Magnification gap

[miguel87]

Thanks in advance for your advice and opinions.

I have been enjoying visual astronomy for almost 10 years. I currently use a SW 200P (focal length 1000mm).

My current eyepieces (6mm, 9mm, 20mm and 32mm plus a 2x barlow) leave a gap between 50x and 100x magnification (that would be an eyepiece of 11-19mm).

Question 1, will it be significantly useful to fill this gap?

Question 2, if so, what should I fill it with?

I recently considered the 15mm BST StarGuider, however, with a 60° AFOV the resulting view would show me 0.9° of the sky, only 0.1° different to the 1° displayed by my 50° AFOV 20mm eyepiece.

Thanks,

Mike

Edited April 5, 2020 by miguel87
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[Timebandit]

 

Hello.

A gap of your 9mm to your 20mm is a large gap on eyepiece spacing when it comes to magnification availability ,and I am not a big fan of Barlow's personally.  So yes I do feel you would benefit from filling this magnification gap.

What to fill it with depends on your budget. But I would consider a second hand TV Delos 14mm with 72d fov or maybe consider a Vixen if you can locate one, a second hand Vixen LVW 13mm with a 65d fov. Two great mid range price eyepieces IMO .

Hope this helps

 

 

Edited April 5, 2020 by Timebandit

[miguel87]

2 hours ago, Timebandit said:

 

Hello.

A gap of your 9mm to your 20mm is a large gap on eyepiece spacing when it comes to magnification availability ,and I am not a big fan of Barlow's personally.  So yes I do feel you would benefit from filling this magnification gap.

What to fill it with depends on your budget. But I would consider a second hand TV Delos 14mm with 72d fov or maybe consider a Vixen if you can locate one, a second hand Vixen LVW 13mm with a 65d fov. Two great mid range price eyepieces IMO .

Hope this helps

 

 

Thanks for the reply,

I get on pretty well with my barlow so I find creating a 10mm eyepiece more than acceptable at the moment. Still a big gap tho.

I have 2 vixen eyepieces (SLV) and really like them so I will definitely have a look at your recommendation for the LVW.

Never looked through a TV but obviously their reputation precedes them! Depends on price, I mean, roughly speaking I dont want to be spending more than about 100 quid.

Mike

[Don Pensack]

7 hours ago, miguel87 said:

Thanks in advance for your advice and opinions.

I have been enjoying visual astronomy for almost 10 years. I currently use a SW 200P (focal length 1000mm).

My current eyepieces (6mm, 9mm, 20mm and 32mm plus a 2x barlow) leave a gap between 50x and 100x magnification (that would be an eyepiece of 11-19mm).

Question 1, will it be significantly useful to fill this gap?

Question 2, if so, what should I fill it with?

I recently considered the 15mm BST StarGuider, however, with a 60° AFOV the resulting view would show me 0.9° of the sky, only 0.1° different to the 1° displayed by my 50° AFOV 20mm eyepiece.

Thanks,

Mike

Boy, has there been a lot written on how to choose magnifications for a scope!  One way is to have a range of exit pupils, but which?  Another way is a % step between magnifications.  Another is to have even steps from a low power up.

I side with this latter approach for a couple reasons:

--it makes the highest powers closer together, %-wise, which is useful when bumping up against the ceiling of your seeing conditions

--a smaller % jump at the high end is necessary to not make the magnification jumps too large.  a 40-60x jump at low power is very close together.  The same % could be 400-600x and that is a huge jump

 

So, what is the even magnification jump to favor so that low powers aren't too close together and high powers aren't too far apart?

I think it varies by scope size.  If you have a 20", it wouldn't be outside the realm of utility to have a 100x jump in between magnifications, i.e. 100/200/300/400x and so on

That wouldn't work for a 4" refractor, where jumps of 30x might be more rational.

So, it seems to me, from owning 31 scopes so far, that a 1X/2X/3X/4X sequence is reasonable, where x = a number appropriate for a certain size of scope.

Perhaps, 50 for an 8", 60 for a 10", 70 for a 12.5", and so on.  And if the jump at some particular place in the sequence seems too large, then fill in with a 0.5x in the middle, like 1x, 2x, 2.5x, 3x, 4x, etc.

 

So, in answer to your question, and having owned an 8" scope for 11 years, I think you do not need a magnification in between 50 and 100x, but each observer has a favorite power that just feels right.  If you are constantly finding 50x too low and 100x too high for some preferred targets, you might definitely add one in between.  What i would do, though, is just to get a wider apparent field at 100x.  The objects would stay in the field longer, and the desire to have a lower power might evaporate.  I think having jumps of 50x on your scope is pretty close to perfect, and the wider field may make the 100x eyepiece more usable and desirable.  Make your 9mm a 100° eyepiece and the 20mm might get a lot less use.

[michael.h.f.wilkinson]

In my C8 I use quite a range of EPs, usually starting with the Nagler 31 mm (65.5x), as a general-purpose finder/wide-field EP. If I need to squeeze the largest possible field of view from the scope, I turn to the Vixen LVW 42mm (48.3x). For galaxy hunts, I go to the Nagler 22 mm (92.3x, which doesn't feel like a big jump from the 31mm). Depending on the surface brightness and seeing conditions, I might insert the Nagler 17 mm (119.4x). That is also an awesome tool for the moon, capturing the entire disc in stunning detail. For compact DSOs, in particular planetary nebulae, I have a Delos 14mm (145x, a fairly new arrival), and Nagler 12mm  (179x, had that for years). We then move firmly into planetary and lunar EPs, and these are the Pentax XW 10 mm (203x), Delos 8 mm (254x), Pentax XW 7mm (290x), Delos 6mm (338x), and Pentax XW 5 mm (406x). The latter two are rarely used in the C8, but see a lot of service in the 80mm F/6 scope. As you can see, the EPs are far more closely spaced at the short end, to adjust to the seeing. Some of my best views of Saturn have been with the XW 7mm.

[Louis D]

I'd say 75x to 100x is my favorite workhorse power range for everyday observing of many objects under typical sky conditions.  As such, I would highly recommend a low-teens eyepiece to fill that gap.

Your 8" Dob is actually 1200mm in focal length, so your current powers are 38x, 60x, 133x, and 200x.  That leaves quite a gap between 60x and 133x, so I'd recommend something around 86x to 100x with a 12mm to 14mm eyepiece.  The 12mm BST Starguider is pretty decent at f/6.  So is the 12mm Meade HD-60, though it's been discontinued and is considerable more expensive in the UK than the Starguiders.

Edited April 5, 2020 by Louis D

[miguel87]

13 minutes ago, Louis D said:

I'd say 75x to 100x is my favorite workhorse power range for everyday observing of many objects under typical sky conditions.  As such, I would highly recommend a low-teens eyepiece to fill that gap.

Your 8" Dob is actually 1200mm in focal length, so your current powers are 38x, 60x, 133x, and 200x.  That leaves quite a gap between 60x and 133x, so I'd recommend something around 86x to 100x with a 12mm to 14mm eyepiece.  The 12mm BST Starguider is pretty decent at f/6.  So is the 12mm Meade HD-60, though it's been discontinued and is considerable more expensive in the UK than the Starguiders.

Thanks Louis,

Sorry I must have made a typo somewhere, my scope is a newt on an EQ mount, not a dob.

 

Yeah I think you are right and it is a gap I would like to fill. I think part of the reason I havent yet is that I use my barlow alot and always try to buy eyepieces that fill two gaps in my range. Any eyepiece around 15mm will not be useful to me when barlowed because I have that whole range covered roughly.

Still, I think it might be worth it.

I think I have boiled it down to...

1. The starGuider 15mm. Lots of recommendations and great price plus an extra 10° AFOV.

2. The Vixen SLV 15mm. I have the 6mm and 9mm and they are great (and parfocal). But it's considerably more expensive and has 10° less AFOV.

Appreciate all the responses

Edited April 5, 2020 by miguel87
Spelling

[Don Pensack]

Just a note: the Vixen SLV is fine in all regards save one: it is a lousy eyepiece for Moon viewing.

There is a bright, shiny, spacer directly under the eye lens that reflects the light of the Moon upside down outside the field stop of the eyepiece.

If you're up to dismantling the eyepiece and blackening that spacer, then never mind.

But if you plan to use it for the Moon, look elsewhere.

On the other hand, for deep sky it is fine and very comfortable to use.

[miguel87]

2 minutes ago, Don Pensack said:

Just a note: the Vixen SLV is fine in all regards save one: it is a lousy eyepiece for Moon viewing.

There is a bright, shiny, spacer directly under the eye lens that reflects the light of the Moon upside down outside the field stop of the eyepiece.

If you're up to dismantling the eyepiece and blackening that spacer, then never mind.

But if you plan to use it for the Moon, look elsewhere.

On the other hand, for deep sky it is fine and very comfortable to use.

That's interesting, how does that manifest when looking at the moon? I havent noticed anything in the 6 or 9mm but to be honest I dont do a huge amount of lunar observing. Probably suggests I can deal with the problem without getting too annoyed.

Still tempted by the price and FOV of the starguider. I just dont want to feel like I'm downgrading.

 

[michael.h.f.wilkinson]

I have a 15mm SLV, and never had any issues observing either sun or moon with it. I cannot fault the three SLVs I have (15, 9, and 5 mm) I use in my travel set-up. Mine may be a later version than Don's example, so the issue may have been fixed. My SLVs all perform similarly to my Pentax XWs, except in terms of field of view. 

[Don Pensack]

I just took apart a 15mm SLV from a fairly recent batch.

The spacer underneath the eye lens is a very light grey color and almost white at a low angle.

There is also a retaining ring holding in the upper set of lenses that is an anodized grey color, also almost white at low angle.

The field stop had a shiny knife-edge as well.

The bottom group of lenses is excellently-housed with dark threads and good baffling.

What happens in use, I found, is that the image of the Moon appears outside the field stop in reflection, upside down relative to the Moon inside the field stop, as if a mirror image with the field stop being the edge of the field and the edge of the mirror.

When the scope drifts over the Moon, a reverse image of the moon outside the field stop drifts in the opposite direction than the image of the Moon inside the field stop.

I think this might not be much of an issue in a tracking scope or if only the center of the field were paid attention to.

My sample also had a lot of debris inside the eyepiece, which I took care of when reassembling.

It was 7 elements in 4 groups, with 8 air-to-glass surfaces.

It's always possible that the internal light scatter has been taken care of in the most recent production, but if so, I haven't seen one yet.

Except for removing the field stop, which is glued in with a few drops of glue (but which dissolves with a tiny tiny drop of acetone), the eyepiece is easy to disassemble and reassemble, so an industrious owner could easily solve all reflection issues and end up with a fine long eye relief eyepiece.

Edited April 5, 2020 by Don Pensack

[bomberbaz]

I think Don Pensack has a great idea to fill your gap with a wider field of view eyepiece but the only problem is doing so brings it's own problems.

I am assuming the 20mm is a SLV or similar with a FOV at 50. On your OTA thats times 50 times and 1 degree fov. or x100 and 0.5 fov barlowed

Using a Explore scientific 14mm/82 degree fov you get x71 with 1.15 degrees fov. or x143 and 0.57 barlowed.

That fixes your something in-between but then your start thinking do I need the 20 or 10 when the 14 & 7 do the same but a little better. (greater magnification and wider field of view)

A great thing about wider field eyepieces is the ability to get the space walk feel but if you buy one you then start looking at your other eyepieces and thinking I like the wider views, do I need the others narrow fov ep's, as good as they are. 

There is always a case to have differing focal lengths and fields of view but the more different types you have, the more complicated it gets. I think you just end up convincing yourself you need a specific eyepiece for viewing this or that object until you have a very lot of eyepieces, some of which never see the dark of night.

Does this sound familiar to anyone?

Certainly happened to me. 

[John]

1 hour ago, bomberbaz said:

There is always a case to have differing focal lengths and fields of view but the more different types you have, the more complicated it gets. I think you just end up convincing yourself you need a specific eyepiece for viewing this or that object until you have a very lot of eyepieces, some of which never see the dark of night.

Does this sound familiar to anyone?

Certainly happened to me. 

Perish the thought baz !

I found with hyper wide eyepieces I needed less focal lengths so I found 31mm, 21mm, 13mm, 8mm and 6mm replaced a somewhat larger range of eyepieces with slightly narrower field of view.

All was going well until I decided that often I wanted smaller eyepieces when using refractors, so I put together a 1.25 inch fitting set. 

Then I decided that I needed an outreach / travel set so added a zoom, a barlow and a wide field for those purposes.

So all my good intentions are undone now

"Confessions of an Ocularholic"  

 

[michael.h.f.wilkinson]

7 hours ago, Don Pensack said:

I just took apart a 15mm SLV from a fairly recent batch.

The spacer underneath the eye lens is a very light grey color and almost white at a low angle.

There is also a retaining ring holding in the upper set of lenses that is an anodized grey color, also almost white at low angle.

The field stop had a shiny knife-edge as well.

The bottom group of lenses is excellently-housed with dark threads and good baffling.

What happens in use, I found, is that the image of the Moon appears outside the field stop in reflection, upside down relative to the Moon inside the field stop, as if a mirror image with the field stop being the edge of the field and the edge of the mirror.

When the scope drifts over the Moon, a reverse image of the moon outside the field stop drifts in the opposite direction than the image of the Moon inside the field stop.

I think this might not be much of an issue in a tracking scope or if only the center of the field were paid attention to.

My sample also had a lot of debris inside the eyepiece, which I took care of when reassembling.

It was 7 elements in 4 groups, with 8 air-to-glass surfaces.

It's always possible that the internal light scatter has been taken care of in the most recent production, but if so, I haven't seen one yet.

Except for removing the field stop, which is glued in with a few drops of glue (but which dissolves with a tiny tiny drop of acetone), the eyepiece is easy to disassemble and reassemble, so an industrious owner could easily solve all reflection issues and end up with a fine long eye relief eyepiece.

Interesting observation. I generally use tracking scopes, so that might explain the different experiences. Is this effect limited to the 15 mm or have you seen it in others too? 

[Piero]

In my Dobson 200mm F6, I never felt the need to have a magnification between 50x and 100x. 

Something in between like a 75x? What can that show that the other two magnifications don't? I don't see an use case, except from collecting one more eyepiece. 

50x is a good low power, 100x is a good medium power. Add a 200x as high power and you are done. These jumps will show different views, worth swapping eyepieces during a session.

 

P.s. the same approach can be used for larger telescopes. In this case though, the medium power will have a higher magnification, and the high power will be subjected to seeing conditions.

As Don suggested previously, I also think that eyepieces should be chosen and bought considering the exit pupil, rather than the mere magnification.

Ideally, something like the following works well:

* 4-5mm exit pupil for low power

* around 2mm for medium power

* 1mm for high power

* 0.7mm for very high power depending on seeing and telescope.

[John]

With my 12 inch dobsonian 75x is my normal low power - its a 21mm Ethos though

[michael.h.f.wilkinson]

I find the intermediate powers useful to match exit pupil to both seeing conditions and surface brightness of the objects

[bomberbaz]

14 hours ago, John said:

Perish the thought baz !

I found with hyper wide eyepieces I needed less focal lengths so I found 31mm, 21mm, 13mm, 8mm and 6mm replaced a somewhat larger range of eyepieces with slightly narrower field of view.

All was going well until I decided that often I wanted smaller eyepieces when using refractors, so I put together a 1.25 inch fitting set. 

Then I decided that I needed an outreach / travel set so added a zoom, a barlow and a wide field for those purposes.

So all my good intentions are undone now

"Confessions of an Ocularholic"  

 

I can definately go along with this John. I had settled one set of EP's, all the widefields listed in my sig. I love my Nikons, they are so easy to get along with plus the views are breathtaking and the Nagler is a great addition to these two. The powermate works superb with these. I no longer even look at new eyepieces. (well ok just occasionally) 😄

However, after buying a mak and frac for use in the garden mainly, I then needed another set. Kept that both simple and cheap though with 3x BST's and a barlow. I find this is perfectly acceptable for intended use.

Out of interest what is your frac set of glass made up of?  (sorry for going off topic OP)

[bomberbaz]

This is my list of eyepieces, magnification, exit pupils and tfov. The 14mm and 10mm are basically the 17mm & 12.5mm (Both Nikon's) with an additional lens screwed into the base. They came supplied with them. Akin to mini barlows. 

The sizes 8.5mm and down are when I make use of a televue powermate with any of the four above them.  The top 26mm eyepiece is a Nagler. So basicallly I use 3 eyepieces.

FWIW my most used is the 17mm Nikon. Pretty much on the same lines as John's 21mm ethos in power.

[John]

37 minutes ago, bomberbaz said:

Out of interest what is your frac set of glass made up of?  (sorry for going off topic OP)

24 Panoptic, 17.3 and 14 Delos, 10, 7, 5 and 3.5 Pentax XWs, 2-4mm Nagler zoom.

[miguel87]

Thanks everyone for your replies. I like to keep eyepieces to a minimum from a financial and practical point of view. My entire collection is 6mm and 9mm Vixen SLV, 20mm Vixen NPL, 32mm 2" Panaview and a Tal 2x barlow.

This gives me exit pupils of 0.6mm, 0.9mm, 1.2mm, 1.8mm, 2mm, 4mm and 6.4mm.

You have made me question my need for another eyepiece.

Mike

[Louis D]

On 05/04/2020 at 04:56, miguel87 said:

I currently use a SW 200P (focal length 1000mm).

 

18 hours ago, miguel87 said:

Sorry I must have made a typo somewhere, my scope is a newt on an EQ mount, not a dob.

No typo on your part, just a misunderstanding on my part.  The SW 200P would refer to the SkyWatcher 8" Dob over here since the 8" EQ version isn't sold under that name here.  The closest would be Orion USA's SkyView Pro 8" Equatorial Reflector Telescope that probably corresponds to your scope.

[Louis D]

3 minutes ago, miguel87 said:

Thanks everyone for your replies. I like to keep eyepieces to a minimum from a financial and practical point of view. My entire collection is 6mm and 9mm Vixen SLV, 20mm Vixen NPL, 32mm 2" Panaview and a Tal 2x barlow.

This gives me exit pupils of 0.6mm, 0.9mm, 1.2mm, 1.8mm, 2mm, 4mm and 6.4mm.

You have made me question my need for another eyepiece.

Mike

As long as you don't mind swapping the Barlow in and out, then you're pretty much good to go.

[Louis D]

7 hours ago, Piero said:

Something in between like a 75x? What can that show that the other two magnifications don't? I don't see an use case, except from collecting one more eyepiece.

If you're using 50 degree eyepieces, I find many open clusters and nebula view better at 75x than 100x by giving them a bit more context.  They're just a bit too small for my liking at 50x.  If you have a 9mm 100 degree eyepiece or ES-120, then that doesn't matter all that much since it provides so much context already and 75x might be a moot point.

[miguel87]

25 minutes ago, Louis D said:

If you're using 50 degree eyepieces, I find many open clusters and nebula view better at 75x than 100x by giving them a bit more context.  They're just a bit too small for my liking at 50x.  If you have a 9mm 100 degree eyepiece or ES-120, then that doesn't matter all that much since it provides so much context already and 75x might be a moot point.

My 9mm is just a standard 50°, so I guess my thinking is that it would be nice to have a wider view other than just my panaview. But it is fair to say that is not a necessity.

I will be getting the scope out tonight (despite the moon) and will see if I feel the need for another piece!

Mike