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Old Monday 8th August 2011, 21:48   #1
Mevcrna
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Celestron Regal 100 ed

Hello all.
I am new to the forum. I have searched but can't find an answer to my problem. I would like to use my Regal 100 ED as a prime focus lens, and hane seen allusions to this use, but I can't get any focus with a t mount adapter to my DSLR. I thought maybe back focus was too short, but I can't get any image of stars or the moon either (infinite focus). All I get is a blur. I added 2 inches of eyepiece extenders and still couldn't get a recognizable image.
Any ideas?

Mevcrna
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Old Tuesday 9th August 2011, 03:52   #2
Smithhill
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The moon is the wrong direction to gain backfocus, get a close object to extend the focus back into your camera, at least for a trial. Get as close as 10 feet? No, you do not want extenders, wrong direction again. Looks like you are stuck with digiscoping. Gene
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Old Tuesday 9th August 2011, 16:29   #3
Mevcrna
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"In order to focus on distant objects, the lens must move closer to the eyepiece." from the Physics Forum.

This made me think I would be able to focus on a distant object if not a nearer one. If the focal point can't reach the camera sensor with the 4" EP removed, how does it work with the EP?

Just wondering.

mike
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Old Wednesday 10th August 2011, 01:23   #4
Smithhill
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The problem you are having is that the image is focused by the scope inside of the eyepiece. When you replace the eyepiece with the camera the sensor is another approx. 1 1/2 inch back and you cannot get the camera close enough with the adapter in place. Focusing on a near object will push that image back to the sensor area. Some new data below:


OK Mevcrna, I set up my scope and camera for prime focus and got some figures. A bit different hardware than what you have. I have a Micro 4/3 Lumix G1 camera with a M4/3 to 2 inch tube adapter that is marked “T2-M4/3”. The eyepiece used for comparison is a 17mm Baader Hyperion, which has both 2 inch and 1 1/4 inch tube for attachment. The Orion 80ED OTA scope has a two inch focuser.

I focused the scope on a distant object, marked the focuser tube position, and then replaced it with the camera with attached T2 adapter for Prime focus photos. I had to move the focuser tube inward towards the objective 1 1/2 inches.

Just for additional information the larger body of the adapter extends 1 3/8 inches in front of the camera body where it reduces to the 2 inch tube. With the eyepiece adapter in place to adapt to the 2 inch barrel, the focused image in the eyepiece is 3/4 inches beyond the end of the scope 2 inch focuser. That is the point at which the T2 adapter also mates.

There is 3 inches more of back focus available with this scope and focuser beyond what was needed to mount the camera. Of course the 2 inch focuser gives good support for a heavy camera. The 600mm x 80mm ED lens of this scope is sold by about 4 different brand OTA astro type scopes and is a real classic for optical quality.

Although not used for this comparison, I like to use a sturdy two inch diagonal to eat up most of the extra back focus and it is nice to be able to place the scope low on a pod for stability and to look down on the camera. Get back to me if you want.

As an added note, when I got the G1 camera I did not have enough back focus to do prime focus on my Celestron Newtonian scope, so I moved the prime mirror forward an inch or so. Gene

Last edited by Smithhill : Wednesday 10th August 2011 at 01:34. Reason: fumble fingers and a slow brain.
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Old Wednesday 10th August 2011, 01:39   #5
Smithhill
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Another thought. Since you have the Celestron scope I guess you use 1 1/4 eyepieces. You might have enough backfocus to insert a 1 1/4 Astro type Barlow lens. Best to be a low power standard and not the Orion "Shorty". This might give you enough added back focus to reach the camera sensor for distant objects. Gene
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Old Wednesday 10th August 2011, 12:38   #6
Mevcrna
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Gene,
Thanks for you time and expertise. It is appreciated.

I took off the adapter and held the camera as close as possible to the scope and could just barely get an image. Clearly not a usable option.

The scope comes with a t mount adapter on the eyepiece. I was hoping to reduce the amount of glass bettween the objective and the sensor.

If I understand the terminology, eyepiece to camera body is "eyepiece projection" and eyepiece to camera lens is "digiscoping". Do people actually digiscope with DSLR through lens to eyepiece?

So far my Lumix TZ10 produced better images than the Canon XS. Motion is a big issue. Does the G1 have in body image stabilization? Do you use it with a lens on?

Thanks again
Mike
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Old Wednesday 10th August 2011, 15:15   #7
Smithhill
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OK, glad you got an image at least for observation.

Yes, your definitions are correct. I and other people have not been able to use an eyepiece for projection photography. We seem go get an image about equal in quality to grandma's plastic magnifying glass. A couple companies, such as Kowa, do make quality lens attachments for projection photos, but it is expensive. Eyepieces are just not designed for that application.

Yes, some people do quite well digiscoping with DSLR cameras. I do NOT like to hang a camera by the lens. Other support brackets are used many times. The micro 4/3 cameras are a bit lighter, but still heavy. I do digiscope with the sturdy 90 diagonal with the weight downward on the lens. It is better to use lenses not longer than 28mm, I like the 24mm. You still lose speed as compared with a compact camera with a 5mm lens for example.

Image stabilization is designed only to be used by hand held cameras with normal lenses, never for digiscoping. I believe the G1 does not have body image stabilization. Gene
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