Ioptron RC6 Focal Length Query

[Tomservo]

Hi everyone!
My previous question about my Ioptron RC6 focus issue, provided me with some great advice, thank you all for that.

It has enabled me to get quite familiar with the mechanics of the scope and also overcome some trepidation about removing mirrors and making adjustments etc.

I believe my focussing problems are over, (although I can't comfim that just yet, until the weather changes)

It has left me with another question: What are the repercussions of NOT having the Focal Length matching the specifications?; the spec shows the focal length for the RC6 as being 1370mm. If, after plate solving, this turns out to be different, should I make adjustments to correct this? does it affect the quality of the image?

I note that there is an eyepiece called Ronchi that will show up incorrect spacing, but if the spacing doesn't matter, I don't see the point of it.

Thanks

Clear skies and a bacon sandwich

[Mandy D]

@Tomservo I have the same scope and from what I understand, it is unusual for the focal length to match the specification. Because this is a two mirror system with hyperbolic mirrors, it is not like a Newtonian with only one active mirror. Both mirrors in an RC play a part in the magnification and effective focal length of the telescope. I understand that the mirrors are produced in volume and paired by selection after production to get a good match. This results in some significant differences in final focal length. What matters is adjusting the scope to it's best performance, not aiming for precisely 1370 mm. So, do not bother trying to achieve precisely 1370 mm. I have no practical experience of any of this, as I have, so far, had noneed to collimate my RC.

[Paul M]

I've read that the only important numerical value for RC optics is the secondary to primary distance. It it is too far from optimum then good collimation will be difficult.

How to find the correct distance? There are diverging views. One view is to adjust the inter-mirror distance so that the Scopes f/l, as given by plates solving an image, is that given in the scope spec. I.E if the scope spec says it's 1600 f/l then adjust the mirror distance until this is true. Then complete the rest of the collimation process.

Other views are that the design spec is just approximate and that the distance should be adjusted to perfect the star image seen through the scope + a lot of extremely technical stuff:

Just to cheer you along: https://www.cloudynights.com/topic/702687-rc-mirror-distances/

I've never measure the inter-mirror distance on y RC10. I just point the laser collimator down the focuser, hit the center spot on the secondary and return through to the target on the laser collimator. I have a fancy collimator thingy for the rest of the process but after getting it right, still found it needed a tweak on a real star!! So I could have saved myself a bit of cash

 

[Tomservo]

9 hours ago, Mandy D said:

@Tomservo I have the same scope and from what I understand, it is unusual for the focal length to match the specification. Because this is a two mirror system with hyperbolic mirrors, it is not like a Newtonian with only one active mirror. Both mirrors in an RC play a part in the magnification and effective focal length of the telescope. I understand that the mirrors are produced in volume and paired by selection after production to get a good match. This results in some significant differences in final focal length. What matters is adjusting the scope to it's best performance, not aiming for precisely 1370 mm. So, do not bother trying to achieve precisely 1370 mm. I have no practical experience of any of this, as I have, so far, had noneed to collimate my RC.

Hi Mandy, thanks for replying, it's reassuring to know that you're happy with the scope, and I'm sure I will be soon!

I understand what you say about the hyperbolic mirrors and it makes a lot of sense, although I will feel better about it when I get to know if it's 'in the ball park' of not when I do the plate solving (now I've got it focusing)

The collimation on my RC6 was miles out (maybe had a rough ride with the courier?), and I just thought while I'm at it, I'll check the distance, but I wasn't sure just how important it was.

Thank you.

[Tomservo]

9 hours ago, Paul M said:

I've read that the only important numerical value for RC optics is the secondary to primary distance. It it is too far from optimum then good collimation will be difficult.

How to find the correct distance? There are diverging views. One view is to adjust the inter-mirror distance so that the Scopes f/l, as given by plates solving an image, is that given in the scope spec. I.E if the scope spec says it's 1600 f/l then adjust the mirror distance until this is true. Then complete the rest of the collimation process.

Other views are that the design spec is just approximate and that the distance should be adjusted to perfect the star image seen through the scope + a lot of extremely technical stuff:

Just to cheer you along: https://www.cloudynights.com/topic/702687-rc-mirror-distances/

I've never measure the inter-mirror distance on y RC10. I just point the laser collimator down the focuser, hit the center spot on the secondary and return through to the target on the laser collimator. I have a fancy collimator thingy for the rest of the process but after getting it right, still found it needed a tweak on a real star!! So I could have saved myself a bit of cash

 

Thanks Paul, and thanks for that link, it does seem that it's been an issue for a few people. 

It's a bit 'Chicken and the egg' with this, and it doesn't seem to be spoken about too much in comparison to collimation which seems to be the priority.

If it turns out my mirror distances are miles out, then what iv'e done so far has been a waste of time, although I've learn't quite a bit!.

Thanks again.