Does a Newt focuser intruding into the light path create artifacts?

[Stonius (Markus)]

I'm looking at replacing the focuser on my Dob and I notice that the current one seems to intrude into the light path by about an inch when fully racked in - and the focuser only has a total travel of about 2"!



Given the fact that even terribly thin spider vanes cause diffraction spikes, I imagine that this must have *some effect?


Can anyone confirm?


Many Thanks


Markus

[Saturnine (Jeff)]

Yes it does have some effect but not enough to worry about. If you point the scope at a bright star and de-focus inwards, as the star image gets larger, more out of focus, you will see the shadow of the drawtube intruding into the out of focus image.
Considering that with most scopes the position of the drawtube with an eyepiece at focus will only be intruding into the light path by 10 or 20 mm, if at all it will be almost unnoticeable and have no effect on your views of the sky.

[Stonius (Markus)]

So if I wanted more focuser travel, could it be replaced with one that had a longer drawtube, knowing it would intrude *further?

[Saturnine (Jeff)]

Most newt focusers only have about 50mm / 2" of travel, that is all that is needed unless wanting to use photographically too.
Do you want more in travel or out travel. If it only intrudes about 25mm when racked in where does it sit when an eyepiece is in focus. If you are talking about more travel that would imply more travel outward unless you are trying to get a camera to focus which would mean intravel, that creates another set of problems.
What are you trying to achieve by having a longer drawtube. Most newts work perfectly fine with a little intrusion by the drawtube. If wanting more out travel just add an short extension tube though if using the same eyepieces etc that would mean racking the drawtube in further so that it would intrude into the lightpath a bit more and defeats the original premise.

[Stonius (Markus)]

It sits between 21 and 44mm with an eyepiece. I have a lot of configurations though that lead to widely varying focus requirements.


Primarily the scope is for visual, with some planetary photography.


But, sometimes I want to throw on an ADC, or a bionoviewer, or a planetary camera which have different requirements.


Best,


Markus

[Rerouter (Ryan)]

The issues are a decrease in contrast due to less light collecting area, possibly some artifacts caused by the shadow shape on bright stars, and if the focus tube is not well coated, secondary reflections off its tube. but that is the most of it usually

[Stonius (Markus)]

Originally Posted by Rerouter

The issues are a decrease in contrast due to less light collecting area, possibly some artifacts caused by the shadow shape on bright stars, and if the focus tube is not well coated, secondary reflections off its tube. but that is the most of it usually

Yep, that makes a lot of sense. Thanks.



Markus