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Why don't more people use beamsplitter guiders??


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I use a 4r/92t beamsplitter to guide my spectrsocope using a Lodestar. I just realised that very few others use the beamsplitter for "normal" DSO imaging. (a 50/50 Cube is neat).

The cube gives a full view of the target object, full field of view for the guide camera plenty of bright stars, un-distorted across the whole field. Take your pick..you can guide on any star!

The downside??

Well a 50/50 beamsplitter looses 0.75 mag to both the imaging camera and the guide camera i.e. if you have a limiting magnitude of 14 mag this will reduce to 13.25 mag. To me, this sounds like a small price to pay for easy accurate deep guiding...

Comments???

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Hmmm. I'm confused.

Is half of the light going to guidecam, in which case don't you need subs that are twice as long for the imaging cam?

It's probably obvious that I have no idea how beamsplitters work, but I'd welcome finding out!

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Basically you're correct....but....

The nett impact to the brightness of the stars recorded is only 0.75 mag.

The magnitude scale is a logarithmic scale.

I've only really used the "cube" for spectroscopy, where it works very well.

Imaging DSO's? Some "actual" user feedback needed.......

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Do you have a link to any beamsplitters Ken....they do sound rather interesting, especially for things that need a long FL, like Planetary nebs, wher it's often hard to find a decent guidestar but the object itself is small and bright.

Rob

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Alan at Sky's the Limit sells a 1.25" beamsplitter cube for 49 gbp....

I end up building my own from beamsplitters from Surplus Shed (You can also get them in various sizes from Thorlabs and Edmund Optics) I use a Vixen flip mirror body (because it has a 2" nosepiece and T threads on both outlet ports.) I have some photos somewhere - I'll upload them ASAP.

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No,

Effectively the light that goes through the cube is just travelling through a "thick" filter with flat square surfaces on both sides.

The size of the cube will affect the back focus distance - it will be lengthened by approx 1/3 the cube size....

Edited by Merlin66
typo corrected!
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  • 5 months later...

My observations would be that a cube beamsplitter would introduce a loss into the system from the beamsplitting coating and the AR coatings (if any) on the input and output surfaces. The BS coatings would also be polarization sensitive, and would also probably act as a passband filter to reject UV and IR. This may affect Ha response. The thick glass block in the optical path would also affect the aberration balance in the system, probably more so in refractors or reflectors utilising coma correctors, or if the block is rotated slightly to form a tilted plate. On the plus side you do get a full aperture image for the guider. However, I've never had a problem finding a guide star with an OAG, so my preference is the OAG.

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For full field work, I'd say the beamsplitter plates- or pellicules (Thorlabs/ Edmund) would be a better bet.

They have details of the transmission curves on their sites and the optical performance is very good.

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This reminds me of what they used to say about the Vincent motorcycle - that it was a solution in search of a problem! Once set up I find that an OAG simply isn't a hassle at all. You have to get the depth of the prism and the focus right, and in the case of the Starlight Xpress version stop the camera from pivoting on its locking bolt, but once done you never touch it again. It takes no light from the imaging camera because it's off axis.

Given the cost in cash and time involved in catching those photons I wouldn't want to lose any of the blessed things!

Olly

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Olly,

In principle I don't disagree!

(I can't effectively use an OAG with a spectroscope - well not easily! The traditional solution is to focus a 30 micron FWHM star image on a 20 micron reflective slit gap and persuade the guider to "guide" on the residual overspill light!!!)

Having said that, the dichromatic beamsplitter may have some real merit for DSO etc - effectively no lost data.....

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