Noisy M20 (improved!!!)

universe24 · Post by **universe24** » Sun Aug 12, 2007 9:54 pm

After taken this image, I feel that a good quality telescope is very important. I found difficulty to get good focus on refractor with colour abberation. It make astro-images always got halo around stars...
What do you all think?

80mm f/7 refractor
Same type as Orion StarShooter : L 18x3mins (TEC off)
Darh frame substracted.

Post by **weixing** » Sun Aug 12, 2007 10:22 pm

Hi,
You mean an Achromatic Refractor??

I think the CA problem should be able reduce if you do tri-colour imaging, but you just need to refocus at every channel colour. If you are not taking tri-colour image, try using a LPR filter or may be a minus-violet which will reduce the CA.

Below is the image of Orion Nebula that I took using my previous Sky-Watcher 120mm F8.3 Achromatic Refractor with a LPR filter using Canon 300D DSLR. I'm quite surprise that the CA issue was not that bad.

Anyway, I also intend to use my Vixen 80mm F5 Achromatic Refractor for some open cluster imaging, so I took some photo of a floodlight sometime ago using LPR filter, minus-violet filter and without filter to see the CA issue. Will post the compare image when I got time.

Have a nice day.

Post by **rcj** » Mon Aug 13, 2007 10:55 am

Hi William,
good attempt on the image. Stars are nice and round. I suppose u did this in the observatory? Like Weixing has pointed out, u need to focus for each colour channel when taking the various colour components individually. Although this will lessen the effects of CA, it will not be completely nullified since the diffractive rings per colour will have various amounts of intensity. Where budget allows, sec-colour corrected refractor will be fine such as the popular Orion 80ED and market variants.
BTW, I notice that the background is furry. U might like to try to remove the hot and cold pixels per sub-exposure before combining into a resultant image.

universe24 · Post by **universe24** » Mon Aug 13, 2007 7:49 pm

Thank you very much weixing & rcj. I agree with you both about to reduce the CA problem. Will try the same object again when sky is clear.

universe24 · Post by **universe24** » Fri Aug 17, 2007 1:45 am

With further processing...now the image appear much better....

Meng Lee · Post by **Meng Lee** » Fri Aug 17, 2007 9:50 am

Using LPR indeed will reduce CA a lot, but exposure times increases a lot. Colour balance will go off quite a lot also. And imaging galaxies, you wouldnt want the LPR there.

Get a nice APO and you will see a significant improvement.

Post by **rcj** » Sat Aug 18, 2007 2:40 pm

here are some further thoughts about using an acrhomatic refractor:

it should be fine using it for narrowband imaging such as H-alpha or even OxygenIII and SulphurII and possible 'simulate' an RGB palette configuration respectively. However, another issue to take into consideration using an achromatic refractor would be its flat-field characteristic. At this point in time, I am not sure whether manufacturers would even bother making flat field correctors for their achromats. Apos do. Just a thought!

Meng Lee · Post by **Meng Lee** » Sat Aug 18, 2007 7:46 pm

Hehe, I think only Vixen NA series is the one where the achromat is made with flat field. Hmmm Vixen NA 140 for narrowband! Ah temptation again!

Post by **rcj** » Sat Aug 18, 2007 10:55 pm

yes, but the neo-achromats from Vixen are slightly better corrected than conventional achromats and would be affordable candidates for narrowband imaging.

Meng Lee · Post by **Meng Lee** » Sun Aug 19, 2007 12:16 am

Hmmmm, NA 140 for NIR imaging !