Scondary size question??
Posted 29 January 2013 - 05:27 AM
I've already have 3.1" secondary.
It is not bad but I want to more samll secondary mirror
for more contrast when observing planets.
I'm considering 2.14" secondary.
I found Lockwood's table for secondary size.
In there 2.14" for my dob meand 0" fully illuminated field size..
My mirror is 13.1" F4.5
and the distance of secondary to focal plane is about 9" long.
What exact meaning of 0 fully illuminated field size?
Is it impossible to use 2.14" secondary?
If it possible for planetary then my plan is
use 2.14" for planet and 3.1" for general observings.
Posted 29 January 2013 - 08:26 AM
The zero field size means that the secondary is too small. Even for a planetary scope you want a field size of about 0.1" (or 2-3 mm). This ensures that the center of the field is not being vignetted by the secondary. Typical designs for a general purpose scope use a 0.5" fully illuminated field size.
Another problem with using a minimalist secondary is that everything must be perfectly centered or you will not see the whole mirror. In other words the spider must be perfectly adjusted, the secondary position perfectly adjusted and the mirror centered in the tube. This is possible to achieve with careful construction, but it is nice to have a slightly larger secondary--it makes everything more forgiving.
To better answer your question we need to know the f-ratio of the telescope.
Posted 29 January 2013 - 10:17 AM
I once had a 13.1 f/4.5. It had some noticeable astigmatism at full aperture, but showed beautiful colors in the Orion nebula. I put a 5" off axis stop on it one time and compared the views with my Quantum 6 Maksutov.
The planetary performance was virtually identical and there was no trace of astigmatism visible while using the stop. Something to consider.
Posted 30 January 2013 - 03:29 AM
Remember the distance is from sec to focal plane not a racked in focuser. Focus the moon on a piece of frosted tape or glass will give you the fp. Try all your eyepieces as some require more in focus and some require more out focus. Take measurements.
It's comman to add a inch to focuser height for differant eyepieces, barlows and paracorr.
So if you measured right your distance 9" will be 10".
I recommend not going below a 2.60" secondary.
That's only 20 percent.
Actually if your sec 3.1 inch (24 percent) is of good quality I would use it.
Posted 30 January 2013 - 01:44 PM
Posted 30 January 2013 - 02:05 PM