Barlow , coma reducer ?

[cotterless45]

I remembered an old thread on cloudy nights or similar and tried out a x2 Barlow with a 32mm ep. This was in my sub f5 10" Lightbridge. I don't normally use 40 or 32 mm eps. It was most pleasing to see a near flat field as compared to the normal looking into a goldfish bowl. This worked very well on M27 with a UHC filter. It was great to see it sitting in a star field.

I often wondered why sub £70 eBay Newts with spherical mirrors come with built in Barlows. 

Well it worked for me, without spending money on uber eps. I might try unscrewing the end and using it , this normally gives X1.6 magnification instead of x2,

Nick.

[John]

I guess it would reduce the visual impact of coma and astigmatism. Not sure if a barlow affects field curvature though ?

[Mark at Beaufort]

Nick I have had the same experience using a barlow especially in binoviewers. I know the APM coma correcting barlow mention this fact in their advert. http://www.apm-telescopes.de/en/optical-accessories/barlow-lenses/1-25-barlow-lenses/apm-comacorrected-1-1-4-ed-barlow-2-7-x

 

[Dave In Vermont]

I'm glad I'm not the only one to 'discover' the merits of larger focal-point EP's like 32mm & 40mm, etc. Our 'More Power!' cultures do us a dis-service.

I'm planning to embark on a Barlow-venture with my larger FP EP's. I just got in a GSO 2" 2X Barlow towards this end. Lordy! The thing is BIG! And very well cobbled together, too. And I was delighted to see the Barlow-lens was threaded and unscrewed easily. Thus making it 2 Barlows-in-one. It also has a 2" to 1.25" adapter with it, compression-bands all around.

Nick - I keep reading that just using the Barlow-lens gives you 1.6X. Then, the next article, says 1.5X. Any paper/link you have which sways this discrepancy one way or the other?

Enjoy the venture -

Dave

[Mark at Beaufort]

7 minutes ago, Dave In Vermont said:

I keep reading that just using the Barlow-lens gives you 1.6X. Then, the next article, says 1.5X. Any paper/link you have which sways this discrepancy one way or the other?

Dave

Dave I seen this as well - sometimes 1.6x then 1.5x. There is a company in the UK called Astro Engineering and they produce this item. As you see it states around 1.5x!!!

[Dave In Vermont]

Thanks, Mark. But I do note the word "...around..." in it.

I guess it's either time for the stop-watch and a green laser, or throw in the towel! Ah well.

'Ta,

Dave

[John]

The amplification given will depend on the positioning of the barlow lens elements in the optical train which in turn would be affected by the physical size and design of the lens housing and the length of the eyepiece / diagonal barrel. This probably accounts for variations such as 1.6x or 1.5x. By using spacers you can deliberately alter this yourself. This is the principle that the Baader Fine Tuning rings use.

Not that the original figures provided by manufacturers are that accurate - the Celestron Ultima 2x barlow was well known to deliver 2.2x in reality.

 

[YKSE]

There're deviations of barlow effect as specified, just like eyepieces have deviations. Also, even if a barlow is exactly as specified, it still has a basic assumption, i.e. eyepiece's foccal plan should be exactly at the eyepiece shoulder, which many eyepieces don't. If a eyepiece has focal plane higher up (closer to eyepiece), it will have higher barlow efftect.