Western Crises

[GazOC]

Thats true Steve,.

[FLO]

Sorry Starman, where are my manners!

Welcome aboard and thanks for sharing some truly stunning images.

Steve

[Astroman]

Interesting but I'm too tired for a fight

So when imagers state that their system is working at f/20, f/30, etc. - they are wrong or is this just convention?

Pete

This is interesting because i have always beleived that if like me you have an F9.4 scope when

i put my 4x image mate on then my imager the image would have been taken at F 37.6 or there abouts.

Could you please talk me through the ins and outs of the F ratio for future reference.

Skyguy: When you put your 4x image mate in, it increases the focal length only slightly-a few centimeters.  The focal ratio will increase maybe to F/9.6 or F/10 at most.

[Jamie]

Cheers Astroman i totaly understand what you are saying.

So could you please tell me how does someone take an image at say f40 ie what type of setup would do this?

I ask this because i have seen planetary images taken by others that claim to be taken at f40 but can't remember the setup they used.

Indulge me here its thicko time.

[Astroman]

heheh no problem, Skyguy.

There's a couple ways to do this.  Aperture mask or eyepiece projection.

Say I'm using a C8 at F/10.  To get F/40, I could stop down my aperture to 2".  My focal length is still 2032mm, but my aperture is 5cm.  F ratio is FL/aperture, so ~F/40.

The other way is to extend the effective focal length.  If I normally focus at 2032mm, I can extend that by placing an ep and projecting the image farther back.  This isn't a straightforward method though.  To calculate this, you need the equations mentioned by James earlier.  What most people do is set up the picture for proper image size on their medium, then calculate, after the fact, what they were shooting at, then rounding to a nice number.  The smart ones, (or, really [removed word] ones), work up a chart or table when they do a setup, so they don't have to calculate every time.

Hope this helps.