M82 from a 16inch Meade in Kildare

M82 pastels on black paper
November 27th 2008
22:20 UT 22:45 UT some cloud made it longer to do
Transparency Poor
Seeing 6/10
No filters used
It was the first time I saw any structure within this object oh boy it's great
have tracking when sketching.

Meade 16 inch FL 4000mm
22mm Nagler eyepiece = 181X
Co Kildare

Deirdre Kelleghan
PRO IFAS

Another fine sketch Dee.
Great detail in there.

Nice one Dee.
Some of the train wreck apparent in your sketch.
Was looking at this last night in Simon's CPC1100 and no great detail could be seen, guess the extra aperature is required.

Another lovely sketch, Dee.

I can't remember if Michael is running a Crayford focuser on the back or not, but if he is the focal length will be a bit longer than 4000mm. I used a fairly large Crayford with a rotator which gave me about 4460mm; I'd guess a Feathertouch would net out at somewhere in the region of 4300.

Cheers,
-- Jeff.

jeyjey wrote:

I can't remember if Michael is running a Crayford focuser on the back or not, but if he is the focal length will be a bit longer than 4000mm. I used a fairly large Crayford with a rotator which gave me about 4460mm; I'd guess a Feathertouch would net out at somewhere in the region of 4300.

The focal length does not change with different focusers - that's fixed by the optics. SCTs often focus by moving the primary mirror along the optical axis to move the focal plane, but the *focal length* still doesn't change. The only way you can change the FL is to introduce more optics into the train, like a Barlow or focal reducer, which change the *effective focal length*... unless I'm missing something!

Phil.

Phil --

Yes, what you're missing is that the secondary of a Cassegrain acts as a magnifier (turning an f/2-ish primary into an f/10-ish system, but that the amount of magnification is dependent on the spacing between the primary and secondary. So an SCT is only exactly f/10 at one particular focus. With different eyepieces this doesn't really make any difference, but when you hang stuff off the back of it, it can make a larger difference.

Figuring out just how much of a difference is more difficult -- it is not the length of the stuff you've inserted into the light path. Chris Lord wrote up a paper on it, but the maths are over my head. (Fortunately, he included a chart at the back for some common systems.)

Chris Lord's paper: www.brayebrookobservatory.org/BrayObsWeb...BOOKS/EFLMAKCASS.pdf

Cheers,
-- Jeff.