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250px with Revelation Astro 2.5x Barlow

richyrich_one

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JamesF

JamesF

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I don't have the standard focuser on mine, but if you have the flextube version you could try not extending the trusses all the way.  If it's the solid tube version then moving the mirror forward with longer collimation screws might help, and/or a low-profile focuser might help.

Although for less than the cost of a decent low-profile focuser you could probably pick up a couple of second hand TV plossls in something close to the required focal lengths.

James

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knobby

knobby

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Agreed, the 250 px is pretty picky on focus distances.

I've got the primary mirror down as far as it goes (almost) and that seems to be ok on most EPs and the same barlow that you have.

You havent got the 2" and the 1 1/4" ep holders both on have you ?

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richyrich_one

richyrich_one

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Agreed, the 250 px is pretty picky on focus distances.

I've got the primary mirror down as far as it goes (almost) and that seems to be ok on most EPs and the same barlow that you have.

You havent got the 2" and the 1 1/4" ep holders both on have you ?

No, just the 1.25".

So I would have to move the mirror down? How do I go about this?

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richyrich_one

richyrich_one

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It would appear that the stock 1.25 eyepiece holder is "too long" and a shorter one would give me the additional inward travel I require.

Would a 2" to 1.25" self-centering adaptor inserted directly into the focuser draw tube work?

I suspect it might cause the opposite problem.

Might I then need an extension tube of some sort so that the overall length in a bit less than the stock eyepiece holder?

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Avocette

Avocette

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I have the 250px and have no focussing issues with a 2x Barlow, nor with eyepieces from a 6mm BGO to a 34mm 2" Maxvision. I would suspect your 'problem' is outward focusing. A Barlow is often used to solve inward focus issues like with Binoviewers. Could you post a couple of photos showing your focuser and eyepieces in place when at appropriate focus points, and I'll compare with mine?

I did have a problem initially reaching prime focus with my DSLR which seemed rather disheartening since SW advertise the feature. I solved this small problem by adjusting the primary collimation screws so that they are held by a couple of turns rather than being screwed almost fully in.

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richyrich_one

richyrich_one

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I have the 250px and have no focussing issues with a 2x Barlow, nor with eyepieces from a 6mm BGO to a 34mm 2" Maxvision. I would suspect your 'problem' is outward focusing. A Barlow is often used to solve inward focus issues like with Binoviewers. Could you post a couple of photos showing your focuser and eyepieces in place when at appropriate focus points, and I'll compare with mine?

I did have a problem initially reaching prime focus with my DSLR which seemed rather disheartening since SW advertise the feature. I solved this small problem by adjusting the primary collimation screws so that they are held by a couple of turns rather than being screwed almost fully in.

Pretty sure it's inward travel I need.

When trying to focus, as I move the focuser in, it reaches it's stop just before being in focus.

When I move the focuser further out the image gets larger until I can make out the mirror clips, spider etc quite clearly.

I was under the impression that a barlow would solve inward focus problems not create them.

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richyrich_one

richyrich_one

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Could you post a couple of photos showing your focuser and eyepieces in place when at appropriate focus points, and I'll compare with mine?

This is everything in place, but not sure what you mean by appropriate focus points.

With this setup and the focuser all the way in it is almost in focus and that is the best point in the focuser travel. Moving out makes it worse.

I even tried removing the 1.25 eyepiece holder and inserting the barlow/eyepiece combo into the 2" opening by hand trying to get focus and I could.

post-36192-0-82866600-1397632934_thumb.j

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Avocette

Avocette

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I see the problem. My focuser has a 30mm length 1.25" tube whereas yours looks to be about 40mm. My focuser is a Crayford from a 300p Flextube that I bought second hand to replace the rack and pinion one originally fitted. Hope knobby's loan piece does the trick.

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AndyWB

AndyWB

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Hmm. Now there's a thought. I really don't like having to cart around both the Skywatcher 1.25" and 2" fittings for the crayford. They're both quite large. I didn't think of getting an adaptor and just putting that into the 2" one.

Knobby, how much taller does that make your focusser tube? How low is 'low profile'?

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Avocette

Avocette

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Here is what my focuser looks like with the same eyepiece.

post-30550-0-71781400-1397653417_thumb.j

I do have a similar 30mm focus tube from another focuser that is an interesting bronze colour! I think it may have spent some of its life in the sunshine.

post-30550-0-79276300-1397654033_thumb.jpost-30550-0-51373700-1397654047_thumb.j

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Reeny

Reeny

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The Revelation 2.5x Barlow will pull the focus point further in towards the tube.

This isn't a problem on my 200p Dob,

But I had to modify the 3-inch travel scope with a similar solution to yours.

It is difficult to blame the scope or the barlow when they both work OK with other equipment.

It's just a shame that they aren't compatable together, without having the hassle of making the modifications.

You will find that it is a great Barlow when it's working correctly.

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richyrich_one

richyrich_one

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The Revelation 2.5x Barlow will pull the focus point further in towards the tube.

This isn't a problem on my 200p Dob,

But I had to modify the 3-inch travel scope with a similar solution to yours.

It is difficult to blame the scope or the barlow when they both work OK with other equipment.

It's just a shame that they aren't compatable together, without having the hassle of making the modifications.

You will find that it is a great Barlow when it's working correctly.

I saw a Scopes and Skies video on YouTube and I'm sure it said the opposite about the focus point. Thats why I wasn't expecting this behaviour. If I can find it I'll post it later.

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Reeny

Reeny

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It definitely pulls the focus inwards

I wasn't chuffed about the situation either.

The plastic lock ring unscrewed from the focuser and spent some time on the belt sander.

Once it was trimmed down by 5mm it worked OK (from 13mm to 8mm thickness)

This is the 3-inch set-up with the focuser up against the stop.

I don't use the Barlow on there now because it blocks the light path.

700.JPG

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AndyWB

AndyWB

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knobby, can you please confirm that your 1.25" adapter sits directly in the focuser drawtube, and not in the SW 2" EP adapter (which is the same length as the 1.25" eyepiece adapter)?

Yes, I'd be interested in that, as a) it'd be interesting to know, and B) I guess the drawtube itself is 2" then, which is something I've not thought about and would be interesting to know.

(I really don't like having to carry the two adapters)

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