Refractor focuser size

[AlistairHowie]

Can someone please explain the implications of using a particular size of focuser on a refractor? For example, on a 80/480 scope, is a 2.5" focuser better than a 2" one, and why? What's the difference from a user point of view and does it make a difference if a visual observer versus doing AP? What actually is the focuser size measuing? Is it the diameter of the drawtube barrel? If it is, as long as it's large enough to not restrict the light cone, does focuser size matter? Am a bit confused!

Many thanks.

[Capricorn]

I suspect that you are refering to refractors that are aimed at astrophotography.

The 2.5" focuser will in general hold a camera with more stability then a 2" one will.

As ever depends on the quality of the unit.

If the camera moves by a pixel or two then an image can be ruined or will be a bit worse.

For visual then not a lot in it really unless you use some of the large eyepieces that are available these days.

[NGC 1502]

The focuser size is the diameter of the accessories that will fit. A 2" focuser will take a 2" diagonal, and will also take a 1.25" diagonal with an adaptor.

For visual that's all you need. Imagers may have other needs.

For an 80/480 refractor, it's nice to have a 2" focuser (most have) to achieve the very low power wide field that short refractors are particularly good for.

Regards, Ed.

[AlistairHowie]

Thanks for the replies.

TFor an 80/480 refractor, it's nice to have a 2" focuser (most have) to achieve the very low power wide field that short refractors are particularly good for.

So how does a 2" focuser achieve that in a way that a 2.5" might not? Are larger focusers basically just more robust for AP because they are heavier/made of thicker metal and less likely to flex under load?

Does the focuser size have anything to do with the light cone, or is that not an issue since all focusers are appropriately sized for the OTA optics anyway? In other words, in my 80/480 example, would it be that a 2" focuser is just fine because it doesn't block any of the light cone and it fits nicely that diameter OTA, a 1.5" focuser if such a thing exists would block some of the light and would be inappropriate, and a 2.5" it might be argued by marketers might be more robust for AP? Is that how it works?

[NGC 1502]

For visual work, the 2" focuser is all that you need, because that will take a 2" diagonal and 2" eyepieces. Apart from eyepieces for very large (and I mean LARGE) telescopes, then 2" eyepieces are the largest generally available. With a 480mm focal length refractor, If you use a (for instance) 31mm TeleVue Nagler, that would give 15x and over 5 degrees of sky,

(480 / 31 = 15x and 82 / 15 = 5.3 degrees - the Nagler has an 82 deg apparent field ).

In a well designed refractor, a 2" focuser will not intrude into the light cone from the objective, a 1.25" focuser could, depends on other factors.

Imagers have other needs and I'm out of my depth here, but large sensors may need an even larger focuser to prevent vignetting that expensive CCD, at least I think that's how it works,

perhaps any imagers reading this could comment please. It's not about robustness, but diameter.

HTH, Ed.

[sologuitarist61]

Apparentlly the larger sizes work better for AP with DSLR's with full sized (35mm) sensors and avoids cut off. For APS-C sized sensors a 2" focuser with be sufficient (and in most cases so will a 1.25", though the camera will not feel as secure on the smaller model)