EP projection

[Gary]

Can someone in beginners terms explain EP projection. What equipment you need, what it is most suitable for and what sort of images can you produce.

TIA

Gary

[Merlin66]

To do eyepiece projection you need a suitable adaptor tube which will hold an eyepiece at the front and your camera at the other end.

The camera connection is usually a T thread. These are avialable at any astronomy shop.

Orion Universal Camera Adapters.

One thing to watch... not all the eyepieces today will fit inside this adaptor! The new wide angle eyepieces have large bulky body shapes ??!! which won't fit.

The system works like a slide projector....

The image produced by the main mirror/ objective is placed close to ( but not at) the focus of the eyepiece and a secondary magnified image is produced at some distance behind the eyepiece... hopefully on the film plane of the camera!

The magnification depends on the focal length of the eyepiece and the distance between the eyepiece and the camera. Say a 25mm eyepiece was used and its 100mm to the CCD, then the magnification is approx 100/25 =4 Which is similar to using a x4 Barlow lens.

This projection method is usually only used on brighter objects like the moon and planets.

Hope this helps.

[Gary]

Thanks Merlin, that helps greatly I think, so what is the difference between EP and I think the experts call it Afocal i.e. when you attach a camera indirectly behind the eyepiece ?

I have done this with a digital camera hand held a few cm's from the eyepiece until I managed to get a basic CCD.

[Merlin66]

The Afocal method is when you use the Camera lens behind the eyepiece, so there's no projection... You're really just taking a picture of the image in the eyepiece..

[Talitha]

There's also 'prime focus'. It's when you connect the camera directly to the OTA without using a camera lens or eyepiece. I've done it with my Olympus OM-1n film camera on the 8"SCT, but to be honest i'm not sure if it can be done with a DSLR... you'd have to talk to one of our imagers to get some info on that.

[Gary]

Now I'm really confused, EP projection, Afocal and now Prime Focus.

Although I have read this forum for ages as a non-member and I'm really a beginner feeling his way I can't work out the advantages or downside of any of the above. Can someone define each?

[brianb]

I can't work out the advantages or downside of any of the above. Can someone define each?

Prime Focus - using the scope as a camera lens - image is small, maybe too small, especially with short focus scopes. But it works well with most scopes - you can run out of backfocus with some scopes, especially budget Newtonians, used with SLR cameras.

Afocal - uses the eyepiece and the camera lens - lots of glass; light loss & distortion issues likely, but it's very simple and can work well, even with compact & phone cameras .

Barlow projection, eyepiece projection - these are similar, use a lens to enlarge the focal plane image. Barlow works best for small increases (around x2 to x4), EP projection works best for large increases (x5 and over). Difficulty focusing , impossible without a reflex viewfinder or a "live view" system. Long focal length gives a good size for small objects but leads to long exposures, a good solid mount & steady seeing are necessary to get good results.

Most deep sky imagers are using prime focus, maybe with a focal reducer / field flattener. Planetary & lunar imaging specialists almost always use a Barlow these days; eyepiece projection was more common with film, when longer focal lengths were necessary to obtain detail.

[Gary]

Thanks Brianp that has made the differences between each much clearer to me.

Gary