Focuser travel requirements

[mura_gadi (Steve)]

Hello,

I have designed and about to commission a focuser I am working on and I'm wonder about the focuser's travel requirements. The current design allows for only the thickness of the tube and the focuser base plate or can extend below the base plate if required.

I am building for a visual scope hence the closer to the tube design, but would like the focuser unit to be able to move to a astrophotography setup as well, as it's going to cost a small motza and I would like to make it as versatile as possible.

How much travel should I work towards to have a focuser that will allow for both visual and astrophotography work? (Min/Max height is really dependant on the flange/safety kerfs of the EP's x2 - which is around 25mm, or 5-50mm of travel. Or I could design for Morpheus EP's and have 5-90mm of travel which seems a lot...)

So, how much travel should I be looking to build into the design?


Thanks
Steve
I'm using Newt for the Web to work out field of illumination and secondary sizes.

[Saturnine (Jeff)]

Hi Steve

You don't mention what size mirror , focal length etc. Also depends on what sort of camera you envisage using with the scope. If an DSLR you need 55mm of extra intravel from the visual focal point. If for an CCD camera then they only need about 12 / 15mm intravel usually. So I would imagine that 20/25mm of focuser drawtube travel should be plenty enough.
Those distances get skewed around if allowing for filter wheels or filter draws as well. Then the focal plane has to further out from the tube wall to accommodate. The filter draw is a bit more friendly to focal travel than filter wheels but it is all a compromise. A lowest profile focuser design as possible while providing 20 / 50mm travel to allow for cameras, good luck.

[mura_gadi (Steve)]

Sorry, good point to have missed...

8" F9, not really a astrophotography scope, but I want to be able to take it to a faster scope later. How much would the travel change between scopes, say the same size but 3 F stops different. Or the same F ratio and 4-6" difference in size?

[Saturnine (Jeff)]

The focuser travel difference for visual vs CCD doesn't change regardless of focal length or ratio, whatever the distance the focal plane is from the secondary for visual ,the CCD chip is set inside the camera body about 15mm from the T adapter . So for the camera you will still need to focus inward about the 15mm for the focal plane to reach the sensor.

[Saturnine (Jeff)]

An 200mm f9 would be a bit of a beast to mount steadily but would make for a decent lunar and planetary scope, presuming it is a good mirror.