Less than a 2x barlow

[iceman (Mike)]

Hi all

My DMK won't come to focus at newt focus in my 10" newt - it needs more in travel, or the mirror needs to be moved up the tube.. similar to if I tried newt prime focus with my 350D I guess.

So I've only got the 2x barlow to use, which gives me too much image scale on some objects, when I want a widefield of the moon, sun or even if I want to try and get a DSO with this camera. Especially with the length of the filterwheel and adapters, it probably gives me more than 2x.

I know there's such a thing as the 1.6x antares barlow isn't there?

Are there any other barlows less than 2x? As close to 1x as possible would be ideal.

I don't want to move the mirror up the tube at this stage, and I've got a moonlite CR2 focuser and don't want to make it any low profile, as it might affect my long focal length imaging.

Thanks

[JohnG (John)]

Takahashi make a 1.6x extender in both 2" and 1 1/4, very expensive though. I dont know how they would work on your scope, my Tak refractor works at f/12 with the 2" fitted.

Sorry, don't know of any others.

JohnG

[gbeal]

Moonlite isn't on a "spacer" by any chance Mike?
If you don't need much, and I suspect you don't, try adjusting the primary as far out on the collimation screws (towards the secondary) as you can stand.
I did this with the MN76, to make it reach focus with the DSLR.

[ausastronomer (John Bambury)]

Mike,

I own the 2" 1.6X Antares Barlow. You are welcome to try it out next time we are observing together if you wish.

CS-John B

[AstroJunk (Jonathan)]

Originally Posted by iceman

As close to 1x as possible would be ideal.

I can sell you a 1x barlow - its air spaced, has ultra high transmission coatings (100%) and I can guarantee it will introduce no optical defects!

On a slightly more serious note, the optical element of the GSO barlow can be removed and screwed directly to the eyepiece to form a 1.5X barlow

[ballaratdragons (Ken)]

Mike,

I found that I had exactly the same problem when I did my Optical Train adjustment the other day. By putting the 2ndry in the correct position meant that I couldn't get full focus with the Toucam.

I fixed it by simply turning the collimation screws 2 full turns which moved the mirror forward about 2mm. It was enough to make the toucam pull focus and have a smidge past focus as well.

Then I just re-collimated with the mirror in the new position. The collimation screws still have a long way to go, so it's not a matter of the mirror cell falling of the end of the screws.

[Harpspitfire]

try this place
http://www.astro-engineering.com/Barlows/barlows.html

[janoskiss (Steve H)]

Originally Posted by AstroJunk

On a slightly more serious note, the optical element of the GSO barlow can be removed and screwed directly to the eyepiece to form a 1.5X barlow

That can be done with other barlows too, including the meade #140 which I believe Mike has, but it won't help with the problem of not being able to reach focus. In fact it will make it worse needing still more in-travel, if I'm not mistaken...

Re the Antares 1.6x barlow, you might have problem reaching focus there too. Because it's a fairly short barlow it does not save much on in-travel either.

What might be a good option is a corrector lens for binoviewer. These have lenses with long focal lengths, so give you low power for given optical path length. E.g. a 2x corrector gives 2x mag when placed about 100mm, or ~1.6x at 50mm from the eyepiece focal plane (or the CCD).

WO 2x corrector can be bought from several dealers locally for around $50, or the Burgess 1.9x for $US29 (mail order from Burgess Optical). They screw into 1.25" filter threads, or you can replace the lens assembly in the meade #140 with one of these. I will check how much in-travel the Burgess 1.9x needs in this configuration.

[janoskiss (Steve H)]

Originally Posted by janoskiss

...WO 2x corrector can be bought from several dealers locally for around $50, or the Burgess 1.9x for $US29 (mail order from Burgess Optical). They screw into 1.25" filter threads, or you can replace the lens assembly in the meade #140 with one of these. I will check how much in-travel the Burgess 1.9x needs in this configuration.

About 4-5mm in-travel is required. Power is ~1.6x. If you want to 'pull' the focal plane out then you need more of the barrel of the barlow inside the focuser.

[iceman (Mike)]

Thanks guys, i'll first try "collimating" the mirror all the way up in the scope as far as it will go, but I don't think it will be enough.

I'll look at the Antares 1.6x or unscrew the bottom of the Meade 2x and see how it goes.

[ving (David)]

heres something i am considering: paul rini makes a .5 focal reducer which looks like and is used like a barlow.

heres a linky mikey!
http://www.weasner.com/etx/reviews/2...alreducer.html

[Don Pensack]

Originally Posted by ballaratdragons

Mike,
I fixed it by simply turning the collimation screws 2 full turns which moved the mirror forward about 2mm. It was enough to make the toucam pull focus and have a smidge past focus as well.

This is quite commonly-given advice, and it works well on scopes where the mirror is collimated by means of a pair of push-pull screws. But if the primary rests on three strong springs and there are no locking screws, this will result in lessening the tension in those springs. The disadvantage of that is that the mirror will no longer hold collimation when the altitude of pointing the scope changes. In a spring-supported primary cell, collimation will require the springs be nearly completely compressed in order to hold collimation.
Just be aware of how your mirror is supported before you try moving it forward this way.
Don