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Pentax 10 / 2 = 5?

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#1 dvb

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Posted 24 April 2015 - 10:02 PM

Can a Pentax XW 10 mm with a good Barlow perform as well as a Pentax XW 5mm?

 

Is a the 5mm just a 10mm with a built a built in Barlow?

 

I have a Televue 2x  1.25" Barlow and a 2x Powermate.  

 

I also already have the 7mm XW, so 10mm with a Barlow/ Powermate would give 5mm and 3.5mm in a tidy package. 

 

Mainly, these would be used with refractors - a 120ED f/7.5  and a 110ED f/7

 

 



#2 Scanning4Comets

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Posted 24 April 2015 - 10:21 PM

Hi dvb,

 

I've used a 10mm XW with a barlow and also a 5mm XW on it's own.

 

I really couldn't see a difference.

 

Best barlow is a TV regular barlow , or TV powermate...or the ES 2x focal extender. You could spend more on a better barlow, but I don't think you need to IMO.

 

Clear skies! :grin:



#3 Starman81

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Posted 24 April 2015 - 11:00 PM

What Mark said... For all intents and purposes, yes. The TV 2x Powermate is a bit large and might affect balance on your refractors, depending your mount and how sensitive it is to balance. The TV 2x barlow has a long barrel and could contact your diagonal mirror. ES 2x focal extender, though heavier than the TV 2x barlow, might be your best bet. The bottom element is removable and could be screwed on your XWs for roughly a 1.6x barlow factor. 



#4 dvb

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Posted 25 April 2015 - 12:21 AM

Thanks for the replies, guys!



#5 russell23

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Posted 25 April 2015 - 05:12 AM

The 20mm XW was as good as the 10mm when used with a quality 2x barlow.

Dave

#6 coutleef

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Posted 25 April 2015 - 08:36 AM

The views are identical with a tv barlow from experience on moon and planet in my refractor and dob

 

boils down if you like using a barlow or not



#7 csrlice12

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Posted 25 April 2015 - 08:53 AM

Barlows are the devil's spawn of the eyepiece universe...... ;)



#8 GOLGO13

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Posted 25 April 2015 - 09:21 PM

I sold my 5mm Pentax XW to fund the 10mm XW for this very purpose. I figured with the 10mm XW and 7mm XW, I could barlow and get the 5mm and 3.5mm. Since they are pricey eyepieces I think it's worked out well.

 

The only discussion point is the size of the combination. I don't think it's a big deal, but it does stick out pretty far barlowed. And it's even crazier with a long barlow like my 3x barlow. But this is rarely much of an issue.



#9 gmartin02

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Posted 29 April 2015 - 02:41 AM

Barlows are the devil's spawn of the eyepiece universe...... ;)

 

+1

 

I used to use Barlows and PowerMates a long time ago before I had enough money to buy full eyepiece ranges, and was never completely satisfied. They worked pretty well, but not quite as good as eyepieces by themselves.

 

Why put even more "exposed" air to glass surfaces in between your target and your eye than you need to get the view that your are looking for (unless you don't have the money to get the full range of eyepieces)?

 

I still have a Powermate 2.5x, but I only use it for prime focus planetary photography. 



#10 paradise

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Posted 29 April 2015 - 06:15 AM

People do not agree about Barlows.

 

However, most often good Barlows can be perfect substitutes.

 

Sometimes it can depend on the set up : long or short focal (Schmidt-Cassegrain or fast Newton,).

 

As a matter of fact, many short focal eyepieces get a Barlow lens.


Edited by paradise, 29 April 2015 - 06:17 AM.


#11 Scott99

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Posted 29 April 2015 - 10:08 AM

I figured out the same thing, why buy any of the shorter XW's, just use a good 2 element barlow with the 10mm.  The shorter ones just add more lenses, and the bodies get excessively long too.  With AP Barlow you're not degrading the image any more than the Pentax lenses



#12 csrlice12

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Posted 29 April 2015 - 11:16 AM

Remember though, not all the XWs are a scaled design, and the "barlow" lenses were specially designed for that particular fl eyepiece.....



#13 Tank

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Posted 29 April 2015 - 03:37 PM

OP

well did these tests now for a while

Answer

NOPE!

 

Its close however if your ok with 90-95% of the image without the barlow than get a barlow

Ive done this exactly with the 10 XW

10 XW barlowed to 7mm vs 7 XW  = barlow combo lost

10 XW barlowed to 5mm vs 5 XW = barlow combo lost

 

Barlows used mainly AP Barcon convertible

 

Its close but if you want a better image stick to not barlowing

IMHO



#14 dvb

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Posted 29 April 2015 - 11:42 PM

Thanks, all, for the continuing comments. 

 

I found a good deal on a 10mm XW so I went for it and it arrived today- along with clouds, of course!  

 

I'm looking forward to trying the 10 with the Televue 2x 1.25" barlow - it's a small barlow that doesn't add much length or mass - I just hope it doesn't impair the image.   The point of going for the Pentax is for a superb image. 

 

When time and skies permit, I'll do a "shoot out" of the Pentax 10 with the Televue 2x barlow and the 2x Powerment with some other 5 mm in my box:  a Nagler T6, the Nagler 3-6 Zoom, the Antares 5-8 zoom, and a Siebert StarSplitter.   

 

 

Of course, the real shoot out would be with the Pentax 5mm XW, but that won't be happening any time soon!  



#15 paradise

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Posted 30 April 2015 - 06:22 AM

Anyway, I use TV Barlow for my Deloses, it is perfect :waytogo: , I shall not buy a 4,5mm or 3,5mm.

 

I think I'll go and buy a Barlow Powermate 2,5x to get 4mm via my 10mm, or buy a 8mm Delos to barlow it with my Barlow 2x.

 

I believe that TV Barlow's are nice for TV eyepieces, maybe be not for all eyepieces ?



#16 swix

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Posted 30 April 2015 - 06:49 AM

ES 2x focal extender, though heavier than the TV 2x barlow, might be your best bet. The bottom element is removable and could be screwed on your XWs for roughly a 1.6x barlow factor. 

Hold on there.... 

 

I understood this magnification of 1.6x to hold true for a "barlow" lens

screwed to the eyepiece.  I also thought that a "focal extender's"

magnification stays the same no matter the placement in relationship

to the eyepiece ?????????????

 

Am I cloudy ????



#17 BillP

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Posted 30 April 2015 - 07:54 AM

... did these tests now for a while  Answer  NOPE! ... Its close but if you want a better image stick to not barlowing IMHO

 

Indeed there is a difference, but it is very slight with a quality Barlow.  I can usually pick out the Barlow impacts on-axis only on critical planetary viewing in very good seeing/transparency conditions.  Turning to star fields and DSO doubt I could easily see any difference on-axis.  However, the off-axis can be a different story as many Barlows out there generate a small amount of lateral color so that is easy to find when you look for it.  My rule of thumb is that if doing critical planetary observing then I try not to Barlow if possible, and for all other observing Barlowing is just fine.  Caveat is of course that you use a quality Barlow.  I've had some Meade's and GSOs and other non-premium ones and they can have an inordinate amount of scatter which then impacts all the observing. 

 

FWIW, when I was building my XW collection I too wanted to Barlow instead of getting the shorter focal lengths.  I tried it but in the end purchased the shorter ones.  Not that the view was an issue, just the ergonomics of having to switch Barlows in and out and have that long tree growing out of the focuser.  Was just a hassle.


Edited by BillP, 30 April 2015 - 07:56 AM.


#18 Starman81

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Posted 30 April 2015 - 10:52 AM

 

ES 2x focal extender, though heavier than the TV 2x barlow, might be your best bet. The bottom element is removable and could be screwed on your XWs for roughly a 1.6x barlow factor. 

Hold on there.... 

 

I understood this magnification of 1.6x to hold true for a "barlow" lens

screwed to the eyepiece.  I also thought that a "focal extender's"

magnification stays the same no matter the placement in relationship

to the eyepiece ?????????????

 

Am I cloudy ????

 

 

The ES 2x Focal Extender is a 4-element design with a doublet in the main body and then a doublet in the 'field lens group'. When you use the removable bottom element, you are only using that 2-element doublet--think of it as a "mini" barlow. It's telecentric properties are gone, like consistent magnification factor and that's why it's more like 1.6x rather than 2x. Also, vignetting may occur for eyepieces with too large a field stop as well. 



#19 swix

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Posted 30 April 2015 - 01:24 PM

 

 

ES 2x focal extender, though heavier than the TV 2x barlow, might be your best bet. The bottom element is removable and could be screwed on your XWs for roughly a 1.6x barlow factor. 

Hold on there.... 

 

I understood this magnification of 1.6x to hold true for a "barlow" lens

screwed to the eyepiece.  I also thought that a "focal extender's"

magnification stays the same no matter the placement in relationship

to the eyepiece ?????????????

 

Am I cloudy ????

 

 

The ES 2x Focal Extender is a 4-element design with a doublet in the main body and then a doublet in the 'field lens group'. When you use the removable bottom element, you are only using that 2-element doublet--think of it as a "mini" barlow. It's telecentric properties are gone, like consistent magnification factor and that's why it's more like 1.6x rather than 2x. Also, vignetting may occur for eyepieces with too large a field stop as well. 

 

Thanks for the explanation.




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