Astrophotography - Unable to achieve focus with current setup

Question

I am new to astrophotography and am looking for guidance. I have the following equipment:

• Orion StarBlast 9814 4.5" Altazimuth Reflector Tabletop telescope
• 1.25in Telescope Adapter Extension Tube T Ring for Canon DSLR SLR Camera DC618
• 17mm and 6mm lenses

When I use the lenses in the Telescope Adapter Extension, focus is not achievable. My theory is that the focal plane of most DSLR are 55mm back focus plane. Reflectors are positioned farther back than where the human eye would normally be placed, and hence it is usually difficult if not impossible to bring the camera into focus.

Is this true? Also, would a Barlow lens help achieve focus or be a waste of money? Any other suggestions?

Answer 1

Yes, the vast majority of small telescopes made for visual observations (like most dobsonians and other newtonian-based architectures) have very limited back focus. In other words, with the focuser retracted all the way in, the focal plane of the primary mirror is sticking out only a short distance. A large back focus is not typically seen, nor is it desirable, in a visual scope. (However, most astrographs have a large back focus.)

This is compounded by the fact that most DSLR have a flange distance (focus-flange distance) which is pretty large - over 45...50 mm in many cases.

Furthermore, an additional distance is wasted by the classic T-ring adapters, which tend to push the camera way back from the focuser's shoulder.

All these factors conspire in your case to prevent you from achieving focus.

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Some ideas and solutions:

Adapters

I tell everyone who is trying to do prime focus photography with visual scopes (dobsonians, regular newtonians) to ditch the old T-ring system and just buy dedicated adapters. All you need is a piece of metal that goes in the bayonet mount on the camera on one side, and into the focuser on the other side. No need for T- components.

Some such adapters are made by telescopeadapters.com:

http://www.telescopeadapters.com/

I have two adapters like this, they save a very large amount of back focus, compared to the classic T-ring / T-adapter system. In other words, they allow the camera to get much closer to the focuser, and therefore catch the focal plane on the sensor. You just need to choose the kind of adapter that works with your camera.

Most of these adapters are 2", so in a 1.25" focuser a size adapter is needed - but that would waste some back focus distance.

Camera

Classic DSLR cameras have a very large flange distance, so a large back focus is needed to overcome it.

Other removable-lens, large-sensor cameras have a much smaller flange distance, if they are mirrorless. Examples: Micro Four Thirds cameras (Panasonic G series), Sony NEX, Samsung NX, etc. All these systems have large sensors (comparable to DSLR) which work well for astrophotography, but don't have moving mirrors, so their flange distances are tiny.

https://en.wikipedia.org/wiki/Mirrorless_interchangeable-lens_camera

I have a Panasonic G1, used with the adapter mentioned above. I only need 25 mm of back focus for this to work, which is an extremely small value.

Use a barlow

A barlow would increase the amount of back focus available. Try and see what happens. Keep in mind it cannot work miracles - if you need a very large amount of back focus, it may not work.

Use eyepiece projection

With some eyepieces, you can thread adapters on the user side, and then put the camera body (without lens) on the adapter. Move the eyepiece WAY out of focus, and it will generate a real image which can be projected on the sensor. This is not prime focus photography anymore, but it works and it's better than nothing.

However, such adapters are hard to find, and many eyepieces won't support them.

Use a dedicated USB "eyepiece camera"

You can find relatively cheap USB cameras made for imaging, some are less than $100. With one of those, you could achieve focus relatively easy. Example:

http://www.telescope.com/Astrophotography/Astrophotography-Cameras/Orion-StarShoot-USB-Eyepiece-Camera-II/pc/-1/c/4/sc/58/p/102083.uts

Would not offer the same performance like a DSLR, but it's better than nothing.

Use a telescope made for imaging

Visual scopes, like a small newtonian, are not really that great for photography, for a variety of reasons. A dedicated astrograph would always work better. There are many Cassegrain systems out there made specifically for imaging, some are relatively inexpensive (not much more expensive than a DSLR).

In many cases, you need very long exposure, and you need to track the sky while the shutter is open. Many such scopes come with a tracking mount, or are made to be installed on a tracking mount. That would make it very easy to do tracking well.

It would be very hard to do tracking with other kinds of scopes (tracking platforms for dobsonians do exist, but are not very common).

Move the primary mirror up

Think about it - if you move the primary mirror up in its cell, the focal plane will stick out. This is a way to gain back focus only if you know what you're doing. Additional spacers in the primary mirror cell could do it. Modifying the cell is also doable.

Beware, you could ruin the scope if you make a mistake. Also, if the secondary mirror is too small to begin with, you'll increase the amount of clipping.

Afocal photography

Put the eyepiece in the scope, just like for a normal visual observation. Now put the camera at the eyepiece and move it around until you capture the image. Click the shutter. This is called afocal photography.

You'll need a camera mount, that will attach to the focuser, holding the camera at the eyepiece. Something like this:

http://www.telescope.com/Astrophotography/Astrophotography-Accessories/Orion-SteadyPix-Deluxe-Camera-Mount/pc/-1/c/4/sc/61/p/5338.uts

Keep in mind that your camera is very likely too heavy for this trick to work. Also, afocal is usually the lowest quality of all techniques, and rarely works well enough.

(There are special adapters that go on the eyepiece on one end, and thread into the camera lens like a filter on the other end. These would work better, but are hard to find, only a few eyepieces support them, and your camera is too heavy anyway.)

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Keep in mind that your scope has a 1.25" focuser. Even if you do get the sensor in focus, the small size of the focuser will clip the image; you will only be able to use the central part of the sensor. It is recommended to use 2" focusers for prime focus imaging with large sensors.

Also, your f/4 scope would exhibit strong aberrations at the edge anyway, which would blur the image. For prime focus imaging, longer focal ratios are usually desirable.

All in all, your scope and your camera are mismatched. You have a pretty good camera, but a very small scope that is not made for imaging. Either use a dedicated astrograph with that camera, or keep the scope and use a cheap USB "eyepiece camera".

Answer 2

With a similar problem, I was successful by designing and 3D printing my own adapter.

My setting was

• Celestron AstroMaster 130EQ,
• Canon EOS-M mirrorless camera.

I cound only find 1.25"-to-EF adapters (for the SLR range of cameras), to be combined with an EF-to-EF-M adapter, thus wasting precious focus distance, not letting the sensor get into focus.

On Thingiverse, I found an EF-M bayonet model (which I refined for a better fit) and added a 1.25" tube to fit into the focuser.

To check whether this approach can be successful for your telescope/camera combination, you need to know where the focus of the scope is located. You can point at the moon, and with a sheet of paper held somewhere behind the empty focuser (fully-in position), you should be able to find the focus plane. If the distance between paper and focuser's end is more than the flange distance of your camera, an adapter is possible.

Of course, 3D prints are typically made of plastics, which isn't as stable as metal, so the weight of your camera matters as well. The lightweight EOS-M was no problem at all.