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Thread: Barlow Lens Advice

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    Default Barlow Lens Advice



    I haven't used my telescope in forever but lately I've been thinking it's about time to rectify that. Not long after I purchased my scope way back when I also bought two very nice eyepieces to supplement the ones that Celestron provided, but I never got around to buying a Barlow.

    While researching them to help decide which brand I should get I learned something new...that if you put a Barlow lens in front of the diagonal you effectively increase its magnification by 50%, so a 2x Barlow will essentially act like a 3x one. For those of you who already knew about this trick and have employed it, I was wondering what price can one expect to pay by doing this? One guy on another forum wrote that you may experience some vignetting, but other than that I really haven't seen any serious drawbacks to this approach discussed.

    I was leaning toward getting a 3x Barlow, but if this technique really does work fairly well then I will probably just go with a 2x and still have the 3x capability when seeing conditions allow just by simply moving the lens forward. Theoretically a 3x Barlow would give you 4.5x capability if placed ahead of the diagonal, but that might be pushing it even with the extremely clear, dark sky and superb viewing we usually have here at +7000 feet in Flagstaff.

    Thanks in advance for any experienced guidance. This is a terrific forum and I've already learned a lot just by poking around and randomly reading posts. I probably wouldn't have signed up to become a member if they didn't cut off my reading privileges once I had exceeded the limit...a very sneaky but highly effective policy!
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    Default Re: Barlow Lens Advice

    Hi Cornell,

    Welcome to the forum.

    This technique comes up around every 2-3 years and is fun to try. The barlow lens' mag factor is based on the distance from the barlow lens to field stop of the eyepiece (EP). So when a barlow says 2X that is based on the EP being inserted into the barrel of the barlow and giving a set and repeatable distance between barlow lens and the EP field stop.

    Now if you put the barlow in front of the diagonal you have increased the distance between the barlow lens and field stop of the EP. To say a 2x barlow turns into a 3x barlow is only a guess (a WAG really). What you do know is that since the distance has increased then the mag factor is something >2x. How much bigger, depends on the distance.

    The downside to doing this is that you also magnify any abnormalities, distortions, imperfections in your optical train.

    But hey, this is a fun experiment to try. Go ahead and do it and report what you observed.

    Cheers,
    JT
    Last edited by jaetea; 12-01-2016 at 03:12 AM.
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    Default Re: Barlow Lens Advice

    Also if you told us what equipment you currently have (scope, EPs, etc) we can give a much better recommendation for which barlows you should consider.

    JT
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    Default Re: Barlow Lens Advice

    If the conditions are favorable it works reasonably well viewing the Moon and occasionally it will work on planets. Not something I do on a regular basis though it is a nice trick if the conditions warrant it.
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    Default Re: Barlow Lens Advice

    This is a good graph from the Tele Vue Barlow page about how distance from Barlow to EP can increase the overall magnification factor.

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    Default Re: Barlow Lens Advice

    Hi jaetea,

    My scope is a Powerstar II purchased circa 1988 back when I lived very close to the Celestron plant in Torrance. Their Powerstars were basically a Super C8 Plus with the added ability to run the Byers worm drive off of DC power. I've inspected the scope and it looks to be in superb condition thanks to its lack of use and the fact that it has spent much of its time in the carrying case in a low humidity environment. The Starbright coating on the primary mirror looks real good to my eye. Haven't checked the collimation yet...I need to expand my knowledge base before assessing that and attempting any tweaks if necessary. I'll probably clean the corrector plate and EPs when I get comfortable enough to do that but IMO even they aren't all that bad.

    The eyepieces that came with the scope were a 26mm Plossl and a 7mm Ortho...these are both the Circle-V lens made back when Celestron was teamed up with Vixen. I'm certainly no expert but they are most likely marginally adequate if not mediocre EPs. I bought 2 additional eyepieces around that time based on the advice of a TRW work buddy who knew a lot about telescopes. One is a Parks Gold Series 35mm Plossl...

    Parks Gold Series Ocular 35mm

    The other is a TeleVue 12mm Nagler Type 2 which is an optical beast that ways over a pound...

    TeleVue Nagler Type 2 12mm

    Yikes, I had forgotten how much I paid for those lenses at the time. At any rate, researching them they appear to be highly regarded so it seems that my workmate did not steer me wrong.

    Any suggestions regarding Barlows or even other EPs to consider would be much appreciated. Right now I'm looking at the Celestron X-Cel LX 2x Barlow at around 70 bucks but I'm wide open for other recommendations to look at. I've always wanted to take a crack at a little astrophotograhy so I'm also thinking about picking one of the ZWO CMOS cameras a lot of people seem to be raving about.

    As I said, the seeing here in and around Flagstaff is generally spectacular. Our house is a little above 6800' and the surrounding sky is fairly dark. If I go a few miles north I can easily get to 9500' at the Arizona Snowbowl...and if I go a few miles south I can set up at Anderson Mesa which is at 7200' and has dark skies that rival the best in the world...

    The Brightness of the Night Sky

    Lowell Observatory and the US Navy both have equipment on the mesa, probably for a good reason. If any given scope can be pushed to its limit, this is definitely one area of the country to try it.

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    Default Re: Barlow Lens Advice

    Thanks for that graph, Frosty. One good chart deserves a good map in return...

    DarkSiteFinder.com - Light Pollution Map

    Pretty striking how the US is pretty much split right down the middle...in more ways than one.

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    Default Re: Barlow Lens Advice

    Hi Cornell,

    With the EPs in your current arsenal I don't think you need a barlow. I would recommend you fill the gap between 26 and 12 with an 18mm EP. For less than $100 I recommend the Celestron X-Cel LX 18mm and for around $200, I highly recommend the Explore Scientific (ES) 18mm 82° AFOV. With this single EP addition you now pretty much have the full range of magnification best suited for your scope available to you. No barlow needed.

    BTW, I own the Celestron X-Cel LX 2x shorty barlow and it performs quite well.

    Look on the "astro-classified" sites for a gently used ASI120MC. This camera will be inexpensive and will take amazing planetary images. Plus it can also be used as a guide camera should you ever need to go there.

    Cheers,
    JT
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    Default Re: Barlow Lens Advice

    My 3X TeleVue and ES24mm are the main tools I use, love it, great FOV compared to using my 11mm, just end up loosing about an "f" stop
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    Default Re: Barlow Lens Advice

    A couple of bits:

    I have done that some in the past and ran into a couple problems:

    1) On some scopes, with the move of the Barlow plus its change in power, there was enough change in the actual focal position that I ran out of focuser. You will know if that is an issue for you the first time you try it.

    2) I found I needed to put the Barlow before the diagonal at low power to start with and then work up to the magnification I wanted. If I tried to make the change from 2X to 3X instead of 1X to 3X at lower power, I would loose the object a lot and have to start all over.

    3) A two inch extension tube can do about the same thing without having to removed the diagonal. Just stick the extension tube into the Barlow and then insert the eyepiece.

    Eventually, I just went to multiple Barlows. It was just easier that way.

    Right now, while using a really short f-ratio scope, I am actually using a 4X Barlow. When I get down to a 6mm eyepiece by itself and need to go lower, I put in the 4X and go back to the 25mm eyepiece and start back down the chain of eyepieces until I get where I need to go.
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