Focal Length and Barlows

[Alexmar]

Hello again, i have some questions about the Focal length and how it work with barlows, question being, does the barlow make the image quality worse. even if it respects the limits of the telescope? 

Example, if i have a 80/200 telescope and a 80/1000 telescope, same piece of equipamente, only diference is the focal length, can i compensate the 200mm one with a 5x Barlow, them both would have 1000mm of focal length, would i have the same quality image between both telescopes? also assuming both are using the same eyepiece.

Also, barlows work in any type of telescope? (Refractors and Reflectors), or each one need a special barlow, i am in the proccess of buying some lens and barlow, so i want to know if i need look something more specifc. (i know that we have like 1.25 and 2" entries)

[wulfrun]

A Barlow doesn't actually change the telescopes FL...but as far as the eyepiece is concerned it makes the scope behave as though it had a longer FL. In other words, putting a 2x Barlow in a scope of 500mm gives the same result as no Barlow in a 1000mm (assuming you picked suitable scopes, since the focal ratio is also affected).

I don't think there's any optical reason to pick a Barlow based on reflector/refractor BUT there might be mechanical reasons (long Barlow hitting a diagonal's mirror, for instance).

If you avoid a cheapo Barlow, you shouldn't notice a degradation in image. Loss is inevitable due to extra glass but slight.

Generally, for visual use a 2-2.5x Barlow is likely to be the most useful. Higher powers are more often used for imaging than visual. Some Barlows have removable elements that can be screwed into an eyepiece's filter threads, which results in a lower multiplication (usually around 1.3-1.5x). Meaning you get a choice of multipliers in one unit. Be aware that some eyepiece/Barlow combinations won't work like this, for physical reasons.

Give us a budget and scope/eyepieces and someone will soon give you decent options & opinions.

Edited July 30, 2022 by wulfrun
added info

[Alexmar]

4 minutes ago, wulfrun said:

A Barlow doesn't actually change the telescopes FL...but as far as the eyepiece is concerned it makes the scope behave as though it had a longer FL. In other words, putting a 2x Barlow in a scope of 500mm gives the same result as no Barlow in a 1000mm (assuming you picked suitable scopes, since the focal ratio is also affected).

I don't think there's any optical reason to pick a Barlow based on reflector/refractor BUT there might be mechanical reasons (long Barlow hitting a diagonal's mirror, for instance).

Generally, for visual use a 2-2.5x Barlow is likely to be the most useful. Higher powers are more often used for imaging than visual. Some Barlows have removable elements that can be screwed into an eyepiecee's filter threads, which results in a lower multiplication (usually around 1.3-1.5x). Meaning you get a choice of multipliers in one unit. Be aware that some eyepiece/Barlow combinations won't work like this, for physical reasons.

Give us a budget and scope/eyepieces and someone will soon give you decent options & opinions.

I see, thanks. I wans't trying to get opnions for what to buy, since i had another topic talking about that, mainly want to understand the FL with Barlow. So a 200FL * 5 Barlow will give-me the same image as the 1000 native FL would give. Cool

 

First i bought a 70/400 Refractor with azimutal mount, got some good images, but that mount is a trick to use, so i decide to refund and get another one, since i like the view. So

 

I bought that one, 76mm/300mm, just want to know if the barlow would make the image worse, even if i respect the 150X maginification. I got this one for 2 reasons, Size and shipping time. If i dont like that one i wil get a 130mm or 150mm with 750FL with the same Donbsomething mount, and i also buying the eyepieces and barlow, because i can use in both of them anyway.

[wulfrun]

22 minutes ago, Alexmar said:

So a 200FL * 5 Barlow will give-me the same image as the 1000 native FL would give. Cool.

Correct...except that the focal ratio changes too, so the image is dimmer. That's why I said you'd need to pick suitable scopes to compare. You'd need a "fast" 200mm and  a "slow" 1000mm for the exact same view.

Suppose you take your 76/300 as the example, it's focal ratio is 300/76 = f/4,  approx (which is fast). Put in a 5x Barlow and you now have (in effect) a 76/1500, which equals f/20, approx (which is slow) and the image is going to be dim.

However, if you stick to the recommended max magnification, you should be OK.

For your example scope, by the way, the practical limitation is probably going to be the size and weight of the eyepiece + Barlow that the focuser can cope with. I have a slightly larger but similar scope (a 114/500mm) and I know a heavy Barlow and eyepiece would make it sag.

Edited July 30, 2022 by wulfrun
missed a word

[Ricochet]

A good quality barlow will not make the view through your hypothetical 80/200 telescope worse, but that does not mean that it will be as good as an 80/1000 telescope. In general a telescope with a faster focal ratio will have larger aberrations than a slower scope of the same design type, aperture, glass types etc. Once those aberrations from the primary are in the image, a barlow will not remove them. 

Similarly, each eyepiece will have aberrations and each eyepiece will perform better at longer telescope focal ratios. In this case you may find that adding a barlow and using longer focal length eyepieces is preferable to using short focal length eyepieces. For example if you use a generic 4mm plossl in an f4 telescope it will not cope with it well, and the image quality will be terrible at the edge of field due to eyepiece aberrations. If, however, you were to use a 5x barlow then the effective focal ratio the eyepiece sees is f20 so you can use a 20mm plossl and have the same magnification and fielding view as you would by using the 4mm plossl with no barlow. As the light cone is so much shallower at f20 the plossl will have no problem with it and eyepiece aberrations will be minimal so your image will have only primary aberrations. More modern eyepiece designs tend to use what can be thought of as a barlow built into the nose and so shorter eyepieces may be better than longer ones to begin with. In practice it would be very rare to use a 5x barlow visually, in fact even a 3x barlow will often be of no use in anything other than extremely fast telescopes like yours. 

It is also worth pointing out that the 2x aperture limit is an empirical estimate based on observing double stars with very slow achromatic refractors. For other targets and other telescopes it doesn't necessarily apply. I would guess that the celestron firstscope would be very unlikely to be able to reach this limit without the image suffering. 

[Alexmar]

2 hours ago, Ricochet said:

A good quality barlow will not make the view through your hypothetical 80/200 telescope worse, but that does not mean that it will be as good as an 80/1000 telescope. In general a telescope with a faster focal ratio will have larger aberrations than a slower scope of the same design type, aperture, glass types etc. Once those aberrations from the primary are in the image, a barlow will not remove them. 

Similarly, each eyepiece will have aberrations and each eyepiece will perform better at longer telescope focal ratios. In this case you may find that adding a barlow and using longer focal length eyepieces is preferable to using short focal length eyepieces. For example if you use a generic 4mm plossl in an f4 telescope it will not cope with it well, and the image quality will be terrible at the edge of field due to eyepiece aberrations. If, however, you were to use a 5x barlow then the effective focal ratio the eyepiece sees is f20 so you can use a 20mm plossl and have the same magnification and fielding view as you would by using the 4mm plossl with no barlow. As the light cone is so much shallower at f20 the plossl will have no problem with it and eyepiece aberrations will be minimal so your image will have only primary aberrations. More modern eyepiece designs tend to use what can be thought of as a barlow built into the nose and so shorter eyepieces may be better than longer ones to begin with. In practice it would be very rare to use a 5x barlow visually, in fact even a 3x barlow will often be of no use in anything other than extremely fast telescopes like yours. 

It is also worth pointing out that the 2x aperture limit is an empirical estimate based on observing double stars with very slow achromatic refractors. For other targets and other telescopes it doesn't necessarily apply. I would guess that the celestron firstscope would be very unlikely to be able to reach this limit without the image suffering. 

I see, i understand now. Now, since you guys was talking about eyepieces, this telescope will come with standard Hughes lens, which i know is not that good, but i didn't find any images comparations with Plossl vs Hughes, do you guys have some image? reason for, like i said, it comes with Hughes lens(6mm and 20mm), if the diference is big, i will buy thoses 6mm and 20mm in Plossl, if not i will buy some diference eyepieces, if you guys dont have images, i will appreciate some opinions.

 

Also, in my understanding, it would be better to get a 5X barlow with a 10mm eyepiece, to reach 150x, them use a 3x Barlow with  a 6mm eyepiece, is that right?

One more thing, does Barlows have types? such as Plossl or Hughes?

Edited July 30, 2022 by Alexmar

[Zeta Reticulan]

You might find this and this useful.

[Gabby76]

6 hours ago, Alexmar said:

I see, i understand now. Now, since you guys was talking about eyepieces, this telescope will come with standard Hughes lens, which i know is not that good, but i didn't find any images comparations with Plossl vs Hughes, do you guys have some image? reason for, like i said, it comes with Hughes lens(6mm and 20mm), if the diference is big, i will buy thoses 6mm and 20mm in Plossl, if not i will buy some diference eyepieces, if you guys dont have images, i will appreciate some opinions.

 

Also, in my understanding, it would be better to get a 5X barlow with a 10mm eyepiece, to reach 150x, them use a 3x Barlow with  a 6mm eyepiece, is that right?

One more thing, does Barlows have types? such as Plossl or Hughes?

Plossl are much better quality eyepieces than Huygens, no contest. 

Barlows are barlows and there are no differences, though some are called apo because they have a third lens they are not actually apos and only take more € out of your pocket.

Tele-extenders and tele-reducers are similar to barlows but are normally 4 lens and really not needed for visual observing. 

Your question of using a 5x barlow with a 10mm would give you a 2mm eyepiece focal length which in  99% of regular observing sessions would be empty/ useless magnification. Same with the 3x with 6mm. With a newtonian these powers would be useless as you would be seeing the secondary mirror as a black dot in your view. 

Once your exit pupil goes smaller than 0.5mm you will be looking at eye floaters and what looks like a firefly in your eyepiece. 

 

For visual observing you will likely find a 2x is more than enough once you start picking eyepiece focal lengths that work in your telescope. 

Edited July 31, 2022 by Gabby76

[Ricochet]

9 hours ago, Alexmar said:

Also, in my understanding, it would be better to get a 5X barlow with a 10mm eyepiece, to reach 150x, them use a 3x Barlow with  a 6mm eyepiece, is that right?

In the very specific example that you are going to use Plossl eyepieces then yes, a 10mm would be more comfortable to use due to the greater eye relief of longer Plossls. However, if you were going to use a more modern design, for instance BST Starguiders, which are a standard upgrade eyepiece, then all of the line is fine at f12 so a 3X barlow would be totally fine with any of them. In the Starguider line the 5/8/12 are fine with no barlow at f6, and I think the 5/8 might even be fine at f4 but I need to have another look through a similar system to be sure.

However, you need to get this idea of 150X magnification out of your head. For planets the empirical value is 1x - 1.2x aperture and for DSOs it is 0.5x aperture. On top of this the telescope you have has a spherical mirror instead of parabolic and at f4 the spherical aberration will blur the image which will lower the optimum magnification even further. I would not be surprised if you had to consider 50x as the maximum for this telescope.

[Alexmar]

Thanks for the informations that you guys provided, i bought a 6mm plossl eyepiece and i will try it when it arrives with the scope, i'will try with some high magnifications, if indead 50x will be the limit i will refund it and order my 150/750 scope.

Really appretiate yours help.

 

One more thing, i was considering buying this thing.

Although, it dons't say if it's a Plossl design anywere, does you guys know about that? also, do i lose quality with that, or would be better just buy separeted eyepieces

Edited July 31, 2022 by Alexmar

[Louis D]

Typical zoom eyepiece designs have two moving groups of lenses, one at the bottom and one in the middle.  The upper, final image forming lens group generally is more similar to one of Konig's designs than to a Plossl.

[Louis D]

1 hour ago, Alexmar said:

if indead 50x will be the limit i will refund it and order my 150/750 scope.

Those 76mm/300mm scopes are insanely difficult to sight onto a target due to their diminutive size and table top use.  I messed around with one at a star party that a fellow astro enthusiast had set out for the public to play with on a long, folding table.  He'd picked up a couple of them from thrift stores over the years for about $20 each.  I ended up lining it up on Jupiter by shooting from the hip since I couldn't get behind the scope to properly sight long the tube.  I wouldn't expect a newbie to be able to do that.  Once on Jupiter, I could see at least one band on it and the Galilean moons, but the view wasn't all that sharp even on axis.  I think it had a decent Plossl in the focuser, but I don't recall noting what it was.  The outer 50% of the field was noticeably blurry thanks to the f/4 spherical mirror and massive coma.  They're really intended as gifts for little kids who have low expectations.

[Zeta Reticulan]

I thought most zooms fundamentally had a moving Barlow element. One problem with zooms is the limited field of view at low magnification.

I recommend the Orion/SvBony 7-21mm. For the money it really rocks.

[Louis D]

22 hours ago, Zeta Reticulan said:

I thought most zooms fundamentally had a moving Barlow element. One problem with zooms is the limited field of view at low magnification.

That's the way varifocal eyepieces like the Speers-Waler 5-8mm and 8-12mm eyepieces work.  They have constant AFOV and are nowhere close to parfocal.

Zoom eyepieces have a Barlow-like group at the bottom that moves downward while a middle group moves upward to maintain some semblance of parfocality.  The downside of this approach is a non-constant AFOV.

[AstroMuni]

On 30/07/2022 at 18:24, Alexmar said:

Example, if i have a 80/200 telescope and a 80/1000 telescope, same piece of equipamente, only diference is the focal length, can i compensate the 200mm one with a 5x Barlow, them both would have 1000mm of focal length, would i have the same quality image between both telescopes? also assuming both are using the same eyepiece.

Others have answered your basic question. In simplistic terms the more the lenses/mirrors you add to your optical train the image is going to deteriorate, as each of these surfaces will not be perfect and will cause imperfections and also absorb light. And in astronomy, we are aiming to get the most photons from those distant sources. Hope this makes sense.

[Zeta Reticulan]

3 hours ago, Louis D said:

That's the way varifocal eyepieces like the Speers-Waler 5-8mm and 8-12mm eyepieces work.  They have constant AFOV and are nowhere close to parfocal.

Zoom eyepieces have a Barlow-like group at the bottom that moves downward while a middle group moves upward to maintain some semblance of parfocality.  The downside of this approach is a non-constant AFOV.

That explains a lot, thanks.