Barlow 1.25"v 2"

[slice of heaven]

Thanks Truff thats exactly why I've asked this ,Idont want that view.So I'm leaning heavily towards the 2"powermate for some quality ep's(my eyes only) and also the orion shortyplus for the 1.25"s for general family use.

Hi John, That post(above Truff's) really cleared the fog.
One Question though; how does the f/ratio change with the barlow?

Same specs both scopes 12" f5 .The old one has Park optics Pyrex mirror.No real difference in specs.Have plans to remount the older scope later,one dobs enough. As for 2"ep's that I'll want ,that is another dilemna. At the moment I use 20's<30's for hunting, 15 for general viewing & 6<9 for powering up. But add a wider f.o.v to the equation and this makes it so much harder to decide. With what I've read in this thread Im thinking of going for a 2x powermate & 15mm ep & then maybe a 40? to cover my most used magnifications. Im not looking to build a treasure chest of ep's(be nice though) just trying to get some quality ep's to cover most bases.


Geoff

[trufflehunter (Wayne Robinson)]

Thanks for that guys. Think I might have to have a look at the Shorty Plus! Just measured my generic barlow and it's only 22mm aperture!

[ausastronomer (John Bambury)]

[QUOTE]Originally posted by slice of heaven

Hi John,

That post(above Truff's) really cleared the fog.
One Question though; how does the f/ratio change with the barlow?


Hi Geoff,

Thats in fact how a barlow works, a barlow is a telenegative lens that increases the effective focal length of the light cone of the original telescope, thereby increasing the magnification of any given eyepiece by the amplification factor of the barlow. The magnification of a given eyepiece in a given telescope is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. The 2X barlow achieves the effective increase in focal length of the light cone by halving its angle of incidence.

Here is a good article explaining how a barlow works by Stephen Tonkin. There is also a good diagram showing the path of the original light cone and what happens after it goes through the barlow.

http://www.astunit.com/tutorials/barlow.htm

Here are a couple of other websites you could look at:

http://astro.isi.edu/notes/barlow.html
http://www.atmsite.org/contrib/Carlin/barlow/
http://www.astrosurf.com/legault/focal.html

Its also worth being aware that the amplification factor (magnification) of a barlow thats quoted (printed on its side) only holds when the barlow is used at a fixed position in the light path. For example a 2X barlow when used in conjunction with a diagonal and a refractor or SCT or MCT will give approximately 2X when used after the diagonal and approximately 3X when used before the diagonal. You can also increase the magnification of a barlow by using an extension tube which moves it further out in the light path. Something else to note is that the stated magnification of the barlow (eg 2X) is only an approximation and the 1 barlow could provide a range of magnifications between about 1.7 and 2.3 depending on the combination of telescope and eyepieces used with it.

There is a way to measure the exact magnification factor that an eyepiece and barlow is giving, I won't bore you with the details but if anyone wants to know out of curiosity to test their barlow let me know and I will tell you how to do it.

Well you wanted to know the time and I just told you how to build a clock, but I guess thats me


Clear Skies
John Bambury

[ausastronomer (John Bambury)]

[B]Thanks Truff thats exactly why I've asked this ,Idont want that view.So I'm leaning heavily towards the 2"powermate for some quality ep's(my eyes only) and also the orion shortyplus for the 1.25"s for general family use.

Same specs both scopes 12" f5 .The old one has Park optics Pyrex mirror.No real difference in specs.Have plans to remount the older scope later,one dobs enough. As for 2"ep's that I'll want ,that is another dilemna. At the moment I use 20's<30's for hunting, 15 for general viewing & 6<9 for powering up. But add a wider f.o.v to the equation and this makes it so much harder to decide. With what I've read in this thread Im thinking of going for a 2x powermate & 15mm ep & then maybe a 40? to cover my most used magnifications. Im not looking to build a treasure chest of ep's(be nice though) just trying to get some quality ep's to cover most bases.


Geoff,

I replied to this part of your post separately because they are separate issues.

The old scope if it has an old Parks mirror should have damn nice optics, provided all is as it should be, collimation good mirror cell and secondary etc. The old Parks mirrors are excellent.

Don't sell the Shorty Plus short and put it into the poor relations category compared to the Powermate, it is an outstanding barlow and 10 years ago rated with the best available. The powermates are marginally superior but the differences are subtle and it often takes an experienced eye and excellent seeing to even detect a difference.

The only problem I see with your choice of 2 different barrel size 2X barlows is that you are not maximising all available magnification options and there is a lot of overlap. The 2" powermate won't do a lot that the 1.25" Shorty plus can do in terms of 1.25" eyepieces. What are the 2" eyepieces that you own, or plan to buy that you wish to barlow in the 2" powermate ??

The option I took was to buy the 2" 1.6X Antares barlow and the 1.25" 2.5X Powermate. This basically gives me 3 eyepiece sets wheres your option still only gives you 2 eyepiece sets even though you own 2 barlows and the native eyepieces.

Clear Skies
John Bambury

[slice of heaven]

Hi John,
Nice post with nice links...Glad I asked you the time.

It's good to know the theory behind things you dont understand.Will be reading those links in more detail before I ask you some questions on a few points there.(heads a bit foggy, I've been at the hospital nearly all day with my lad who had his tonsils out)

Good point about the having the same Power on 2 different sized barlows. That wasn't a smart choice I made. I was going down the wrong track ,a pretty narrow one at that.
With the 2 different powered barlows / 2 different sizes this would give me :
2 ep sets in the old scope(1.25"focusser)
Barlowed and not barlowed

3 ep sets in the new scope(2 "focusser)
?x 1.25"barlow(adapted)
??x 2" barlow
not barlowed

I dont have any 2" ep's @ the moment simply because the old scope has a 1.25" focusser. Now we'll have the new scope with 2"focusser I have the option of using this size ep. Hopefully with extra benefits e.g. wider fov, easier on the eye.
After considering comments here on the SHORTY PLUS and a few searches elsewhere I ordered one today. It seems to be a decent piece in the price range I was looking at and I can use it in both scopes. So now I have to choose a powermate size to compliment the 2x shorty plus. Hope I haven't stuck my foot too far in the muck. ......More advice please.

The old scope seems to be great. I' ve never compared it to another one so next week will be my first chance to. I enjoy using it... that excludes carrying it...and the large aperture gives the kids a decent view even at twilight. The mirror and certificate of optical precision are hand signed/engraved by George Clements, master optician.


Thanks again , Geoff

[ausastronomer (John Bambury)]

Geoff,

On the basis that one of your scopes only has a 1.25" focuser I would just stay with the shorty plus and be done with it.

Clear Skies
John B

[slice of heaven]

Thanks John
. Music to my ears. Just had a peek in the new scope & I cant justify purchasing 2" equipment and only using it in the gs.
If I change to 2" on the Parks then I'll consider it. But that means more $$$ for a top focusser on top of the ep's & barlow.
Now I can leisurely just add some nice 1.25s to the shorty+ for now.
I've enjoyed the chat John Thanks Geoff