Does a Barlow effectifly extent my useful maximum magnification?

[THL22]

Hello dear Stargazers!

Sorry if this been answered before but I couldn't find a clear answer.

Lets say my scope has an focal length op 1800mm and a diameter of 150mm. Maximum useful magnification with this scope should be double the diameter. So 300x

For instance if I use a 10mm eyepiece I'll be a 180x magnification (1800/10)
But with a 5mm eyepiece i'll reach 360x (1800/5) this would overwrite my useful maximal magnification right?

But what if I use a barlow of 2x. Thus doubling my focal length. in this case to 3600. My maximum useful magnification will still be 300x.
Can I now use a higher power eyepiece? Like 5?

 

[Peter Drew]

As you say, a 150mm  aperture has an upper useful magnification of 300x, if you effectively double the focal length by use of a 2x Barlow you will double the magnification of a given eyepiece. In this case, using a 5mm eyepiece will result in 720x magnification which would be unusable.   

[Ricochet]

As you say, the maximum magnification is determined by the aperture, so modifying the focal length by adding a barlow will not alter the aperture or maximum magnification that you can use.

Also, maximum magnification of 50-60x per inch (2-2.4x per mm) is for separating double stars. If you want the maximum magnification for planetary then 25-30x per inch (1-1.2x per mm) is often quoted.

[THL22]

8 minutes ago, Ricochet said:

As you say, the maximum magnification is determined by the aperture, so modifying the focal length by adding a barlow will not alter the aperture or maximum magnification that you can use.

Also, maximum magnification of 50-60x per inch (2-2.4x per mm) is for separating double stars. If you want the maximum magnification for planetary then 25-30x per inch (1-1.2x per mm) is often quoted.

So in my case with 1800mm and a 150mm diameter. Which eyepieces would you recommend and which barlow ( if any)?

[davyludo]

A 12mm eyepiece would give you 150x and you could 2x Barlow it to give effectively a 6mm eyepiece, which would give you 300x if you really wanted it. So a 6mm gives you maximum mag for your scope.

What eye pieces do you already have?

I think BST do a 12mm, unless you're looking for something more expensive - let us know an idea of price range you're looking at. I've also got their 2x Barlow which I quite like.

I use the BST Barlow with my mak 127 (1500 focal length) to get me from 100x to 200x with a 15mm eyepiece.

[Ricochet]

22 minutes ago, THL22 said:

So in my case with 1800mm and a 150mm diameter. Which eyepieces would you recommend and which barlow ( if any)?

It depends what type of object you want to view. I tend to work from focal ratio, which for your scope is 12. There will be no need for a barlow at this focal ratio to achieve high magnification, only to increase the number of options.

For planetary I would suggest something in the 10-12mm range (0.85-1xFR) but for small extended deep sky objects I would say 2-2.5x FR so 24-30mm.

For my current set of eyepieces to be used with my f6 scope I started with the DSO eyepiece and then divided the focal length by 1.4 to get my increasing powers so I've got steps of 14-10-7-5. For your f12 scope you could just double these numbers to 28-20-14-10. 28mm is already quite long so the only logical longer focal length eyepiece to add would be a 40mm.

[THL22]

1 hour ago, davyludo said:

A 12mm eyepiece would give you 150x and you could 2x Barlow it to give effectively a 6mm eyepiece, which would give you 300x if you really wanted it. So a 6mm gives you maximum mag for your scope.

What eye pieces do you already have?

I think BST do a 12mm, unless you're looking for something more expensive - let us know an idea of price range you're looking at. I've also got their 2x Barlow which I quite like.

I'm going to get the Skymax 150 pro mak with a 25mm eyepiece. i'm looking to replace the kit eyepiece for some better quality BST pieces. I'm want to look a basically everything. But what I want to prevend it buying a eyepiece that will be "too strong" for my scope (to be).|
Thanks for your help so far:)

[davyludo]

Ricochet makes a good point that you shouldn't need a barlow in order to get high mags with you focal length/ratio - the only reason you would need one would be to make use of existing eyepieces for additional magnifications (or to extend eye relief). 

Just to give you an idea, I use the following eyepieces with the mak 127 at the moment: 

• X-cel LX 25mm which gives me 60x (use barlow to give 120x)
• Vixen SLV 15mm which gives me 100x (use barlow to give 200x)
• X-cel LX 9mm which gives me 166.67x

I use the same eyepieces with my ST102 so I like having a barlow as it gives me a decent selection to use with both scopes. I also have a BST 3.2mm that only get used with the ST102 as it is WAY too much for my wee mak. I've only used the one BST but really like it and I'm actually considering getting the 8mm (which would give me 187.5x in the mak). To my rookie eyes, the BST 2x barlow is perfectly acceptable and I use it quite a bit - but I understand that not everyone likes a barlow. 

[davyludo]

I use the following website to try out combinations of eyepieces and scopes to give me a general idea of what I'll be able to see: 

http://astronomy.tools/calculators/field_of_view/

 

[Buzzard75]

Theoretical maximum magnification is often quoted as 50x the aperture in inches. The USEFUL maximum is about half that. In your case, 150mm=5.9in, the theoretical maximum is roughly 350x, but the useful maximum is only 175x. With a focal length of 1800mm, you should look for an eyepiece or a combination of eyepiece and Barlow that will give you this useful maximum or something as close to it as possible, so roughly 10-11mm. You could get a 10mm eyepiece or a 20mm and a 2x Barlow and achieve the same magnification. If you buy the Barlow, then any eyepiece you buy after that would essentially be like buying two eyepieces. You also don't want to go too far below a 1mm exit pupil. The lower you go the more uncomfortable the viewing can be and you may begin to start focusing on floaters and things in your eye rather than your object. Take your eyepiece focal length and divide by your focal ratio to get your exit pupil. Your focal ratio is 1800/150=12. So 10mm/12=.833mm.

On the other end of the spectrum, I start looking at exit pupil sizes more closely because I have a Newtonian with an obstruction in the center of the aperture. Anything that will give you an exit pupil larger than 7mm will start to show a darkening in the center of the image if there is a secondary mirror obstructing the aperture. Also, the human eye isn't capable of dilating much more than 7mm. Some people aren't even capable of that and it gets worse as we get older. With a 7mm exit pupil and an f/12 scope, the longest focal length eyepiece you'd want is 84mm. Good luck finding one that long. Even if you did, it'd probably be ridiculously large and expensive. I'd say 50x is probably as low as you'd want to go, so a 36mm eyepiece would probably do you well. And if you bought the 2x Barlow, you'd also essentially have an 18mm eyepiece as well.

As for what goes in between, anything that really tickles your fancy. Tele Vue, Explore Scientific, Plossls, Naglers, etc., etc. Each manufacture and type of eyepiece will have pros and cons and it's really up to you to decide what you really like about each brand and type. You also have to decide what's in your budget as each manufacture and type will vary greatly in price.

[THL22]

1 hour ago, davyludo said:

I use the following website to try out combinations of eyepieces and scopes to give me a general idea of what I'll be able to see: 

http://astronomy.tools/calculators/field_of_view/

 

That site is indeed very helpful! But I always found that it showed a green circle indicating OK status even if the magnification is greater than two times the diameter of the scope. 
I'm not sure what to make of that honestly

[davyludo]

5 minutes ago, THL22 said:

That site is indeed very helpful! But I always found that it showed a green circle indicating OK status even if the magnification is greater than two times the diameter of the scope. 
I'm not sure what to make of that honestly

I thought it just used different colours so that you could add multiple eyepieces to the same view.... I didn't think the colours meany anything other than that.

[THL22]

4 minutes ago, davyludo said:

I thought it just used different colours so that you could add multiple eyepieces to the same view.... I didn't think the colours meany anything other than that.

I think you are right! Basically the 8mm is my maximum EP judging from the magnification level. 

[davyludo]

19 minutes ago, THL22 said:

Basically the 8mm is my maximum EP judging from the magnification level. 

For the BST range then yes. A 6mm would be the theoretical maximum for the skymax 150. But a 6mm would only be used on nights of exceptionally good seeing, and even then probably limited by atmosphere.

Something in the 10-12mm range would get more use and would give you 150-180x mag. 

Personally if I was getting a skymax 150 and had no eyepieces, I'd go for a 32mm (56x) for finding things and DSO stuff, an 18mm (100x), a 12mm (150x) and a 2x Barlow. The Barlow would give me 200x when used with the 18mm, and 300x (with 12mm) for when I was feeling ridiculous. But that is purely my opinion (fairly new to astronomy) others who are more experienced may have a different/better view.

[THL22]

16 minutes ago, davyludo said:

For the BST range then yes. A 6mm would be the theoretical maximum for the skymax 150. But a 6mm would only be used on nights of exceptionally good seeing, and even then probably limited by atmosphere.

Something in the 10-12mm range would get more use and would give you 150-180x mag. 

Personally if I was getting a skymax 150 and had no eyepieces, I'd go for a 32mm (56x) for finding things and DSO stuff, an 18mm (100x), a 12mm (150x) and a 2x Barlow. The Barlow would give me 200x when used with the 18mm, and 300x (with 12mm) for when I was feeling ridiculous. But that is purely my opinion (fairly new to astronomy) others who are more experienced may have a different/better view.

Very helpful mate! Much appreciated! 

[ronin]

6 hours ago, THL22 said:

So in my case with 1800mm and a 150mm diameter. Which eyepieces would you recommend and which barlow ( if any)?

I would also go with the no barlow option, and as you mention BST's get the 8mm, 12mm, 15mm, 18mm, 25mm.

If you did get a barlow then drop the 8mm and 12mm, I think the resultant magnification of either of those and a 2x would be too much. So in a way 5 eye pieces or 3 eyepieces and a barlow. Likely a small saving in cost but I would expect a slight loss in performance with the barlow.

1800FL and 8mm gives 225x, doubt that you will get to use 300x very often, if at all.

[25585]

Is eye relief increased proportionately by the Barlow's magnification?

TV 's Powermates and rivals don't do that so much. A 1.5x seems good for large to mid fl eps as an alternative to lower er, higher power eps.

[John]

Barlow lenses do push out the eye relief and there is a formula to work this out. I think you need to know the barlow lens focal length though, which is not always easy to find out.  Bill Paolini devised this approximate formula:

New ER = Old ER * ( 1 - (Barlow Mag / Barlow FL) * 5)

 

 

[25585]

1 hour ago, John said:

Barlow lenses do push out the eye relief and there is a formula to work this out. I think you need to know the barlow lens focal length though, which is not always easy to find out.  Bill Paolini devised this approximate formula:

New ER = Old ER * ( 1 - (Barlow Mag / Barlow FL) * 5)

 

 

My ep I want to extend er on is 13mm Nagler T1 smoothie. Native is 13mm er. 

So really I want a Barlow, mainly for my Genesis and then dob. The 13mm has a 2 inch skirt, which would give more support, but there are more 1 1/4 Barlows. TV does not recommend their Big for diagonals, so using the info here https://www.cloudynights.com/topic/538194-how-do-you-measure-the-focal-length-of-a-barlow-lens/  and https://www.cloudynights.com/topic/357584-effect-of-barlow-mag-on-eye-relief/

[Louis D]

38 minutes ago, 25585 said:

My ep I want to extend er on is 13mm Nagler T1 smoothie. Native is 13mm er.

That eyepiece is known for having SAEP or kidney beaning.  Barlows are known to increase SAEP for many eyepiece types.  I'd be reluctant to try the combination.  YMMV.  Let us know how it works out for you.

[Stu]

6 hours ago, Louis D said:

That eyepiece is known for having SAEP or kidney beaning.  Barlows are known to increase SAEP for many eyepiece types.  I'd be reluctant to try the combination.  YMMV.  Let us know how it works out for you.

Had to look that one up Louis, hadn't heard it described like that before. Some useful info here, on Spherical Abberation of the Exit Pupil. Thanks Louis  

http://www.telescope-optics.net/eyepiece_aberration_2.htm

[Merlin66]

25585,

There's a much easier way to measure the focal length of a barlow (or any negative lens)....

Measure the diam of the barlow lens, draw a circle twice this diameter on a piece of card project an image of the sun through the barlow lens onto the card and vary the distance until the image fills the circle. The distance from the lens to the card is the focal length of the lens.

 

[25585]

13 hours ago, Louis D said:

That eyepiece is known for having SAEP or kidney beaning.  Barlows are known to increase SAEP for many eyepiece types.  I'd be reluctant to try the combination.  YMMV.  Let us know how it works out for you.

Maybe I'll scrap the idea. Its a good ep otherwise. I have an emotional attatchment to it otherwise I would have sold it while it was still worth something.

[Louis D]

1 hour ago, 25585 said:

Maybe I'll scrap the idea. Its a good ep otherwise. I have an emotional attatchment to it otherwise I would have sold it while it was still worth something.

Absolutely hang on to it if it brings back good memories of times gone by.  It's still a landmark eyepiece.  It's the one that started the wide field eyepiece revolution almost 40 years ago (1980 actually, man do I feel old).

Pick up a barlow for around $50.  There are many threads on SGL with recommendations about them.  They're useful to have in your kit.  There's no harm in trying the old Nagler in the barlow to see what happens.  Report back here if and when you do give it a try.

I thought the 13mm Nagler Type 1 had more than 13mm of eye relief.  It was the one advantage it had over the 12mm Type 2 which had 12mm of eye relief.

[25585]

It did have better eye relief than the 12, as I compared them side by side. My first wow! sighting was with the 13. The nebula in Orion's sword with an OIII filter, at just over x100 in my 12 dob, first Nagler moment!