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Doing things the hard way, or how many teleconverters can you stack?

March 27, 2018

A long while back, before I got the telescopes I now have, I was sitting around wondering what I could do to take a full frame picture of the moon. Of course the obvious answer is get a telescope and attach your camera to it. But what could I do with the equipment I had?

Well it turns out I have a lot of misc. camera gear that I have just collected over the years. Including several teleconverters for the two film bodies I have.

So what is a teleconverter? A teleconverter is a lens you put between the lens you use on a SLR and the camera body. It magnifies the image. So if you have a 300mm lens, you can slap one of these on there and get a 600 mm lens. Cool right? They are generally a lot cheaper then buying an actual lens with the higher focal length. (For you astronomy folks, this is a barlow. Same idea, different name.)

Well nothing is free so there are trade offs with using a teleconverter. One is a loss of contrast. You get this whenever you put more glass between you and the thing. Each time the light hits the glass some of it scatters and things happen. Designers making a lens can mitigate this, but they haven’t got a clue about that extra hunk of glass you are going to add later. Besides loss of contrast, there is simple loss of light. For a 2x teleconverter you can generally count on losing 2 stops of light. In non-photography speak, this means you 1/4 the light on the backside of the teleconverter as went in. (every stop is around 1/2 the amount of light of the previous one.)

The pro to all this con was that I could use equipment I actually had right then instead of equipment I was thinking about getting.

So I rummaged around in my gear. I chose to use the Olympus OM2n for this exercise. The long lens I had for this camera was a Soligar 28-200mm 1:3.8-5.5 zoom. On top of that I stacked a Telesor 3x teleconverter and then a CPC 2x teleconverter. On top of that I stacked yet a 3rd teleconverter this one also a 2x. I think it may have been a Vivitar 2X macro focusing teleconverter.

Here are a couple of sample shots from that night:

scan0035

scan0032

Both of these are pretty bad. The first one has such a long exposure the moon has moved. The second also has some chromatic aberration as you can see in the orange rim near the top.

But, they demonstrate two important things. Firstly, the moon is so bright that even through the darkness of stacked teleconverters you can get enough light. Second, 2,400 mm is a good focal length for 35mm film for the moon.

This later detail I confirmed by reading the book “Astrophotography” by Barry Gordon. This is a good getting started guide for film based astrophotography.

If you want to know why you should use film for astrophotography, well, let me just say that the windmills other people tilt at never seem as interesting as our own.

In any case he explains how to calculate the amount of focal length needed given the degrees of arc across the phenomenon is. The moon is 31 arc seconds which is just over 1/2 a degree. For 35 mm film, or full frame DSLRs, the full frame focal length is 2,539.5 mm.

I also learned something of a con in my setup. At the time I had a simple pan and tilt tripod head. This was not easy to use to photograph the moon. For one thing when you had the head locked in position, it would sag under the weight of the camera. This meant to frame the shot, you had to over correct and take into account the sag once you let go. This quickly becomes frustrating. It explains why neither of these shots is framed well. I have since corrected this with a gear head as I have mentioned in other posts.

What I did not learn from this but which was a problem was that it is difficult to focus on the moon. For me at least, it is absolutely necessary to use the focusing aids that manual cameras have to get a clear shot at full frame.

So what focusing aids do manual cameras have. The most common is a split prism screen. Some DSLRs have the ability to use these because some high end lenses do not have auto focus abilities. Also auto focus can be difficult to use, or simply fail in low light conditions or when teleconverters are used with certain lenses.

The way I figured out that focusing was the problem will be covered in another blog post, but it turns out that the lens I was using was not able to focus on the moon when stacked on the teleconverters. I wonder if the lens used for the above shots had the same issue. I certainly wasn’t being as careful to focus as I have learned to be.

So lessons:

  1. Use a tripod head you are comfortable using to track the moon. It should not sag under weight when you let go of the controls. For me this is a gear head. Ball heads are notorious for sagging.
  2. 2,400 mm is a reasonable focal length for 35 mm or full frame cameras.
  3. 3200 ISO film is overkill. The moon is a bright object.
  4. Plan on using focusing aids to get that critical focus.
  5. Don’t take it for granted that your lens can focus in the given configuration.
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First Head to Head

December 14, 2017

So a blog dedicated to exploring how to do something would be pointless if it did not compare methods from time to time. With that in mind it is time for the first head to head.

In this corner we have the Youniker lens, and add for smart phones that let’s you, yes you, get that perfect shot. In the other corner we have a Celestron Travel Scope.

Youniker 18x Zoom

vs.

Celestron-D70

While the Youniker kit came with a smart phone clamp, the Travel Scope did not. So an accessory, a Gorsky universal telescope adapter, was also necessary.

First the pics:

On the left is the Youniker shot. On the right is the Celestron shot.

IMG_20170905_225636

moonshot-d70-oneplusone

To me the winner is clear. The Celestron shot has better detail. (It is also better exposed, but that hasn’t anything to do with the lens.)

Another point is that both shots have chromatic aberration. What’s that? That’s when you see a colored fringe near and edge in your picture. It happens because different colors bend by different amounts as they pass through the glass of the lenses. All lenses have this to some degree or another, but higher end optics have coatings on them to correct for this kind of thing. It can also be corrected by software such as Photoshop. The aberration is more apparent in the full size version of this image. There is a green fringe in the Celestron shot and a bluish fringe in the Youniker shot.

So better lenses right? In the case of the Youniker, there isn’t anything you can do except post process it out. In the case of the Celestron, I am not sure if the issue is in the main telescope lens, or the eyepiece used. If in the eyepiece, then a multi-coated eyepiece would be a simple solution. Otherwise a new scope would be required.

So the Celestron wins the first part. It has better detail, even though both fail for chromatic aberration.

But now the second part. Ease of use.

The Youniker was difficult to use. It is difficult to mount to the smart phone in the first place. It was difficult to aim at the moon. (And if you can’t just look up and see the moon, what is going on?) The little table top tripod it came with was useless for this task. Once pointed, it was difficult to get focus as this required turning the barrel of the Youniker and then tapping the screen to autofocus.

The Celestron was not a picnic either. Like the Youniker, the tripod it came with was not useful for this task. (I have since learned that these kinds of tripods work better if you do not extend the legs.) Nevertheless, the Celestron was easier to aim then the Youniker.

So the Celestron wins in both image quality and ease of use. Not that this means much. There are better ways to get a moon pic.

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