Advice using lenses for imaging

[astromerlin]

I decided to try using a lens on my modded SPC900 in order to get a wider FOV when imaging DSO's. Not knowing anything about lenses I decided to purchase a Photax-Paragon 70-220mm zoom lens off a well known auction site.

I made an adapter to screw in the webcam and fit the M42 thread of the lens and monted on the EQ6.

Due to the weather I haven't had alot of imaging time but from the experience I have had I'm not too impressed with the images I'm getting.

With reference to the images, I carefully setup the focus to get the stars as small as possible and also used the focus score in Sharpcap. When setting the webcam to long exposure the brightest stars developed a bloated red ring around the star.

So changing my technique and setting the webcam to a 3 sec exposure I adjusted the focus so I cant see the red rings at all.

The result is pic 2 where I get blue tails instead. My question is, is there any (collimation ?) adjustments that I could do to the lens or is it just a poor lens or something else.

[Merlin66]

I think you're seeing the chromatic aberrations from a poor lens. Zoom lenses are always a compromise..fixed focal length lens are usually better.

I use my ol' Oly Zuiko 135 f2.8 and 200 f4 for spectroscopy and they seem to work very well.

Try to find an ol' Pentax or Oly lens......I think you'll find they perform much better.

[neilmack]

Even the best zoom lenses of today lag behind the performance from 'prime' lenses, and those made years ago were less good still. And, to be truthful, the lenses branded as Photax-Paragon were generally sold on low price rather than optical quality. Manual prime lenses from any of the major camera manufacturers - Nikon, Minolta, Canon, Olympus, Pentax would most likely be better; some East German lenses made in the Praktica screw-fit, eg the Zeiss 135mm S f/3.5, are also very good indeed. Also worth looking out for are the premium ranges of lens from independents Tamron (the SP range) and Vivitar (the Series One range). Prime lenses will also generally offer faster f/stops, and therefore shorter exposures.

[astromerlin]

Thanks for the advice guys, a great help for my next choice. To be honest I chose it out of all the others because it had a mount built in, newb mistake!

[Looking Up]

You can buy old canon fd mount lenses very cheap, I bought a 200mm 2.8 for £49! I converted a 500mm to ef mount very easily, it works well!

[chiltonstar]

Some of the best corrected lenses are said to be macro lenses. You can find some obsolete models (eg Tamron SP90 Adaptall) for quite low prices - these give cracking astro images, and as the mounts are made of metal, and they don't do funny things at low temperature unlike modern plastic lens mounts!

Chris