I have an 8 inch Dobsonian and I'm taking shots of Saturn through my 35 mm film SLR camera with adapter.

The highest mag I can use with the camera is using a 25 mm with a 2 X magnifier (that extends the light path), giving the equivalent of looking through a 12.5 mm.

This gives about 100 power which is still small photographically.

I'd like to use higher powers to highlight Saturn's rings but I don't have enough focuser travel in the inwards direction. Using higher power eyepieces even with the 2 X magnifier, I can't get into focus cause the focuser doesn't go down far enough.

Is the solution a more low profile focuser? Yet the focuser on it now seems pretty low profile. Is there a solution?

It is so funny what that title looks like on the main index page...

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If your focuser will not go in far enough with the eyepiece you wish to use, you will need one with a lower profile.

To get an image with anywhere near the telescope's resolution limit, you will need a guided equatorial mount for any exposure longer than about 1/15 second, because the image will drift about one arcsecond in that amount of time. A Dob would be limited to snapshots of bright objects, and Saturn almost surely is far too faint for such work. My rough and dirty estimate for your 100x attempt is about 1 second with ISO 400 film. An unguided exposure that long would cause a prohibitive amount of blurring.

Hi,

A Newtonian telescope mounted on a Dobson mount, is just not capable of the sort of photography you are trying to so, for reasons explained in a previous post. Before the Dobson alt/az mount became popular, anyone interested in astrophotography started with an equatorially mounted telescope.

As far as increasing your magnification goes, use a decent Barlow telenegative lens in conjunction with an eyepiece, This will put to focus well into your focuser travel, a 2x Barlow lens will increase the magnification range of all your eyepieces.

Clear skies...

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Shevill Mathers, Southern Cross Observatory - Tasmania 42 South.
Hon. Associate School of Maths & Physics
University of Tasmania
http://www.astrotasmania.com-a.googlepages.com/home

well where do I find this magical telenegative lens? I already have a 2 x Barlow that I screw with the eyepiece..

I can't get a more low profile focuser; heck as it is now, the focusing knobs are almost flush with the tube!

Then you move the mirror forward and add some weight to the back to put the center of gravity back where it belongs.

Also for Saturn you'll need a tracking platform when using film. Back in my film days I needed at least f/50 for fine grain film on Saturn, that took a 4 second exposure using the hat trick to avoid mirror flop vibration. In 4 seconds the earth's rotation will move saturn more than its own diameter so it will be a fuzzy streak. You'll need a tracking platform under it. They can be bought or made depending on your circumstances. Google the term <"tracking platform" telescope> without the brackets but with the quotes and you'll turn up info on building one as well as commercial units. Try "Poncet platform" as well Check the ads in the back of Astronomy and Sky and Telescope magazines as well. Lots of them out there in all price ranges. Simple single axis ones are all that's needed for planetary film photography.

Rick

Hi,

I have to say this does not make sense to me somehow, when you put a Barlow into the focuser - it extends the focal plane away from the focuser outwards and with a camera attached, one normally would have to rack the focuser out, especially if the camera is dome distance from the Barlow lens. Without seeing how it is all set up I am shooting in the dark.

I can understand it if you were trying to do prime focus with your camera, then you do need in-travel.

Try the following: Insert your long tube Barlow (2x telenegative lens) (not a shorty) into the focuser attach your camera say 4" away from the lens if you can (no eyepiece) and you should get an image in focus, start all the way racked in then rack out and you should get an image at some point (racked out - not in).

The Barlow lens produces a diverging virtual image which you cannot see with your eye but it will be seen if you hold a piece of frosted glass or thin greaseproof paper over the end and adjust the focuser. The farther away from the lens - the larger the image gets, also much dimmer. This is well worth doing with the Moon which has heaps of light which makes it an easy experiment.

Let me know what happens before going any further.

If you can post some quick pics of your set up, it will help.

Regards
Shevill

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Shevill Mathers, Southern Cross Observatory - Tasmania 42 South.
Hon. Associate School of Maths & Physics
University of Tasmania
http://www.astrotasmania.com-a.googlepages.com/home

Here are pics of my setup; sorry about the blurriness in the first one. I'm using a 2 x barlow. The lens is screwed out of it to show you. I cannot attach the whole 2 x barlow into the camera adapter as it makes the 18 mm eyepiece stick out too far such that it would hit the camera's reflex mirror so I just screw the 2 x lens into the 18 mm eyepiece or whatever eyepiece I'm using.

I can't get Saturn or the Moon or anything at Infinity into focus. And as you see in the second photo, the foruser is racked in all the way. I want to be able to use a 9 mm eyepiece with the 2 x barlow on Saturn.

How about I use .96 eyepieces? Does a .96 eyepiece give a different focal point geometry? Help me?!

It looks as if you have the Barlow lens assembly in the focuser and the camera adaptor inserted into the back end of the Barlow.

If an eyepiece in place of the camera adaptor shows a focused image at the same focuser setting, then the film in the actual setup is 3 or 4 inches behind the focus. If you could rack the focuser in enough farther with the Barlow in place, you might be able to move the focus out to the film, but the optical correction might suffer in the process.

My hunch is that the camera adaptor was intended to be used with a typical Schmidt-Cass, with which you easily can move the focus back many inches to reach the film. I don't see any way to make this Newtonian reach the film in this setup without remounting the primary mirror farther up in the tube. Then you will need some sort of extension tube to move the eyepiece out for visual use.

Hi Spaceboy,

Thank you for taking the time to gets photographs, each is worth a thousand words. The news is not good, I'm afraid.

At a glance I can tell that you are too far inside focus. The shorty Barlow is defeating its purpose by a, being too short in tube length, b, pushed too far into the focuser. With your short Barlow set up as it is, you cannot rack it out far enough to come to focus.

Also, if you use a 9.0mm EP in the Barlow, the camera needs to be a lot farther away from the EP to the point that the whole thing becomes unwieldy. You would need the same weight stuck out the opposite side of the focuser as well as more weight added to the mirror end.

What you are attempting to do will not work with a Newtonian telescope on a Dobson mount. The resulting image will be so highly magnified and very dim, which even if you could get it in the field & focused, would be out of the field in the blink of an eye, due to the stationary mount. The exposure needed would be too long and end up being a blur.

I have a friend with a reasonable level of photo experience, with an 11" SCT whole is struggling with a web cam, which produces a much bigger image of Saturn than your 8" Newtonian, so the task is not an easy one, even with the right size EQ driven telescope.

The next Moon is coming along next few days, please do the little experiment with just the Barlow lens and the tissue/tracing paper - it will show you just how far away the camera needs to be. Don't push the Barlow all the way in to the focuser as long as it is secure, and rack the focuser out and leave enough travel to focus.

Please let me know how your Moon experiment goes.

Regards
Shevill

__________________
Shevill Mathers, Southern Cross Observatory - Tasmania 42 South.
Hon. Associate School of Maths & Physics
University of Tasmania
http://www.astrotasmania.com-a.googlepages.com/home

yes but how about using .96 inch eyepieces? I have an older 2.4 inch Celestron refractor with a .96 inch neck. I found the 25 mm .96 ocular gave a different focus distance than the 25 mm 1.25 inch ocular. I also have a .96 inch camera adapter.

There is no general rule. It depends on the position of the eyepiece's entrance pupil when the eyepiece is inserted all the way into the focuser tube or the Barlow lens tube as the case may be. The entrance pupil is a ring in the tube just forward of the front lens of the eyepiece, and it forms the sharp edge of the field of view.

So far it is not clear as to what lenses are in the optical path between the camera body and the diagonal as shown in your second picture. I am guessing that the Barlow lens is in its normal position. Have you placed another lens, such as one of the eyepieces, inside the adapter between the Barlow and the camera? If so, is it accurately centered and aligned with the optical axis?

No the only way this system can work physically in terms of all the camera parts connected together is with the 2 X barlow lens screwed into the eyepiece. The adapter that the 2 X lens comes with is not part of the mix in the camera setup.

Are you saying the distance between the 2 X lens and the eyepiece is of importance here?

It appears that you are attempting eyepiece projection onto the film. What still is not clear is the location of your eyepiece-Barlow combination. Is it inside the Camera Adapter tube? If so, it looks as if it will be too far back from the telescope mirror's focal point to have any chance of coming to a focus.

The spacing between the Barlow lens and the eyepiece is important. In normal visual operation, the field stop of the eyepiece will be approximately abreast of the locking screw on the back end of the Barlow tube, and the Barlow lens will be a little over two inches in front of the stop. If you screw the Barlow lens directly into the eyepiece the spacing will be much less, reducing the multiplication and upsetting the optical correction.

yep, the 2 X is indeed screwed into the eyepiece and both are inside the Camera adapter

It won't work there. Put it back in its holder then put the camera adapter into that as if it were an eyepiece. Then, if its holder puts the barlow down inside about 2" it should work fine.

Rick

Okay have done an experiment. I focused the 8 inch Dob on a tree on a mountain about 2 miles away.

I used an 18 mm eyepiece in both situations.

I marked with a felt pen the point on the focuser tube where the image came to focus with the 2 X screwed directly into the 18 mm eyepiece.

Then repeated with the 2 X lens inside the 2 X adapter and 18 mm eyepiece inside that combo.

In the latter, with the 18 mm eyepiece inside the 2 X adapter and 2 X lens inside that combo, the focuser was 14 mm further in than with the 2 X lens screwed directly into the 18 mm eyepiece.

I got a sharp focus in both situations.

The latter configuration will give you the full 2X multiplication and the best optical quality. The former will give you less power and might introduce some spherical aberration that could degrade your images.

You have given the settings for visual use. If you rack the focuser out slightly, the telescope can be made to project a focused image onto film several inches behind the eyepiece. The trouble is that if you put the eyepiece/Barlow combination into the camera adapter tube, it will be at least an inch and a quarter, maybe more, farther out from the scope, and the focuser may not be able to go in enough farther to achieve such a focus. There is no way the eyepiece barrel can fit into the small end of the camera adapter from the inside, because both tubes have an outside diameter of 1.25 inch.

You would need a custom-made tube that would slide into the front end of the camera adapter, with a suitable lens centered in it. A 5x microscope objective would be a good choice, because it is corrected for just that optical geometry. You would need a machinist with a lathe to make the adapter.

As has been pointed out before, all of this is a moot point unless the telescope is mounted on a guided equatorial mount.

in other words, to get proper 2 X power, the Barlow lens must remain intact with the lens where it's supposed to be and the eyepiece attached at the predesigned position?

A 2x barlow is 2x only at the spacing it is designed for. Decreasing the spacing reduces the power by the amount you reduced the spacing. Cut it in half and its 1.5x. Double it and its 4x. Put it in the eyepiece and it is just about 1x, hardly does a thing for you. If the eyepiece focuses beyond the barlow it will reduce the power to less than 1x and likely not even focus. Never tried it. I never had a barlow that would screw into an eyepiece.

Barlows alone can be used to project an image onto film or a CCD. Often with better results than using an eyepiece. I never tried both in all my years of film work! Altering the distance the eyepiece or barlow projects to the film gave me all the image scale variation I needed. One eyepiece, a 12mm symmetrical so I could use it upside down as well as right side up to change projection distance was all I used for higher powers. For lower I used barlow projection alone. Both work like a slide projector. Think of the barlow or eyepiec as the projector and the film as the screen. Put the screen farther back and the image is bigger but dimmer. Enlarge it much at all and you will need a drive to track the earth's rotation. You might get away with barlow projection this way but certainly not eyepiece projection. The image scale will be too great for the available light, especially with film.

Rick