RC 10" F8 can't get sharp focus after collimation and back focus at 264mm; help with GN rochi eyepiece images interpretation.

[Peter_P]

I am new to RC after buying a second hand Altair truss RC 10" f8 that was in need of attention; read and followed to the best of my abilities David & Nicolas instructions and believe to have achieve a fairly decent collimation using a cheshire, a HG and diffused star to fine tune. However, the RC does not get into sharp focus. The best focus is at 264-265mm, but the drawings say is at 239mm. I also got a GN rochi eyepiece to see if the root cause of the problem are aberrations in the mirrors, but unable to interpret the images. Please refer to the attached images. Many thanks in advance for your help.

Imaging system distances.pdf

[skybadger]

Can you take a set of ronchi images that are three or 5 bands only,  both inside, at and outside focus. The ones above are alarming but it might just be thermals and shows some sort of regular pattern from NW to SE that might be confusing. 

Also, what does an out of focus image look like on a bright star without the ronchi ? Ie how well collimated is it actually ?

I guess you already know you are looking for parallel lines to indicate the correct mirror spacing and theres a good amount of contrast on the lines so the spacing looks fine to me... Other eyes will help.

 

 

[Peter_P]

SB thanks for your reply. I only have the 10 lines ronchi and do not know if/how can do 3 or 5 lines images without buying another eyepiece?

The 1st picture above is Arcturus out of focus from my ZWO camera; does this help?

I will definitely work on the inside and outside focus images of an artificial star and post when completed (the weather is what it is....), hopefully tonight.

I would absolutely love to see the parallel lines or something resembling that... I have not considered until now that the contrast in the images above may have been enhanced by the DSLR camera setting used to take the picture.

[skybadger]

You get to fewer number of bands by being closer to the point of focus, ie defocus only a little bit either side, you get more lines crowding in the further away you are from focus. At focus you've got zero lines and you're essentially taking  a Foucault-gram.

In taking a picture of a star I'd be looking for the airy rings to check they are centred, you can have the illuminated secondary centred in the beam but the primary and secondary reflections still be out.

 

[Staring]

You want that star to be far, far closer to focus to check for collimation on axis. You also need to check the off-axis star shapes to be symmetrical. This is because both mirrors are hyperbolic, meaning their optical axes must be very carefully aligned to each other and to your focuser. You can have a good on-axis dtar and still terrible field. Google „DSI collimation method“ for an explanation. This is no SCT or Newtonian and tips and experience with them will lead you astray.

Can‘t help much with the Ronchi, but I‘ld you‘re just not collimated enough to get a meaningful reading.