Looking for images from LX200 scopes

[bobmoss]

Hi,

I am trying to put together a list of the actual focal lengths of Meade LX200 telescopes.

So I am after photos taken through LX200 scopes at prime focus so that I can calculate the focal length from them.

They don't have to be good (slightly out of focus, poor polar alignment etc. is fine), but they do have to be un-processed (converting them from raw to JPG is fine).

Full size would be best but they might be a bit large so they can be re-sized (as long as you know how much you have re-sized them by!).

They also need to have some stars in them to measure (M42 is a good one as it has some easily distinguishable stars, but nearly anything will be OK).

If you can tell me what scope you have (including whether it's a classic, GPS, R or ACF), what camera/CCD was used (so I can look up the pixel size) and what was in the optical train (including the microfocuser if you used it).

I would prefer ones which you haven't used a focal reducer on (although a couple with a FR would be interesting for comparison).

If the photo is too big to attach here or PM me then let me know and you can either e-mail me it or I'll set an FTP site up.

I've already asked this on another forum (sorry if you've seen the request on there) but have had no photos sent to me at all!

Thanks.

[Merlin66]

Why do you want to do this?

On the SCT's, as you probably know, the focal length varies with the back focus distance.

All you have to do is mount your camera on various lengths of spacer/ adaptors and re-focus.

Having done that, I'm puzzled, a bit, with what you DO with it?

Every set-up I've seen is slightly different; different cameras, different T rings, filter-wheels, spacers, flip mirrors, focusers etc etc etc.

[bobmoss]

Hi,

The reason I want to do this is because from reading on forums etc. it seems that the general consensus is that on LX200 scopes, the "designed" back focus distance is around 120mm and this should yield the native focal length (2500mm in my case for the 10").

However, I have measured the focal length of my scope at 118mm spacing and I get a calculated focal length of 2736mm. Then, using different adapters, microfocuser etc. I measured the focal length at different spacings. In fact, if I use the standard Meade camera adapter I get a focal length of just under 3000mm!

After graphing this data, I estimate that on my scope the native focal length would be at about 50mm spacing which is practically impossible to achieve.

I did the measurements originally with a Canon 50D and repeated the test with a 450D and got the same results.

So, to cut a long story short, I am trying to find out if my telescope is somehow wrong, or, is the designed back focus distance actually different to what everyone thinks it is (or, did Meade change it with the R/ACF revision and not tell anyone).

Thanks.

[Merlin66]

I don't know how Meade grind and polish their secondary mirrors, but they "normally" have about a x5 magnification ie the primary mirror is f2 and the end result is f10. I could well understand if there is some "manufacturing" variability which could easily cause the differences you are seeing. It wouldn't take much to change the effective focal length by 236mm (about 10%)