Is this normal?

[azrabella]

Just a quickie. After collimating a reflector and doing a star test by intra and extrafocus, I noticed what appears to be the shadow of the focuser intruding into the image. I've never noticed this befor and I'm not sure if it is correct or not. On re-focusing normally there seems to be no ill effect. I managed an easy split of a 6th mag pair with 1" separation, so clearly not too detrimental. Here is a visual mock-up of what I'm ranting about. Any opinions welcome, because collimation paranoia is bound to set in before long.

[Ben the Ignorant]

This is normal in some telescopes. When you defocus a lot to do your star test the focuser drawtube is farther in than usual so it enters the light cone. In case you still see that very close to focus you might have to move the primary mirror a few millimeters toward the secondary to push the focal plane toward the focuser. This is done by shimming the cell or changing the collimation screws and/or springs, depending on what system you have.

[azrabella]

It's a Skyliner 200p. Would it not be easier to shift the secondary a few millimetres toward the primary instead?

[John]

If you are splitting 1 arc second doubles you have no collimation problems at all !

I've rarely managed that with my 12" newtonian from across the river from you (my collimation is also spot on).

[Ben the Ignorant]

30 minutes ago, azrabella said:

It's a Skyliner 200p. Would it not be easier to shift the secondary a few millimetres toward the primary instead?

That would decenter it relative to the focuser.

[NGC 1502]

9 hours ago, azrabella said:

It's a Skyliner 200p. Would it not be easier to shift the secondary a few millimetres toward the primary instead?

To move the secondary closer to the primary means shifting the focuser as well so that it’s in line with the secondary.

So you would need to either enlarge the hole in the tube to take the focuser tube or make a new one, plus drilling new holes to fix the focuser to the tube.  Of course this means you have unused holes to deal with. If there’s not enough adjustment on the secondary you’d have to reposition the spider - more new holes and unused original ones.

So on balance it’s usually simpler to shift the primary. Or just accept it if the focuser tube intrudes into the light path, or perhaps shorten the inner end of the focuser tube.

However if when in focus the inner end of the focuser tube is not intruding into the light path or only by a small amount, then leave well alone.

All the above is possible if you are average at DIY.  But if you do this you’ll probably void any guarantee that applies.

Ed.