Advice regarding eyepieces and barlows\powermates

[Dyptorden]

Hello,

I need and advice, especially somebody who used powermates "known to be exponentially" better than barlows.

Due to lack of money I intend to buy those 2x and 4x powermates from TV so that I won't need to buy smaller eyepieces... this way the 12mm would become 6mm and 3mm, the 16 would become 8mm and 4mm and so on.

How much quality\light would I sacrifice by using those barlows? Should I better wait more and buy smaller eyepieces (in mm) and use no barlows?

And second question, if I want to go for planet photo, would the 2x or 4x powermate be too much magnification (some say that the best planet photos need no barlows).At the moment I have no t2 adapter so that I can test, so I need your experience

I own an 8" DOB / 1200 FL / Nikon D40.

Thank you,

[John]

I've owned a couple of the 2x 2" Powermates and currently have the 1.25" 2.5x PM which I use with Tele Vue plossls. Optically the Powermates do exactly what it says on the tin - you just get the amplification of the image and no aberrations of any sort. With the 2" 2x Powermate I could turn my 8mm and 3mm Ethos eyepieces into 4mm and 3mm Ethos with no loss of field of view, clarity, or additional light scatter. Wonderful devices

The downside with the 2" PM's is that they add 500 grams or so plus 150mm of extension to the focuser. If your focuser is up to it then no problem.

This is a 20mm 100 degree eyepiece in a 2x 2" Powermate compared with the mighty 31mm TV Nagler and an 8mm plossl for scale

[Dannae]

Hi,

I concur with John about Powerrmates, they do an excellent job and the exit pupil remains the same (in a barlow it does not). For image amplification visually they are 'better' than barlows.

I was ultra serious a few years ago with planetary astrophotography just as the toucam mods became available and I compared Powermates and eyepiece projection using low element eyepieces (TMB Monos) at the same magnification using an 8" f/20 Mak-Cass. Eyepiece projection was better with the monocentrics compared with a x2.5 or x5 Powermate, approximately the same with a Televue Plossl and worse with all the other eyepieces I owned (Pentax XF and Naglers at the time). I put this down to the amount of glass in the optical train.

So in all but one case the Powermates gave the best possible planetary imaging possibilities. If you can find a x1.8 TMB Barlow that was extremely useful too but difficult to compare directly with the powermates.

I usually got best results at f/40 - f/50, diminishing returns after that. Cameras have moved on so unsure as to whether this alters that part of the equation but I remember keeping a large planetary image centred on a small webcam chip was challenging in slight wind even with a good mount.

Regards

Dannae

[Dyptorden]

Thank you very much for the answers...yet regarding my second question... if I use ... a 4x Powermate to make pictures to Jupiter, or Saturn, will they fit the image? or will I have any other sort of problems?

[Dannae]

I think that using a powermate would be a necessity at f/6 to gain any image scale. A x4 powermate would give you an f/24 which would almost certainly fit onto any ccd camera chip I know of.

When using a 4x Powermate you will have 4x the focal length and 4x the magnification.

If you are using a conventional digital camera (Nikon?) rather than a 'webcam' type where you are collecting large number of video type frames then the necessary exposure time grows as the square of focal ratio: the lower your focal ratio is, the longer exposures you'll have to get - in square! So, when going from f/6 to f/24 with a 4x Powermate you'll need 4^2 = 16x the exposure time to get the same brightness. Jupiter has a fast moving rotation so you want to keep exposures to 60 secs max (preferably less)  to reduce any possibility of blurring. I think this will be more than enough to grab a few frames to increase the signal to noise ratio. I have never used a digital camera for capture but I am guessing you could grab maybe 20-30 frames with an autoshoot function on timed for the increased exposure time when using a powermate?

Hope this helps

Regards

Dannae
 

[John]

Thanks for handling the imaging questions Dannae - it's an area where my knowledge is very thin