Jump to content

Narrowband

GSO RC8 primary - secondary mirror distance


Recommended Posts

Hi all

Ever since I took apart my GSO RC8, I'm not sure about the distance there should be between the secondary and primary mirror.

This distance is set by driving the central screw further or closer to the plane where the branches are attached. See picture below.

On a stock RC, there seem to be only 1 or 2mm on the "B" distance. Another friend did some tests and calculated there needed to be 74,5mm for the A distance. That's much more than what seem to be the stock distance.

So the question is, what is the proper distance ? Can anyone please upload pictures of their front area on the RC to compare ?

10772-1401531311.jpg

Link to comment
Share on other sites

  • 2 months later...

Hi Philming,

I was struggeling with the same question for my GSO RC 6 inch telescope. I think the best way to check it to use a astrometry program on one of your CCD images without any reducer. The astrometry program will be able to calculate the focal length and therefore confirm the correct distance. 

To check the focal distance distances I used the astrometry program "Pin point Astrometry" to analyse my CCD images. From my oldest images (before I took the telescope apart) it calculated for my 6 inch RC a focal length of 1396 mm without flattener. With a flattener at a little to short distance it calculated a value of 1379 mm. This is close to the published value of 1370 mm. So I assume it is correct to include the flattener in the focal lenth check. If  the distance between the two mirrors is increased with 2 or 3 mm, the calculated focal distance decreased from 1379 mm to 1330 mm. So a factor 10 or more as confirmed in http://interferometer-tests.blogspot.nl/2013/06/2542000-10-ritchey-chretien-gso.html.

For your GSO RC 8" the focal length should be 1624 mm. Analysing your images should produce this value plus or minus within maybe 15 mm. Your secondary mirror distance should then be correct within maybe 15/10=1.5 mm. For images without a flattener, I estimate that an astrometry program will calculate a focal length about 20 or 30 mm above the 1624 mm.

A little more info you can find at my webpage:

http://www.hnsky.org/RC_collimation.htm

If think there are astrometry programs online.  If you can't find one, you could send me an image for anlysis via my webpage email

Regards, Han

Link to comment
Share on other sites

Here an example what an astrometry analysis will tell you:

Locating stars in image
Looking up stars in catalog
Solving using 4031 image and 766 catalog stars
Matched 188 of 4031 image and 766 catalog stars;
Average residual 0.4 arcsec; order 4
RA 16h 41m 52s,  Dec +36° 25' 29"
Pos Angle +02° 58', FL 1359.1 mm, 1.18"/Pixel

Focal length in this case is 1359.1 mm

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.