hello guys! i have a quick questions. i have a 10 inch (254mm) skywatcher collapsible telescope. the f/ratio is 4.7   (its fast)  and the secondary mirror it came with is 58mm(minor axis).  focal length is 1200 mm . i put smudges on the secondary with my fingers accidentally and seems like i scratched the mirror when trying to wipe it off, i would like to get a new secondary mirror, what would you guys suggest in size for the best optimal performance with this size of a telescope. what are my minimum to maximum sizes?

p.s. if you think 58mm is best, i cant seem to find that size because it is unusal and this scope is pretty fast i guess . thank you guys

You should be able to get a replacement from Skywatcher. If thats the size their using they should have them.

I notice the Orion version (XT10/Intell10) which has the same focal length and focal ratio uses a 63mm secondary.

I found this web page which has a chart of secondary sizes as well as a pretty decent write-up on the whys and hows of sizing a secondary.

http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html

yea , i tried emailing the maker....... and they dont have a phone number on there website, i havent heard back from them......sorry i am a newbie and dont understand that chart. can you maybe help me choose a size that would be the best? thanks again guys

Their phone number is on their website. It is 1-888-880-0485.

Their address is 2835 Columbia Street, Torrance, California 90503

oh really , hmm interesting. i went to this web site for them http://www.skywatchertelescope.net/swtinc/index2.php

 do they have another website?

could not seem to find a number, or an address

Ok, from what I can tell the chart is showing the size of the primary mirror on the left column. Based on the numbers you've given I suggest looking at the data for 10" F5.

To determine D2, it's the distance from your secondary to the focal plane. I think you should be able to figure it out, since you know the scope is 1200mm FL ie. from the primary to the focal plane. Measure from primary to secondary along the outside of the scope, and subtract this from 1200mm. This should give you D2. If the exact value for D2 is not listed in the chart, choose the next higher value. The author suggests for general viewing you use the bottom row of data, ie. Field in focus= 0.5".

Just guessing that in your case D2 should be about 5 inches + 2 inches for the focuser plus some more for where the focal point is, so around 8-10 inches. According to the chart, the secondary should then be between 2.02" and 2.40". You said your secondary is 58mm- thats 2.3 inches

 Very good Telenoob.

sam85:

oh really , hmm interesting. i went to this web site for them http://www.skywatchertelescope.net/swtinc/index2.php

 do they have another website?

 

could not seem to find a number, or an address

I went to this website;  http://www.skywatcherusa.com/home.php

thanks guys i appreciate all the help , !!!

 Hubble Optics on ebay are selling a 10" f4.7 mirror and as part of the ad they specify the secondary size. To quote:

A = d + (D-d)L / F

A=diameter of the minor axis of the secondary
F=focal length of the primary
d=fully illuminated fields desired
D=Diameter of the primary
L=distance from the secondary to focal plane

Normally, a secondary with about 2.48" minor axis diameter can be used with this mirror.

zachsdad:

I found this web page which has a chart of secondary sizes as well as a pretty decent write-up on the whys and hows of sizing a secondary.

http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html

 

okay i am very new to the whole idea of making a telescope

and i that link doesnt work for me

can someone post one that is close to it?

thanks

check out www.howtelescopeswork.com  thanks!