Eyepiece Projection and Skywatcher PDS200

[JeffM]

New member question. 
I can image the moon at prime focus with a canon DSLR no problem. 
I have a basic set of adjustable extension tubes, intending to try some eyepiece projection but I just can’t seem to get focus, as if there’s not enough INWARD focus travel to achieve a sharp image.

I can’t see why I have the problem as using the same eyepiece visually i have no problems focusing it. 
Does anyone else use EP on the PDS200 (or differently sized siblings) and if so, what might I/my equipment be doing wrong?

[vlaiv]

Not sure what is going on exactly, but here is "theory" in the nutshell - it might help you figure out what is going on.

There are three different configurations that you can use to image with your scope:

- prime focus (that is just camera sensor at focal plane of telescope)

- EP projection - that one is eyepiece between sensor and telescope

- Afocal imaging - that configuration has both eyepiece and camera lens between telescope and sensor.

First one I presume you understand.

Third one is like using telescope for visual observation with difference that camera lens acts like eye lens and camera sensor acts like human retina.

In that configuration beam exiting eyepiece is collimated - or parallel and it is eye/camera lens that does the focusing. That gives proper EP focus position.

If you are using EP projection, eyepiece acts as a simple lens and you no longer want light to exit as parallel rays as that would give you blurry image on sensor - you want EP to focus light to sensor.

This can be achieved in two different ways - you can have EP act as focal reducer and you can have EP act as "regular" lens (re-imaging lens). Here are simple diagrams of light rays to help you understand:

Upper diagram shows EP acting as "focal reducer", while lower diagram shows EP acting as re imaging - lens.

If we take regular EP focus position as "baseline", then two above cases can be summarized as:

- for focal reducer, you need sensor to be closer than focal length of eyepiece (so if you use 32mm EP for example you need sensor to be less than 32mm away from EP, or "inside" its focal point). This configuration also moves focus position "inwards" with respect to baseline focus position - it will act as regular focal reducer - reducing size of the image. Reduction will depend on sensor-EP distance.

- For regular EP projection, or bottom diagram, you want sensor to be further than focal length of EP away from EP. This configuration will move focus point further away from telescope (outward focuser position compared to baseline) and it can result in different magnification depending on where you put your sensor. If you put your sensor at twice focal length you will get 1:1 - or no change in scale, and it also means that you will need to get "one focal length" outward focuser travel as well.

Depending what you want to achieve - you want second scenario, first one is rather difficult and in general you don't have enough inward travel to get it.

You can use online calculator for distance and focus travel needed - like this one:

http://www.wilmslowastro.com/software/formulae.htm#EPP

It will give you approximate results (but good enough for orientation).

For example, using 25mm eyepiece on your scope and placing sensor at 80mm from it will give you:

2200mm effective focal length.

You can also use lens formula to calculate outward focus needed:

1/object + 1/image = 1/focal_length

so

1/object = 1/focal_length - 1/image = 1/25 - 1/80 = 0.0275

So object distance = 36.4mm and since regular FL of eyepiece is 25mm, difference is 11.4mm - that is how much outward focus you need in above case.

Hope this helps.

[Alien 13]

I have dabbled with eyepiece projection but always found it fiddly and the results highly dependent on the quality of the eyepiece in use. A better method in my view is to use a barlow lens instead, the magnification factor can also be played around with by extending or reducing the distance to the camera sensor with those tubes you have.

Alan

[JeffM]

Thanks for the advice all. After perseverance and a bit of tinkering, I managed to take some images of the moon, with eyepiece projection tonight.