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XX12G / Mark V worked with ES Barlow

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#1 Eddgie

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Posted 18 August 2013 - 08:53 AM

Went out last night to measure back focus on my new Orion XX12G (I have 40mm of in-travel from the focal plane which is good).

Anyway, after making the measurement, I was looking at the moon in Monoview and I just could not stand it.

I did not think I had any way to hook up the Mark V, but then I realized I had an Astro-Tech 2" ED Barlow around.

I thought it would be interesting to try using the Barlow element screwed right into the filter threads of the 2" nose of the Mark V.

Happy to report that it worked perfectly and I had about 8mm or so (just a guess) of in-travel left over.

Ahhh. Was able to binoview the moon and felt sooooo much better.

I used 35mm Ultima, ES 2468s, and Hyperions 17s and all reached focus.

I guess I have just become so used to using binoviewers for the moon that I just can't bear to do it any other way.

It was a lot of power though. I estimate that the Barlow in normal configuration is about -90mm of focal length (measureing from the rear of the lens elements to the seat of the 1.25" eyepeice holder).

The light path with the binoviwer and 2" nose was perhaps 148mm, so this would give (as I understand it) about 3x from the Barlow?

Does that sound about right???

That would mean that in the 1500mm scope, the focal lenght was maybe 4500mm and the 35mm Ultimas were giving 128x???

It sure seemed higer to me than 128X though. In fact it seemed much higer than that. I would have guessed that it looked more like 150x.

Could have just been the "Binoviers make everthing look bigger" effect.

Anyway, just wanted to report this. If you want to use a Newt or Dob with a binoviewer with a 2" nose and standard filter threads, the AT 2" ED Barlow will likely get you there if you have about 30mm of in-travel (which I think is pretty common in most dobs).

#2 Pinbout

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Posted 18 August 2013 - 09:27 AM

One of my dobs doesn't come to focus. I haven't bothered to figure it outy yet. I'll have to try my 2" barlow...uhm.

#3 REC

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Posted 18 August 2013 - 09:33 AM

Good news Ed, glad you where able to use your BV in your new 12" Dob....must have been a wonderful sight! I'm in a similar situation with my new 10" Dob as I could not get the focus tube down enough to come to focus. I'll have to try the barlow out on the next trip if the clouds ever part!

I am thinking about another solution for this. I see Agena and Orion just came out with a new low profile focuser and it is 47mm with the tube collapsed. I measured the standard Crayford that came with my scope and it is 65mm from the flange. Do you think this gain of 18mm would be enough to get the BV in focus now without adding a barlow? That would be great on a 1250mm scope I would think?

Thanks,

Bob

#4 Astrojensen

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Posted 18 August 2013 - 10:14 AM

Hi Eddgie

To find the magnification of a barlow lens: Divide working distance of barlow lens with barlow lens focal length and add one.

F/f+1 = x

Here, F is 148mm, f is 90mm

(148mm/90mm)+1 = x

1.64 + 1 = x

2.64x


The reason it feels higher is the well-known "bino effect" which, to most people, makes everything look 50% larger than in mono view. A bright view from a large scope will also appear larger than the same magnification in a smaller scope, despite being exactly the same.

The only way to be 100% sure of the actual magnification factor, is to measure it.


Clear skies!
Thomas, Denmark

#5 Hothersale

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Posted 18 August 2013 - 12:49 PM

Thank you, Thomas! That's the formula I was looking for. I'm using the same barlow element as Ed in front of my new bino, and the formula's calculated magnification factor agrees precisely with my observed estimate of 2.5x.

The formula also tells me I shouldn't waste my time adding spacers to try and achieve 5x because I would need about NINE more inches!

Cheers,
Chris

#6 Eddgie

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Posted 18 August 2013 - 01:42 PM

Ok, not as high as I thought. Than means I was only using 115x with the 35mm Ultimas and only 165x with the ES 2468s.

Sure seemed like a lot more than this.

Oddly, that is about the same magnification I was getting with these eyepieces out of the C14!!!

Well, at some point I will buy a Binotron 27 and that will let me get a lot lower powers than I am getting now.

For the time being, I am going to get the Baader 1.7x Newt GPC though.. I have already spend too much money on eyepeices and stuff for the time being.

Thanks!

#7 Astrojensen

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Posted 18 August 2013 - 03:12 PM

I can vouch for the Baader 1.7x newtonian GPC. It is really excellent. Superb image quality. It needs a bit of in focus travel, but you seem to be good to go, from what I can read.


Clear skies!
Thomas, Denmark

#8 junomike

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Posted 18 August 2013 - 04:07 PM

Hi Eddgie

To find the magnification of a barlow lens: Divide working distance of barlow lens with barlow lens focal length and add one.

F/f+1 = x

Here, F is 148mm, f is 90mm

(148mm/90mm)+1 = x

1.64 + 1 = x

2.64x



Thomas, I don't see a place within the equation to differentiate between different magnification factors?
Is this formula strictly for a 2X Barlow and how would on calculate for a different Barlow?
Does one add "+1.5" in place of "+1" to calculate a 2.5X
and add a "+2" in place of "+1" for a 3X Barlow?

Mike

#9 Astrojensen

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Posted 18 August 2013 - 04:32 PM

Thomas, I don't see a place within the equation to differentiate between different magnification factors?
Is this formula strictly for a 2X Barlow and how would on calculate for a different Barlow?
Does one add "+1.5" in place of "+1" to calculate a 2.5X
and add a "+2" in place of "+1" for a 3X Barlow?



A barlow lens always produce 2x, when the working distance is equal to the focal length. The formula is F/f+1=x , where F is the working distance and f is the focal length. 1 is a constant and cannot be changed. x is the magnification factor.

A 2x barlow with 100mm focal length and 100mm working distance

100/100 + 1 = 2

To make it a 3x barlow, we just increase the working distance with exactly one focal length distance:

200/100 + 1 = 3

Many barlows sold are not exactly 2x or 3x, but vary considerably. Also because different eyepieces have their focal planes in different places relative to the barrel shoulder. A variation of 10% across different eyepieces is not unusual at all, especially in short focal length barlows, where a small variation in working distance will have a greater impact.

Consider a 1.25" 2x barlow with a working distance of 70mm exactly at the top of the barlow. Now we insert a 25mm UO eyepiece with the field stop 7mm forward in the barrel.

63/70+1 = 1.9

It now works as a 1.9x barlow.


Clear skies!
Thomas, Denmark

#10 junomike

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Posted 18 August 2013 - 04:47 PM

Thanks Thomas, that makes sense.
So in reality, an eyepiece with a Field Stop near the "working distance" would be closer to the native/true Barlow magnification as compared to a 5mm XO or TV Zoom that have the FS placed near the bottom of the eyepiece.

Mike

Mike

#11 Astrojensen

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Posted 19 August 2013 - 01:51 PM

So in reality, an eyepiece with a Field Stop near the "working distance" would be closer to the native/true Barlow magnification as compared to a 5mm XO or TV Zoom that have the FS placed near the bottom of the eyepiece.


Yes.


Clear skies!
Thomas, Denmark

#12 Hothersale

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Posted 19 August 2013 - 02:04 PM

I tested a Hyperion Zoom Barlow (nominally 2.25x in native mode) attached to the nosepiece of the WO bino last night and it yielded at least 4.5x. It was too much magnification for the seeing, but still within the realm of usability for shorter scopes with longer eyepieces. Not much in-focus was required using such as a strong barlow, as you'd expect.






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