Barlow reducing visibility?

[MBSteve]

Hi fellow gazers,

Over the weekend I set out with the intention of seeing the E and F stars in the Trapezium. I have a 10" dob with a 1250 mm focal length, but up to now I haven't been able to see them. The reason being that I didn't really know what to look for.

I was in my backyard in the suburbs of a big city, but the sky was clear and seeing conditions were pretty good, considering.

With some research I now knew what I was looking for. I was able to just make out the E and F with my 30mm stock plossl and with my standard GSO 9mm plossl. I was pretty happy with this effort up to this point 

Next I used a GSO 2x barlow with the 9mm. This is now where the trouble started. With this setup I was not able to see the E and F stars at all, even though I could see it without the barlow!

So the question is, why? I have narrowed it down to the following and would like some help on this please:

1. The barlow optics are not good enough. I am not so sure if this is the case because, as I understand it, barlows are generally pretty standardized and reliable.
2. The fact that the light had to pass through additional lenses dimmed the image sufficiently to make the light from these stars undetectable. I guess this is possible, but the E and F stars weren't that dim without the barlow.
3. I did have a harder time focusing with the barlow than without. I assume this is because the greater zoom narrows the range of a good focus a lot. Because the image moved out of the eye piece view pretty quickly, I wasn't able to focus the image in time before I had to nudge the telescope to track the object. I repeated this exercise until I got the best focus I could, but still nothing.
4. There is some inherent limitation to either optics or light that I am not recognizing.

I think item (2) is the most likely reason, but I guess (1) is also a possibility.

I do not think the additional zoom would have affected the brightness. I understand that more zoom means less light, but that is only because we're disregarding the light captured by the telescope that is not in the immediate field of view of the eye piece. If we can see a given object at one power, we should be see it at a greater power as well, but not vice versa.

I am actually interested in the science of the issue I'm having and any help will be very much appreciated.

Thanks

Paul

[Earl]

using a barlow to get effectivly 4.5mm is going to cause problems especially at f5 unless you have top quality eypieces and barlows.

[John]

I can't really help you with the science Paul but I have found that seeing E & F Trapezium regularly seems to be helped by finding the "goldilocks" eyepiece for the scope. In my case I find 8mm hits the mark with my 12" f/5.3 dob (199x) but with my ED120 refractor 6mm (150x) is the one that does the trick best. I don't use barlows.

Using more or less magnification often seems to cause the E & F stars to become a lot more challenging for some reason - I don't know exactly why but I've found it to be the case very often.

While not terribly faint stars themselves (around 10th and 11th mag) there are factors that add to the challenge I feel

- The nebulosity of M42 seems to obscure them, possibly because it brightens the background you are viewing them against ?

- They are quite close to brighter stars A and C Trapezium (around 4-5 arc secs). F in particular is close to C which is a much brighter star and the glare from C can make F hard to pick out.

Here is some useful data on these stars:

 

[mikeDnight]

I think its the seeing that's at fault!

The E & F stars are stubborn and strangely difficult to see despite being relatively bright. A sharp medium power will often show them but increasing the power washes them out. When that happens its usually because the seeing isnt good enough for high power stellar observation. 

Mike

[newbie alert]

Your seeing conditions will limit your mag on your eyepeice..you was effectivley using a 4.5mm eyepeice..only once have i been able to see with a 7mm.. i tend to try and steer clear of barlows..

[MBSteve]

Thank you everyone for the responses.

I have a 4mm plossl as well, but I don't tend to take it out much. There are some good sky conditions coming up over the next week or so. I'll go for a drive to a dark site and compare the barlowed 9mm with the 4mm. If I can see E and F with the 4mm and not with the barlowed 9mm, I'll know it's the barlow causing the issue (whether bad optics or just because of more lenses).

If both show nothing, and I can see it through the 9mm un-barlowed, I'll blame the seeing.

My TV Delos 17mm is arriving soon. It will be interesting to see it's effect on the trapezium with and without the barlow.

Thanks again for the help.

Paul

[John]

The Delos might help (I have the 17.3 and it's excellent !). My favourite E&F eyepieces are the 8mm and 6mm Ethos. I'm sure Delos of the same focal lengths would do the job too though

[brantuk]

2 hours ago, mikeDnight said:

I think its the seeing that's at fault!

I couldn't agree more - amongst other factors you need to consider that a 2x barlow doubles magnification of the object, but it also magnifys the crud in the sky (heat waves, moisture, particles of dust, whispy cloud, etc). So the view can get very grainy if you magnify too much. Finding the right combination of lenses for the right conditions on the night is the key to successful views.

[MBSteve]

I'm really looking forward to the Delos. It cost a damn arm leg and kidney, but luckily I have a spare of each... 

Thank you everyone for your insights.

[Dave In Vermont]

I see I've been lax here, and not trotted out my shameless plug on adding some 'green-letters' to the ol' eyepiece-case. So.....

 

Poor quality Barlows' can be the worst enemies to the cause for using a Barlow - ever again! A Barlow adds more glass in the optical-pathway from the primary mirror or lens. A lesser Barlow-Lens not only can dim the target and bring a loss of clarity - it will magnify any imperfections the eyepiece may have, or the lens, filter, mirror, etc. So you want a Barlow that becomes as close to invisible itself. Many would-be Barlow lovers end up buying Barlow's over & over again - looking for 'the one' that has the least problems, while resisting buying the 'top-priced' Barlow. The TeleVue® Barlows. And a few others are now right up there, too, but I'll let other folks talk about these. I have personal experience with the TV's so I'll stick to these here.

The result of buying one mid-cost Barlow, selling it, and trying yet another is either giving-up altogether. Or having a drawer full of Barlows - collecting dust. Untouched. But after all the disappointment, buying the TeleVue 2X or 3X seemed throwing more good money - more than the dust-collecting one's cost - on finding a good Barlow. If it even existed. Until you either get lucky at a star-party and try a TeleVue® Barlow, or inherit some extra money and buy one. And then it all changes.....

The TeleVue seems to have vanished - just adding pure magnification without dimming or a loss of resolution. And the price really wasn't much more money than the mid-line (dusty) Barlows. And if you'd started with these, you wouldn't have bought another Barlow because you started at the top-end. The only direction from there is down. I usually hesitate to suggest TeleVue® to folks - especially recent converts to astronomy and observing as the high-costs of these can appear 'elitist' - but in the case of Barlow-lenses I make the exception. A little 'green-lettering' in the eyepiece-case is okay to me. And if you start at the top, what's that 'only direction' thing...? Oh yeah - down. And the price of these isn't really that much more. Look for sales.

Due to their becomming invisible (almost), I've never a reason not to throw it into my optical-train. And the TV 'PowerMates' are even better in most observer's opinion. But I won't go there now. Those aren't a true Barlow.

I hope this saves you some money, and increases your fun!

Dave

 

image by Mak the Night - the local 'Barlow-Nut'

[SonnyE]

Dave, Is a Barlow-Nut anything like a cola-nut, or uncola nuts?

https://youtu.be/AXmc7DG4uu8

Ducks... Runs...

 

[Dave In Vermont]

<Dave throws a blueberry-pie at SonnyE & gets him.>

- Sorry - I honestly couldn't resist the urge. -

Excuse me while I go play with my filter-collection. But just to clarify 'Nut' in the context I was using it within, a form of Obsessive-Compulsive Disorder. With Barlows regards Mak (a good friend of he).

Dave - a Filter-Nut

 

 

[MBSteve]

@Dave In Vermont I have to be honest, circa $120 on Amazon for one of these 'green lettered' toys does not seem too outrages or elitist. Especially considering how much the Delos 17.3mm cost me

I'll be adding one of these to my wishlist. 

Thanks for the help

[SonnyE]

10 hours ago, Dave In Vermont said:

<Dave throws a blueberry-pie at SonnyE & gets him.>

- Sorry - I honestly couldn't resist the urge. -

Excuse me while I go play with my filter-collection. But just to clarify 'Nut' in the context I was using it within, a form of Obsessive-Compulsive Disorder. With Barlows regards Mak (a good friend of he).

Dave - a Filter-Nut

 

 

I liken myself to a common ordinary back yard nut. Garden variety.

This Blueberry Pie is good, even with the lint in it!

Not a gatherer of too many things, but a procurer of specialized things I want.

2 Barlow's, maybe a dozen filters, and a Partridge in a Pear Tree.

[glowjet]

As has been said, the seeing conditions may have been a contributory cause of the problem. However, it is quoted that a poor Barlow is better off used as a door stop. Optical perfectionists, such as Tele-Vue have laid down the ground rules concerning the use of optical magnifiers, that they should neither add anything to, or detract from the field of view from the eye piece in use. In other words they should be an invisible compliment in the optical train.

Bearing the foregoing in mind, it should be possible for you to check your Barlows optical quality, when good seeing condition allow. It may seem excessive to pay the price for optical quality, such as Tele-Vue, but many will tell you that for the results, it is; in the long run, worth the outlay

 

[Stu]

On 18/12/2016 at 20:34, MBSteve said:

I do not think the additional zoom would have affected the brightness. I understand that more zoom means less light, but that is only because we're disregarding the light captured by the telescope that is not in the immediate field of view of the eye piece. If we can see a given object at one power, we should be see it at a greater power as well, but not vice versa.

Much good advice has been given already. I will only add that this particular quote should be correct, but is in fact not!

You would naturally think that applying more magnification to an object would make it easier to see, or at least that it would be visible at all higher mags. That just doesn't work in practise. Often with these faints stars you need to get just the right mag to optimise the contrast and make them visible. I don't know the science behind it but I know it works!

As an example. A while ago I was observing Saturn with a 4" refractor. At lower powers I could see 5 of the moons surrounding it. When I upped the power they disappeared and another moon further out appeared. This is a similar example to the E & F stars as they are faint point sources next to brighter objects, it is about maximising the contrast, and minimising the light scatter. All fascinating stuff!

[Stu]

Here is my original post:

 

[MBSteve]

9 hours ago, Stu said:

Often with these faints stars you need to get just the right mag to optimise the contrast and make them visible.

I would have thought conventionally that higher mags would automatically mean better contrasts since we're capturing less background light, meaning darker backgrounds overall relative to the object we're observing.

I know higher mags also magnify the atmospheric conditions. I wonder, though, if this is all proportional. For example, when doubling the magnification, the object is double in size, but is the atmospheric conditions also double the magnification, or is the conditions, say, 2.1x magnified?

If this is not linear/proportional, then I can see why a sweet spot exists between increased contrast and decreased seeing with more power.

In my beginner mind, the above would be my hypothesis as to why I could not see E and F at higher mags.

Of course, it could still be the barlow. Time, and experience, will tell. Last night was cloudless, but seeing was poor and I could not see E and F under any magnification.

[Stu]

17 minutes ago, MBSteve said:

I would have thought conventionally that higher mags would automatically mean better contrasts since we're capturing less background light, meaning darker backgrounds overall relative to the object we're observing.

I know higher mags also magnify the atmospheric conditions. I wonder, though, if this is all proportional. For example, when doubling the magnification, the object is double in size, but is the atmospheric conditions also double the magnification, or is the conditions, say, 2.1x magnified?

If this is not linear/proportional, then I can see why a sweet spot exists between increased contrast and decreased seeing with more power.

In my beginner mind, the above would be my hypothesis as to why I could not see E and F at higher mags.

Of course, it could still be the barlow. Time, and experience, will tell. Last night was cloudless, but seeing was poor and I could not see E and F under any magnification.

I did say that I didn't understand the science behind it

Basically the point source should not increase in size but the background brightness dims as it is spread out more. I guess though that the star is actually an airy disk rather than a point source so perhaps this is a factor?

As said, I don't know but perhaps someone else does.

[John]

I'm sure I don't understand the science but I have found from experience what works for me on such targets and what does not which is what I posted earlier in the thread

When I try and understand "why" on some of this stuff my head starts to hurt so I go and look through a scope again. Practical experimenting is good fun !

 

[Stu]

9 minutes ago, John said:

I'm sure I don't understand the science but I have found from experience what works for me on such targets and what does not which is what I posted earlier in the thread

When I try and understand "why" on some of this stuff my head starts to hurt so I go and look through a scope again. Practical experimenting is good fun !

 

Good job I didn't try to bring exit pupils into it then John  

[LukeSkywatcher]

There's probably other contributing factors, but in regards to barlows:

There are some good ones and some bad ones. A good one will not/should not hamper the view. A bad one most likely will. Most people when using a scope for the 1st time think that a 2x-3x barlow is an essential piece of kit to have. It really isnt. It can be substituted by buying an eyepiece which will offer you the same magnification.

Example:

A 16mm eyepiece + 2x barlow = 8mm

OR (without a 2x barlow) 

An 8mm eyepiece

Introducing a barlow into the optics is putting more glass in between the scope and your eye. A good barlow should not have any ill effect. Most visual observers (i think) learn that the less glass between the scope and your eye, the better....so dont use a barlow.

It really is a personal choice. Ive used 2-3 different 2x barlows over the yrs and the best i have used (that was within what i was willing to pay for) was a TAL 2x. They have been discontinued now (heavens only knows why.....they could have whacked the price up by 100% and they would still sell like hot cakes).

You get what you pay for, but in the case of the TAL barlow.........you got more than you paid for.

 

[SonnyE]

If you'd like to see abominations, do what I've done...

Stack your Barlow's.

The image being viewed does lose as the stack climbs, but it did work for me. My first summer I was showing the telescope to the Step-Son and we got onto Saturn... (Doesn't everybody?) Then I stacked in the 2X Shorty Barlow. Humm, better. Darker, but bigger anyway. So we tried the 3X Barlow, that's more like it! Darker still, but we could see Saturn better. So why not? BAM! Stacked the two Barlow's. Darkest yet, but the scene before our wondering eyes was a live view of Saturn, including the atmospheric hash mixed in.

I got a kick out of the wife's excitement as she ran into the house to bring the Daughter-In-Law out for a peer through the telescope.

One could quibble about clarity. One could surmise about the darkening interference. But the excitement of folks having their first ever live view of things never seen before? Priceless!

And every time I see new objects, or old friends from last year, I can back away and look up, and realize how much I've missed in my life.

Great view, or not so great view, I find myself drawn to look up.

[glowjet]

"Stack your Barlow's. " Wanting to get that bit closer, reminded me of one Astronomers good wife who told her other half, "if you want things to be a bit larger, then take the scope down to the bottom of the garden and get closer to what you are looking at"

 

[Mr Spock]

I never use Barlows even though I have three...

I prefer the view without all the extra optics in the way - always seems much a cleaner image with a higher powered eyepiece.