How much is image quality affected by a Barlow?

[dannybgoode]

As per the title really - seems the value way to extend ones eyepiece collection is via a Barlow or two but how much is image quality affected by them?

I appreciate of course that it would somewhat depend on the quality of the Barlow but if I were spending say £60-£120 per EP then it does not seem unreasonable to spend the top end of that on a good Barlow if it means I can buy fewer EP's.

With that in mind also - what are considered to be good quality Barlows?

Thanks all

[Crossway]

I   was interested to see your  post,  dannybgood,   because  I had just been about to pose the same basic  query and hoping that  someone who knows would be in touch with some advice. 

My Astromaster kit comprises  two EPs ....a 20 mm and a 10mm.  The 20 mm gives excellent  sharp views and I am happy with it.  The 10 mm on the other hand is disappointing  and appears to give a sort of "furry" image  for want of a better description.   I put this down perhaps to the limit of quality which Celestron obviously would take into consideration  with their cheaper scopes. 

Therefore, rather than lash out for a  better 10mm I  wondered if a  2X Barlow used with my 20mm would  be a better option. It  would give the same Mag. as the 10mm after all.  By necessity I work at budget levels !!..... and therefore interested that Maplins are offering the Celestron

"T"  ring Adaptor plus a 2X  Barlow for just under £30. 

Hoping that we hear something!

Crossway

[NGC 1502]

There are some really iffy barlows around that give poor results.

I'm definitely no equipment snob, but my early model TeleVue 2x barlow adds nothing but magnification.  As long as it's used so as not to push the power too high, it works great.

In fact, it can actually improve an eyepiece - I have a cheap 'n cheerful Antares 14mm wide angle eyepiece that gives fuzzy views towards the edge of field with my F 4.8 Dob, but when I combine it with the TV barlow, it makes a good 7mm equivalent eyepiece with much improved performance.

A barlow has another advantage - the eyerelief of the original eyepiece is maintained or improved.

There may well be other good barlows, but if you want a very good one, the TV is well worth paying extra for ( I was fortunate to get a second hand one ).

Many premium eyepieces have what is effectively a built in barlow within the chrome barrel section, that speaks volumes I think.

Regards, Ed.

[John]

As an alternative to a good quality barlow it's also worth considering a Tele Vue Powermate or the similar tele extenders by Meade and Explore Scientific. They are more expensive than a barlow but seem to have no negative impact on the image quality or eye relief and just add the magnification.

They have more glass elements in them but modern coatings and glass types have done wonders for making glass seem to "get out of the way". 

[AlexB67]

There are some really iffy barlows around that give poor results.

A barlow has another advantage - the eyerelief of the original eyepiece is maintained or improved.

Regards, Ed.

I don't always see this as advantage, for plossls and orthos in short focal length it can be a nice benefit. On the other hand eyepieces that already have a long eye relief like 20mm and longer focal lengths it can actually push out the eye relief too far, which I find a tad annoying. I notice it slightly with my pentax XL 10.5 where I find myself having to back away slightly with the eyecup is as far out as possible, though useable. Barlowing  long focal length eyepieces of say 15mm and over with long eyerelief I would imagine this would get too much for comfort perhaps.  Shorty barlows the push on eyerelief  will be even more pronounced.

If I were in the market for a 2x 1.25 inch barlow in that sort of 100 pounds range for barlowing wider angle long eye relief eyepieces, personally I would strongly consider the ES scientific 2x focal extender, perhaps not quite powermate quality, but from what  I read but does a very decent job by all accounts of getting out of the way and providing good optical correction, it can currently be got from sky's the limit for < £100.   

[Sandraj]

I bought a cheap Seben barlow when I first got my scope but it was useless with the 25mm that came with the kit and I I was ready to bung it, later I bought a 32mm TV Plossl and its brilliant with that which surprised me.

[NGC 1502]

I don't always see this as advantage, for plossls and orthos in short focal length it can be a nice benefit. On the other hand eyepieces that already have a long eye relief like 20mm and longer focal lengths it can actually push out the eye relief too far, which I find a tad annoying. I notice it slightly with my pentax XL 10.5 where I find myself having to back away slightly with the eyecup is as far out as possible, though useable. Barlowing  long focal length eyepieces of say 15mm and over with long eyerelief I would imagine this would get too much for comfort perhaps.  Shorty barlows the push on eyerelief  will be even more pronounced.

If I were in the market for a 2x 1.25 inch barlow in that sort of 100 pounds range for barlowing wider angle long eye relief eyepieces, personally I would strongly consider the ES scientific 2x focal extender, perhaps not quite powermate quality, but from what  I read but does a very decent job by all accounts of getting out of the way and providing good optical correction, it can currently be got from sky's the limit for < £100.   

Fully agree, with an EP that already has long ER, the barlow can make it hard to place your eye in the right spot, another possible issue is vignetting towards the edge of field. The Powermate mentioned in post # 4 avoids those issues ( or so I've read in lots of reviews ).

Regards, Ed.