How high a magnification can I go and still focus w barlow?

Ask your fellow astronomers to solve your telescope queries

How high a magnification can I go and still focus w barlow?

Postby newmodelarmytrooper » Mon Apr 25, 2016 6:26 pm

Hi

I am now proud owner of a Skyliner 200p dobbie. Before I buy extra eyepiece \barlow etc I tried out my old SR4mm lens on the new dobbie. I found I was able to focus it successfully which is something I could not do on the old scope the SR4 came with. The picture was poor as the lens is a cheapo though.

It got me thinking however. If i bought say a 2.5 barlow and a decent plossl 5 or 6mm lens, would I still be able to focus the new lens on the dobbie? The scope is 8 inch mirror with 1200 focal length ie f6 or so. Does barlow interfere with the focusing ability of a lens?

Thanks in advance.
newmodelarmytrooper
 
Posts: 2
Joined: Mon Apr 25, 2016 6:12 pm

Re: How high a magnification can I go and still focus w barl

Postby Aratus » Mon Apr 25, 2016 7:04 pm

I'm not sure if you would notice if it was in focus or not. :o Let me explain . . .

1200/5 gives a magnification of 240x. You then multiply that by 2.5 and you get a theoretical magnification of 600x. I say 'theoretical' because the image will be so dim you would have great difficulty in seeing it. An 8" telescope might have a top magnification of 50 x 8 or 400x on a very good night. More like 40 x 8 or 320x on reasonably good night. 600x in the UK isn't going to work even on a much bigger telescope. :(

Your 4mm eyepiece alone would give you 1200/4 or 300x, which on the face of it seems ok, but Dobsonians are pretty critical when it comes to higher magnifications. (That might be one of the reasons you had problems with the eyepiece you have) A top notch eyepiece might give you good results, but a lot will depend on the quality of the rest of the telescope and its collimation. I would consider borrowing a high quality eyepiece and trying it out before buying it.

Personally I find a 4mm eyepiece very difficult to squint through. I find an 8mm with a x2 barlow much easier. (Old guy! :( )
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus
 
Posts: 809
Joined: Tue Jan 12, 2016 2:17 pm
Location: East Lincolnshire

Re: How high a magnification can I go and still focus w barl

Postby newmodelarmytrooper » Mon Apr 25, 2016 7:13 pm

Thanks

This is very helpful. One more question. Is a 4mm plossl image really likely to be that much better that the cheapo SR4mm image? My SR4mm is a 0.965" that I fudged into my 1.25" mount. Pleased with the results given that tho.
newmodelarmytrooper
 
Posts: 2
Joined: Mon Apr 25, 2016 6:12 pm

Re: How high a magnification can I go and still focus w barl

Postby Aratus » Mon Apr 25, 2016 8:04 pm

A genuine good quality 1.25" eyepiece will work better than a 0.965" eyepiece on your Dobsonian.

A plossl is potentionally a good eyepiece, but quality tends to vary. A good, well made plossl tends to be expensive. A cheaply made one will be poor because corners will have been cut on the glass quality. Go for a good make.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus
 
Posts: 809
Joined: Tue Jan 12, 2016 2:17 pm
Location: East Lincolnshire


Return to Equipment Advice

Who is online

Users browsing this forum: No registered users and 1 guest