in Sunnyvale! I’ve taken pictures of Lick with my telescope but not prime. They turned out horrible. I’m hoping a prime optical system will turn out a lot better.

    • Dave Anderson Says:

      The photo was taken from the top of a parking garage where i work, not far from Lockheed in the Moffett business park(NE of where 101 and 237 meet).

      For anyone reading along, the photo of the observatory is here.

  • Alex Says:

    Thanks for this great post. Is it possible to achieve this without the low-profile focuser? I don’t want to spend $200 on a $300 scope. I’m thinking I could remove the focus knobs so I could push the focus mount in and attach it as you did. What do you think?

    • Dave Anderson Says:

      Alex, looking at the focuser that I removed from the scope, it seems possible to mount it the same way I did my low profile unit. It is a bit more wobbly than the low-profile unit, so I did realize an advantage there. Maybe you could put smaller focus knobs on if clearance is an issue, but focusing would be even more fiddly. Having a large knob gives finer control, and after experiencing the 11:1 fine control on the new focuser I would be hesitant to go back to the old focuser at all, let alone with smaller knobs.

      Another option is to put a barlow between the camera and scope, that will increase magnification and move the focus point back some, but there again you’re looking at $200+ for a decent 2″ barlow.

      Another option, not for the faint of heart, is to figure out how much you need to move the focus point then cut that amount off of the bottom of the tube to move the mirror up.

  • Burns Doran Says:

    Great photos, It is too bad you ended up blocking so much of the incoming light with the focusing assembly being mounted on the inside of the tube. Typically you will see a loss of contrast when blocking the path to the primary mirror.
    The old fashioned fix, so that you can use the telescope as a primary lens, is to move the primary mirror forward. I have done this with my 8” reflector, Meade 1200mm f/6, for some photos and it allowed the use of the original focuser with no further modification.

    • Dave Anderson Says:

      It actually blocks very little light. So little, in fact, that my camera meter suggests exactly the same exposure as before when the rig is pointed to an artificially-lit wall. I don’t see any added diffraction artifacts or detectable loss of contrast. The diffraction that is noticeable is still dominated by that produced by the secondary mirror.

      I did consider moving the primary mirror forward, but this scope is not intended to be a dedicated photography-only scope. I still want to be able to use eyepieces for direct viewing, and moving the mirror forward would require additional eyepiece extensions. Like everything in photography and astronomy(and life), it’s a series of tradeoffs.

      I am OK with the few very minor(even imperceptible) downsides, and I am very happy with the new focuser which is far superior to the original.

    • Chris Franks Says:

      Burns Doran – is it possible to move the secondary mirror and focuser/eyepice assembly down the tube towards the primary mirror, rather than move the primary mirror up and get the same effect?

      • Dave Anderson Says:

        Doing so would require repositioning the supports that hold the secondary mirror further down the tube and cutting a new set of holes for the focuser.

  • Marty Says:

    Wow thanks! You did a great job on this! I’m kinda doing research on my own Zhumel 10″ which has I think the same focuser, but I have not looked at the base plate yet to see if its going to have the same issue. I’m just starting out getting the camera and adapters so I’m trying to figure out how to best set up my 10″ dob for prime focus. I love the Zhumell dob for the price but I’m thinking now I should have maybe gone with an astrograph or something designed for prime focus. Great work!
    Marty

  • Doug Says:

    Thanks for this info Dave, I have a Meade 10″ Schmidt Newtonian which is a 1016mm F/4 and it drives me nuts that I can’t focus it either…the “low” point on the stock focuser is about 5 1/2 inches!! At 2 3/8 on the low profile focuser I will be 3″ closer! Hope that will be enough!

    • Dave Anderson Says:

      You can remove your existing focuser and just hold the camera in front of the resulting hole in the side of the unit to evaluate this. You won’t be able to get a sharp enough focus this way to be useful for photos but you will be able to approximate the distance needed. I suggest trying it in daylight with the scope pointed at the horizon.

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