focus

[TheThing]

Having replaced the focuser on my homemade dob (see the DIY section), I find I don't have enough in focus.

What's the best method to cure this? Lower profile focuser or move the main mirror? If the latter, up or down?

[terrym2002uk]

Hi

Moving the mirror up the tube will push the focus point further up the focuser by the same distance I believe.

As to swapping the focuser or moving the mirror depends on funds. Is there an option for replacing your focuser tube with a shorter one or reducing the length?

[TheThing]

I can't see that the focuser itself can be reduced in length. I've tried much smaller 2" and 1.25" adapters, but with little success. It seems to be the base - the bit that screws onto the tube and has the knobs on - which is too tall.

I don't have a big budget at all, so getting a lower profile focuser would have to be on the cheap - i.e. 2nd hand, not a nice new Moonlight!

Does moving the mirror up increase in focus or out focus? I'm not clear from your description.

[Stephen]

Moving the mirror up the tube, move the focus point "out" on the focuser.

I used to use a 35mm ext. tube to focus my EPs, since moving the mirror up the tube I now need to use a 50mm tube.

Hope I made some sense there

[TheThing]

Ah, got it, thank you.

Moving the mirror up the tube is no option for me then, as I need more in focus. I can't move the mirror down, as it is already at the bottom of the tube and I can't extend the tube...

...so I need a lower profile focuser then!

Measure up tomorrow then I'll have to post some wanteds.

[Stephen]

Ah, got it, thank you.

Moving the mirror up the tube is no option for me then, as I need more in focus. I can't move the mirror down, as it is already at the bottom of the tube and I can't extend the tube...

...so I need a lower profile focuser then!

Measure up tomorrow then I'll have to post some wanteds.

Revelation Newtonian Crayford Focuser Dual Speed

That's the one I got (amazed the price hasn't gone back up yet ...)

[kniclander]

I may be wrong but would a low power barlow push the focus out a little bit - that's the "standard" solution for binoviewing when there often isn't enough "in" focus...

[Stephen]

I may be wrong but would a low power barlow push the focus out a little bit - that's the "standard" solution for binoviewing when there often isn't enough "in" focus...

Quite true, the barlow will relieve the problem - but could bring with it others? (FOV not as expected, more glass in train, centring issues?)

[barkis]

Hugh, are you unable to focus, even your lowest power eyepiece?

If so, I'm not sure a lower profile focuser is the answer, so the only alternative is to locate your main mirror cell further up the tube towards the secondary. Doing so, will alter the cone of light diameter at the secondary, so be careful you don't lose light due to vignetting.

If you can focus using your lowest power, then you won't need to move the cell much, perhaps an inch or so, unless you intend to use a camera for prime focus shots, then you will need to move the mirror further up, 3 inches plus.

Again, the main disadvantage may be possible vignetting if it has to be moved too far. It depnds on the minor axis diameter of the secondary in relation to the mirrors focal length.

Ron.

To clarify for you. If you can't rack you focuser in far enough to reach focus, then you need to move the main mirror nearer the secondary mirror, this in turn places the focus of the mirror further up the focuser, and towards the eyepiece.

Moving your main mirror further back in the OTA will make your problem even worse.

[terrym2002uk]

Ah, got it, thank you.

Moving the mirror up the tube is no option for me then, as I need more in focus. I can't move the mirror down, as it is already at the bottom of the tube and I can't extend the tube...

...so I need a lower profile focuser then!

Measure up tomorrow then I'll have to post some wanteds.

Are you running out of inward focus, ie the focuser is screwed all the way down towards the ota? If so I don't see why moving the mirror up would not be an option as this will push the focus point out further within the range of your current focuser. But as stated depending on some of your ep's you may need to use an extension tube. Depends how far you move the mirror.

[great_bear]

Moving the mirror up the tube is no option for me then, as I need more in focus.

Erm... Moving the mirror up moves the focal point out, thus giving you the in-focus that you need.

[ollypenrice]

Yes, if at the moment you cannot get the EP close enough to the tube then moving the mirror up will push the light cone further out through the focuser and solve the problem. However, as Ron says, the danger is that the present secondary may not be large enough to cover the beam if it meets it closer to the primary.

Olly

[TheThing]

Thanks gents - I get it now.

Yes, using a 40mm EP, I can't get the focuser to go in far enough to achieve focus. It's nearly there, but not quite. If I take the EP holders out and hold the EP in the focuser tube I can move it in towards the secondary enough to find focus.

It seems I got it the wrong way round and moving the mirror up the tube will be the solution.

I think I need to delete my wanted ads and see if anyone has a mirror cell to fit instead!

Thanks gents.

[yesyes]

Looking at the pictures in the DIY section I don't think you need a new cell. Just put something underneath the existing cell to raise it.

[Stephen]

Looking at the pictures in the DIY section I don't think you need a new cell. Just put something underneath the existing cell to raise it.

I don't know if it's practical for your cell, but I replaced the rubber o-rings under my cell with springs (for another reason) and ended up with a 15mm shift up the tube

Maybe worth a look to see if you can replace any screws or introduce spacers and longer bolts?

[yesyes]

If you post a few more close-up photos showing how the cell is attached to the back board, we could suggest ways of moving the mirror up.

[TheThing]

Ok, thank you.

I'm note sure any more pictures are needed, but can post if required.

Simply, the mirror sits at the bottom of the square box. It rests on the wooden bottom of the box with nothing underneath it.

Internally, there is a square fame to hold it in place, with supports accross each internal corner. There are shims of cardboard and paper in between these supports and the mirror to make it snug.

On top of each support there is a small piece of wood screwed in, which points to the centre of the mirror. This just touch the mirror at the edges - on overhang of a couple of mm - and stop the mirror falling out forwards.

Hope that explains it?

[great_bear]

Yes, using a 40mm EP, I can't get the focuser to go in far enough to achieve focus. It's nearly there, but not quite.

Don't forget to add some "slack" since eyepiece focal planes vary a lot, whilst also leaving room to accommodate Barlow use.

I think that getting the ideal amount of focus width is probably quite a tricky thing, and there's probably some combinations of things that still won't work out. However, having a focal plane too high can at least be accommodated by using extension tubes, whilst one too low would need further modification.

Good luck

[TheThing]

Some photos to help the above explanation.

Note that the string under the mirror is just aid removal.

TheThing.

Sent from my iPhone using Tapatalk

[yesyes]

Hmmm, I think your best bet would be to try and remove that wooden frame, put something underneath and attach it again. This should move the mirror upwards...

Problem could be the collimation bolts. You might need longer ones...