I did a short experiment last night with barlows and focussing distances.

The results baffle me so I am going to have to try it again.

As a general rule of thumb, I had believed that using a focal reducer
would bring the focus point in closer to the telescope and that a
barlow would move the focus point further out from the telescope.

In addition, I believed that the more powerful the barlow, the further
out the focus would be.

So. I have a motorised focuser with a numerical counter. I moved the
focuser all the way in, had a 90 deg diagonal and the zwo asi290mc
camera installed, the following are the results

focuser all the way in to start, position 0
no barlow -23000 to focus – quite a ways out
x1.5 antares barlow +3000 almost all in
x2 antares barlow -15000 maybe 1/3 out
x2.5 televue barlow -4000 maybe 1/4 out
x3 televue barlow +1000 maybe 1/4 out

Note that we started at zero and went negative the further out (that is
just the way the numbers went). So we should not have had any positive
numbers at all.. and yet we did.

How to account for that? slippage in the focuser motor, connection,
etc? It was pretty cold (-4C) and it was moving pretty slow at times.

In addition, the belief that the focuser would move further out with the
power of the barlow turned out to be not so.

I think I will repeat this again. Then also repeat it with the starting
point of the focuser all the way *out*.

In any event, the whole purpose of this was to help with the catch22 of
imaging.. trying to find your out of focus target. you need focus to
see the target but you need the target in order to focus. So I was
going to write these numbers down on the wall and use them to achieve
“close enough” focus in the first step, in aid of finding the target,
then do a fine focus afterward.