Eyepiece projection

[Demonperformer]

I hope to get out this evening to try my new "eyepiece projection" set on Saturn & Mars. The only suitable (relative term!) eyepiece I have is a UW 80° 11mm - not ideal, but the only ep that will fit in the tube. Camera is ASI224 colour (actually the cooled version, but don't think that is important for planets?) and scope is an 8SE.

If anyone has any experience of taking images using this method and has any tips that are likely to increase my likelihood of getting good results, they would be most welcome.

Thanks.

[Cornelius Varley]

A formula for calculating the effective focal length using eyepiece projection can be found here . The critical factor is the distance between the eyepiece and focal plane.

[Macavity]

I would be interested to see what you achieve! I did once (rather casually!) try this
technique with Baader Hyperions (which have a screw-on adapter). The result was
(shall we say) "a little indifferent"... But it would seem to be a useful technique! ?
It is a rather easier to achieve significant image "magnifications" though...

[Demonperformer]

5 hours ago, Cornelius Varley said:

A formula for calculating the effective focal length using eyepiece projection can be found here . The critical factor is the distance between the eyepiece and focal plane.

Using the calculator, I can get an ep/chip distance of 97.5mm (which is within the advised 90-120mm range) with 80mm of extension tubes and the 17.5mm nose/chip distance within the camera. This gives me a staggering focal length of 15727mm, or just over f78.6! Will I even be able to get Saturn on the chip at that focal length? And what sort of exposures am I going to need? No idea. This is either going to produce some very interesting results, or very little in the way of results.

Other reasonably easy options (all less than the 90-120 range) are:
32.5mm => 3909mm (f19.5)
47.5mm => 6636mm (f33.2)
67.5mm => 10273mm (f51.3)

I feel that this evening could involve a lot of experimenting ...

[Cornelius Varley]

1 minute ago, Demonperformer said:

Using the calculator, I can get an ep/chip distance of 97.5mm (which is within the advised 90-120mm range) with 80mm of extension tubes and the 17.5mm nose/chip distance within the camera. This gives me a staggering focal length of 15727mm, or just over f78.6! Will I even be able to get Saturn on the chip at that focal length? And what sort of exposures am I going to need? No idea. This is either going to produce some very interesting results, or very little in the way of results.

Other reasonably easy options (all less than the 90-120 range) are:
32.5mm => 3909mm (f19.5)
47.5mm => 6636mm (f33.2)
67.5mm => 10273mm (f51.3)

I feel that this evening could involve a lot of experimenting …

With conventional (prime focus) planetary imaging the usual focal ratio to aim for is about f20. At f78 Saturn will probably look like a mushy blob. The f19.5 option probably looks like the best but you can easily achieve that with a 2x barlow.

[Owmuchonomy]

Here are my 2 efforts from 2014 using a Canon 60D and a Celestron 15mm Plossl inside an unbranded T2 eyepiece projection tube placed in the diagonal of my SCT 9.25.  I have no idea of the FL achieved but I was actually quite pleased with the outcome.  It's a very simple method.  The pixels of the 224 are much smaller than the Canon I think.  You should aim for a f/ of 4 to 5 times your pixel size.  Good luck!

[Demonperformer]

26 minutes ago, Cornelius Varley said:

The f19.5 option probably looks like the best but you can easily achieve that with a 2x barlow.

That is my thought and the barlow is a lot simpler.

 

8 minutes ago, Owmuchonomy said:

 You should aim for a f/ of 4 to 5 times your pixel size.

The pixels in the 224 are 3.75, so that would give me (roughly) f15 to f19, so that pretty much matches the above.

All of which begs the question: why does anyone ever do it this way ...

[Demonperformer]

I've even got a 3x barlow so even if I wanted to take it up another step, the barlow would still probably be easier.

So unless I try taking it up to the f50 option ...

Mmmmm

[Stub Mandrel]

3 hours ago, Demonperformer said:

Will I even be able to get Saturn on the chip at that focal length?

I doubt it!

I image planets at the equivalent of 3.6 meters and a VERY accurately aligned finder is needed. That also give me a pretty much optimum pixel scale. At 15 meters you probably ought to be imaging with a 60cm scope...

Edit for reference: 150PL is 1200mm + x3 barlow for 3600mm = f24 used with cameras with 3.75um pixels

[martinl]

f20 is pretty much optimum for 3,75um pixels, so if you have a 2x barlow just use that. Getting the planet on the chip is not trivial. If you center the planet using a standard eyepiece it will be so out of focus when you replace the EP with the camera that you’re likely to not even see it on the computer screen. Either be aware that you need to refocus a lot to account for the ~12.5mm back focus of the camera or, as i did, make an EP parfocal with the camera. I added an extension to a TV Plossl and ~6mm of rubber O-rings (TV EPs already focus ~6.3mm out from the shoulder) to do this. 

[Demonperformer]

Well conditions last night were far from perfect ... a lot of moisture in the air and the image was jumping around all over the place. Based on the above, I avoided trying eyepiece projection and settled for some F10 and F20 shots with 2x barlow. I am not particularly optimistic about the results, but will see. Will post the results (maybe!) later.