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10mm XW & 13mm Vixen LVW.....too close?

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#1 Scanning4Comets

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Posted 09 December 2012 - 08:55 PM

Like the title says, I have a 10mm Pentax XW and I also just picked up a 13mm Vixen LVW. I have tried both and I wasn't sure at first, but it seems as though the 13mm LVW is very close to magnification and field size to my 10mm XW.

The are both really nice eyepieces.....but....

10mm Pentax XW = 105 / 40'
13mm Vixen LVW = 90 / 43'

I'm wondering if it would be better if I traded the 13mm Vixen LVW for a 17mm Vixen LXW, because the 17mm would be right in between my 22mm LVW and 10mm Pentax XW ?

With the 17mm Vixen LVW, I would get 71x / 55' which seems more logical to use in between.

Any suggestions here?

#2 paul hart

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Posted 09 December 2012 - 08:59 PM

The eyepieces are pretty much redundant so it makes sense to trade for the 17 LVW. I have both the 8 and 22mm LVW's (great eyepieces).

#3 jrbarnett

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Posted 09 December 2012 - 09:38 PM

I suspect the 17mm LVW is closer to the 22 LVW than the 13mm LVW is to the 10mm XW. Obviously you need to scrap the 10mm XW in favor of an 8mm LVW. Problem solved. :grin:

- Jim

#4 ryanlu92

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Posted 09 December 2012 - 09:44 PM

If your 10mm is for 105x, then your 13mm should be for 80x.
I have both 13mm and 10mm and they are just right to me.

#5 Scanning4Comets

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Posted 09 December 2012 - 11:41 PM

I suspect the 17mm LVW is closer to the 22 LVW than the 13mm LVW is to the 10mm XW. Obviously you need to scrap the 10mm XW in favor of an 8mm LVW. Problem solved.

- Jim


I have a 7mm Pentax XW, :lol:

If your 10mm is for 105x, then your 13mm should be for 80x.


How do you figure 80x? I drift timed the 13mm and it gives me 90x. Even if I divided 13mm into 1200, it would still net me more magnification.

#6 Starman81

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Posted 09 December 2012 - 11:42 PM

I suspect the 17mm LVW is closer to the 22 LVW than the 13mm LVW is to the 10mm XW. Obviously you need to scrap the 10mm XW in favor of an 8mm LVW. Problem solved. :grin:

- Jim


Yeah, it seems like the LVW's are beginning to encroach on Pentax XW territory! If Mark adds the 8 LVW, the XW 7 is being threatened. Then you're gonna have to add a Delos 6, again, Mark.

#7 Scanning4Comets

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Posted 10 December 2012 - 12:29 AM

I may have to look into a 15mm or 16mm instead !!

16mm Uwan
15mm Luminos

:tonofbricks:

#8 planet earth

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Posted 10 December 2012 - 12:50 AM

10mm Pentax XW = 105 / 40'
1200mm fl.
:question:

#9 Scanning4Comets

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Posted 10 December 2012 - 12:58 AM

10mm Pentax XW = 105 / 40'
1200mm fl.


Yep.

I drift tested both it and a Pentax 10.5mm XL. The 10.5mm Pentax XL gave me 109x working effectively as am 11mm eyepiece. The 10mm Pentax XW works out as an 11.4mm eyepiece. Not all eyepieces work at the magnification people think they do as there are discrepancies in the the eyepieces themselves,(+/-), in exact FL.

You'd figure that my 7mm Pentax XW would give me 171x right?

1200mm / 7mm = 171x

Well it doesn't !!! it gives me 160x, and it also did in my 8" F/6 I had in 2007, so my 7mm XW works as a 7.5mm EP.

Drift timing gives me a nice ball park figure.

In any event, a 17mm Vixen LVW will be the choice !!!

Cheers,

#10 ryanlu92

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Posted 10 December 2012 - 01:45 AM

I suspect the 17mm LVW is closer to the 22 LVW than the 13mm LVW is to the 10mm XW. Obviously you need to scrap the 10mm XW in favor of an 8mm LVW. Problem solved.

- Jim


I have a 7mm Pentax XW, :lol:

If your 10mm is for 105x, then your 13mm should be for 80x.


How do you figure 80x? I drift timed the 13mm and it gives me 90x. Even if I divided 13mm into 1200, it would still net me more magnification.


I mean if 1050mm/10mm = 105x then 1050mm/13mm = 80x.

But... your measure number surprises me. I need to see what's the difference between 13T6 and XW10. :question: Thanks!

#11 Scanning4Comets

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Posted 10 December 2012 - 02:27 AM

I mean if 1050mm/10mm = 105x then 1050mm/13mm = 80x.

But... your measure number surprises me. I need to see what's the difference between 13T6 and XW10. Thanks!


The focal length of my telescope is NOT 1050mm, it is 1200mm.

1200mm / 254mm = F/4.7

If it was 1050: My telescope would be a 10" F/4.1!

1050mm / 254mm = F/4.1

I drift timed the 10mm XW before and got a different number than 105x as well, which I forgot to mention. When drift timing, one needs to do it about 3-4 times, and use the average to come up with a magnification. I also did not use COS in my calculation which throws things off a little, but I am willing to live with that and get a rounded number that is close enough. My previous drift test of the 10mm XW gave me around 108x, which is probably closer to the real magnification for that particular eyepiece.

#12 Damo636

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Posted 10 December 2012 - 04:36 AM

I haven't the first clue how to drift time an eyepiece but I have found the views through the 7mm XW to be visually, almost indistinguishable from the 8mm Ethos! (in terms of magnification)

#13 HoriaC

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Posted 10 December 2012 - 07:45 AM

Markus

I do not think that drift timing the eyepiece gives you a better estimate of the focal length. For this you would have to:

- trust the specified apparent field of view more than the specified focal length

and

- assume that the eyepiece has zero angular nonlinearity, which is mostly not the case.

But maybe I am completely wrong as I do not know how you are actually doing it. You might want to cross check using this method (the "hard" one):
http://bobatkins.com...cal_length.html

Horia

#14 russell23

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Posted 10 December 2012 - 08:48 AM

I haven't the first clue how to drift time an eyepiece but I have found the views through the 7mm XW to be visually, almost indistinguishable from the 8mm Ethos! (in terms of magnification)


You're absolutely right - there isn't much difference. With your 12" f/5 the 7mm XW gives 218x and the 8mm Ethos gives 191x. The 7mm XW only gives a 14% higher magnification which for our eyes is not that significant.

Even the gap mentioned in the OP - 13mm to 10mm - is not that significant. The 10mm XW gives 120x in Markus's scope and the 13mm LVW gives 92x - a difference of only 30% and really not that significant. Now if he goes with the 17mm LVW that will give 71x and a 69% increase in magnification which is more worthwhile.

Small incremental magnification changes are more useful at higher magnifications as one approaches the seeing limits.

If I was Markus I would go with the 20mm XW or a 22mm LVW and then the 10mm XW. 2-2.5x gaps at lower magnifications are really ideal IMO.

Dave

#15 REC

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Posted 10 December 2012 - 09:32 AM

17mm is a very nice comfortable EP, you will love it!

#16 Mike B

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Posted 10 December 2012 - 01:16 PM

I do not think that drift timing the eyepiece gives you a better estimate of the focal length. For this you would have to:

- trust the specified apparent field of view more than the specified focal length


This is true. I've measured the *true* field of the LVWs i've owned at about ~63* net... but as was stated, this is assuming the FL values to be near their advertised values.

But if the Pentax AFoV values are fairly close to advertised, then i can see where the LVW's narrower AFoV would create less distinction *seen* on the sky between the two; the LVW, if a 70* EP like the Pentax, would show field as tho it were an 11.7mm EP.

If that's truly the case, then a case for a 13mm-to-17mm swap can be made.... and the step b/t the 17 & the 22 LVWs would then be comparable.
:grin:

Small incremental magnification changes are more useful at higher magnifications as one approaches the seeing limits.


This is an excellent point, and likely wouldn't come into play with the EPs being discussed... so another vote for a larger jump b/t FLs at this range.

#17 Scanning4Comets

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Posted 10 December 2012 - 02:22 PM

If I was Markus I would go with the 20mm XW or a 22mm LVW and then the 10mm XW. 2-2.5x gaps at lower magnifications are really ideal IMO.

Dave


I already own the 22mm LVW as I stated in my first post!

This is true. I've measured the *true* field of the LVWs I've owned at about ~63* net... but as was stated, this is assuming the FL values to be near their advertised values.


Jonathan Hoof did the same and he got 69 degrees AFOV! To me it looks more like 65-68 compared to my XW's.


I do not think that drift timing the eyepiece gives you a better estimate of the focal length. For this you would have to:

- trust the specified apparent field of view more than the specified focal length

and

- assume that the eyepiece has zero angular non-linearity, which is mostly not the case.

But maybe I am completely wrong as I do not know how you are actually doing it. You might want to cross check using this method (the "hard" one):
http://bobatkins.com...cal_length.html

Horia


Horia,

When I drift time my eyepieces, I choose a star that is almost right on the Celestial Equator, (0 degrees declination). I let the star drift from center of the eyepiece to the following edge of the field stop.

I then divide the seconds by 2.

EG: (My 13mm Vixen LVW):

Drift Time: (1 min / 31 sec).

= 91 sec / 2
= 46'

If I divide 69 by 46' I get = 90x

69 deg AF / 46' = 1.5 x 60 = 90

I was only going by what Jonathan Hoof had said in an archived post as to what he drift times the apparent field of a 22mm LVW was. (69 degrees).

Mike B. just said he found the apparent field in his at 63 degrees. So....If I plug in Mike B's apparent field into my calculation I get:

63 AF / 46' = 82x

If I plug in the apparent field at 65 degrees, I get:

65 AF / 46' = 85x

So, more than likely, my calculation of 90x is incorrect.

I'm going to go with 65 degrees apparent field @ 85x.

Cheers,

#18 russell23

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Posted 10 December 2012 - 03:47 PM

If I was Markus I would go with the 20mm XW or a 22mm LVW and then the 10mm XW. 2-2.5x gaps at lower magnifications are really ideal IMO.

Dave


I already own the 22mm LVW as I stated in my first post!


Yes - and that is one of the options I would go with in your situation. Either keep the 22mm LVW and go with the 22mm LVW and then straight to the 10mm XW, then smaller gaps from there up ... or get a 20mm XW and go with that and the 10mm XW (60x and 120x) and you have things covered.

It is just my opinion, but I don't see where 60x to 120x is a gap that needs to be filled. As you already noted the 13mm LVW does not seem that different from the 10mm XW. And if you get a 17mm LVW you are going to find that it too is not that different from what you have with the 22mm LVW (55x vs 71x). When I had 20mm and 14mm XW's at one point with a 1000mm FL scope (50x and 71x) the difference in magnification was not enough to be useful. The jump to 71x was so small that the desire I had was to keep on going.

Again - that's my feelings about it. If you really want that 71x, then the 17mm LVW is a nice eyepiece. I had one once.

Dave

#19 Scanning4Comets

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Posted 10 December 2012 - 05:04 PM

Yes - and that is one of the options I would go with in your situation. Either keep the 22mm LVW and go with the 22mm LVW and then straight to the 10mm XW, then smaller gaps from there up ... or get a 20mm XW and go with that and the 10mm XW (60x and 120x) and you have things covered.

It is just my opinion, but I don't see where 60x to 120x is a gap that needs to be filled. As you already noted the 13mm LVW does not seem that different from the 10mm XW. And if you get a 17mm LVW you are going to find that it too is not that different from what you have with the 22mm LVW (55x vs 71x). When I had 20mm and 14mm XW's at one point with a 1000mm FL scope (50x and 71x) the difference in magnification was not enough to be useful. The jump to 71x was so small that the desire I had was to keep on going.

Again - that's my feelings about it. If you really want that 71x, then the 17mm LVW is a nice eyepiece. I had one once.

Dave


I guess you're right about the magnification jumps. Going from the 22mm to the 10mm is fine, however, there are some targets that require something in between so it will fit nicely in the EP, but prob not enough to justify keeping the 13mm LVW.

I *WAS* warned before I did buy the 13mm LVW, (I can just use the bottom of my barlow lens element on my 22mm LVW to get me roughly a 16mm EP), but I was curious about the 13mm LVW to see how it would perform!

Thanks for the tips Dave! Much appreciated !

#20 Mike B

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Posted 10 December 2012 - 06:13 PM

Hi Markus- for the record, the way i measure EPs (& Barlow effects, too) is hanging a carpenter's tape-measure on a streetlight pole, during daytime, & then scope on it using the EPs and/or Barlows from ~100 feet distance. I center the tape in the view, so it represents a full diameter. Then i read the tick marks to a 1/16" at top & bottom. Take that net dia. distance, with the scope-to-tape distance, and use ArcTangent to figure true angular field (TFoV). All that's left is to multiply that value *UP* by the magnification the EP produces in the scope you're using. There's your AFoV.
:grin:

What i find happens with drift timing is you end up missing a true "diameter" drift... ending up with a "chord" off to this side or that. So if i take 5 drifts, i'll throw out the lowest 3 results, figuring they're chords (not true diameters), and maybe average the longer two. You're FAR more likely to get a false *short* drift than a false LONG one.

That's why i use the tape-measure method. Very little wiggle-room there. ;)

As an aside- in having done this several times, with several batches of EPs, i've found Naglers hit 81-82* regularly, ES-100's hit 100*, some of the Meade UWA's hit 78-79*, and most older-style SWA's hit low-to-mid 60's degrees.

#21 eklf

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Posted 10 December 2012 - 10:18 PM


It is just my opinion, but I don't see where 60x to 120x is a gap that needs to be filled. As you already noted the 13mm LVW does not seem that different from the 10mm XW. And if you get a 17mm LVW you are going to find that it too is not that different from what you have with the 22mm LVW (55x vs 71x). When I had 20mm and 14mm XW's at one point with a 1000mm FL scope (50x and 71x) the difference in magnification was not enough to be useful. The jump to 71x was so small that the desire I had was to keep on going.

Dave


I agree 100% with Dave and jrbarnett. I extensively compared 22 LVW,17 LVW, 13 LVW and 10 XW in my 8 nch F6. The 17 LVW was too close to 22 LVW and 13 LVW was , likewise, too close to 10 XW. I almost went with 10 XW, but the 13 LVW was just too comfortable (even more so than 10 XW). So I replaced 10 XW with 7 XW to finall give 22 LVW, 13 LVW and 7 XW.

#22 Scanning4Comets

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Posted 10 December 2012 - 10:40 PM

I agree 100% with Dave and jrbarnett. I extensively compared 22 LVW,17 LVW, 13 LVW and 10 XW in my 8 nch F6. The 17 LVW was too close to 22 LVW and 13 LVW was , likewise, too close to 10 XW. I almost went with 10 XW, but the 13 LVW was just too comfortable (even more so than 10 XW). So I replaced 10 XW with 7 XW to finall give 22 LVW, 13 LVW and 7 XW.


Now, there's an even better idea!

I could keep the 22mm and 13mm LVW's and sell off the 10mm XW because I already have the 7mm XW....maybe even spring for a 5mm Vixen LVW !!! :rainbow: :D

Hence:

38mm Q70
22mm LVW
13mm LVW
7mm XW
5mm LVW


#23 ausastronomer

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Posted 10 December 2012 - 10:55 PM

Hi Markus,

Is your scope not a 10"/F4.7 newtonian ?

If so, your calculations on magnification are incorrect. In a 10"/F4.7 scope the 10mm XW gives you 119X, compared to 90X for the 13mm Vixen LVW. That's a 33% increase in magnification and a big enough gap for me.

Cheers,

#24 iluxo

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Posted 10 December 2012 - 11:04 PM

A more sensible set would be 5, 8, 13, 22 mm, or 7, 10,15, 25 mm.

#25 george golitzin

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Posted 10 December 2012 - 11:30 PM

I agree 100% with Dave and jrbarnett. I extensively compared 22 LVW,17 LVW, 13 LVW and 10 XW in my 8 nch F6. The 17 LVW was too close to 22 LVW and 13 LVW was , likewise, too close to 10 XW. I almost went with 10 XW, but the 13 LVW was just too comfortable (even more so than 10 XW). So I replaced 10 XW with 7 XW to finall give 22 LVW, 13 LVW and 7 XW.


Now, there's an even better idea!

I could keep the 22mm and 13mm LVW's and sell off the 10mm XW because I already have the 7mm XW....maybe even spring for a 5mm Vixen LVW !!! :rainbow: :D

Hence:

38mm Q70
22mm LVW
13mm LVW
7mm XW
5mm LVW


I think that's a terrible idea. Unless you're totally in love with the 13 LVW, why the heck would you sell the 10XW?? For one thing, the jump from 13 to 7 is cutting your exit pupil almost in half: way too big a jump for my taste. I really like the "root 2" sequence I have: 20, 14, 10, 7, 5, 3.5. (The 10 and under are XWs, the 14 is a delos, and the 20 is the NT5.)

I think the 13 feels really close to the 10 not because of magnification, but because of tfov: the true fields of the two are very close, which is why you're thinking of selling. But the exit pupil/mag difference makes it worth holding on to the 13. Again, why sell an eyepiece that you like?

But, if you want a balanced sequence, consider that the geometric mean between 10 and 22 is 14.8--and since the 10 has slightly wider afov than the 22 lvw, a 14mm 70-degree eyepiece would feel close to the mean between the two. So save up your funds, sell the 13, and get a delos: that's my recommendation!

-geo.






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