Orion Widefield

[mswhin63]

Hi,

Considering I need to go wide-field for a while, I picked up a low cost 75-300mm canon lens and for what its worth is surprising good. The only thing I believe needs to be looked at is fringing which is quite bad, although there was a UV blocking filter on the front of the lens it has some sort of fault that caused ghosting.

This is one image I took on the night, the fringe has been dampened a bit using a script called Defringe for GIMP.
It couldn't get rid of the star blowout but it did remove an awful lot of the dark blue fringe.

Any further advice would be good eg: filters for the lens etc.

Canon 450D 300mm, 40 x 15 seconds @ 800ISO

[rcheshire]

Hi Malcolm. I would remove the filter and stop down 2 stops from your widest aperture and increase the exposure time - say 20 or 30 seconds. This should improve things. Slightly longer exposure though. And be very critical with focus.

[mswhin63]

Thanks for the advice, will try it hopefully tmw

[Visionoz]

Malcolm perhaps the lens has too much CA it being a "cheapie" zoom and as suggested stopping down might "reduce" it

HTH
Cheers
Bill

[bojan]

Perhaps even better way might be to place external aperture.
This worked surprisingly well with my Canon 200mm and 300mm FD lenses - f5.6 removed the bloating and CA completely, while internal stop just reduced the bloating somewhat but the CA remained.

[mswhin63]

I think i will try all of the suggestion many thanks, but I have also been considering RGB imaging, maybe this is a good place to start.

I split the RGB on the original image and there is definately CA on the blue, it is way out of focus on blue. I think green is bit as well.

[bojan]

If you want to get rid of lateral CA, Gimp has appropriate plugin. This doesn't help on axial CA (out of focus for different colours.. maybe you should try focus magic on R, G & B frames?)
I was using it on the final (8-bit) images.

[rcheshire]

That's an interesting point. Do you mean placing a card with a smaller hole over the front of the lens?

[luigi]

The 75-300 is unusable from 200 to 300mm, I wouldn't put any time in trying to get something from that lens.

[bojan]

Yes.
Have a look here, post #6 specifically:http://www.iceinspace.com.au/forum/showthread.php?t=51866&highlight=canon+200mm

I have similar results with Pentax and Tamron lenses (above 200mm).

[mswhin63]

That is a great example and seem to duplicate my issues.



Some people told me I wouldn't get much from a DOB but I managed to find a way, maybe you are right but I love to experiment and I believe there maybe a way. RGB would most probably be the ultimate. I have another test that I want to try without RGB filters am going to try, I hope it will provide the same effect as photgraphing RGB. I tried to buy some RGB today but nothing in stock state wide at least.



Haven't seen focus magic will search for it, I have a couple of plugins already 2 of them are CA and the Orion has been dealt with, the initial images were really bad.

All in all I dont have the money to buy a $1000.00 lens otherwise I would buy a widefield triplet carbon fibre scope an do it that way. At least when I get my surgery done I can handle a lense on a camera and can get my wife and/or kids to carry the tripod out an set it up when needed, I already have spots marked on the pavement to place the legs of the tripod. 6 to 9 month later when I finally recover I may use my DOB again. :thumbsup:

[rcheshire]

That's amazing - I must try it out on an EF-S lens.

[mswhin63]

FYI, I have done a very quick test as I am quite busy tonight, took a shot with precise focus and as usual red and blue are out of focus while green is sharp as.

I then miss focused by a smidgen and behold blue and red was in focus and green was out of focus. Apart from looking a bit green the star looked very symmetrical and there was no fringing either. This hold to the ability to use RGB and focus refocus for each colour.

Couldn't buy any RGB filters today though. I am though considering filtering the monitor to the individual colours and refocus not sure if it will work but could be a very low cost method.

I want to look at consideration of fitting a filter-wheel between the Canon and lense but that would be quite a chore.

Next step will be increase F/STOP and test that but for another night.

[bojan]

Yes, that is axial CA.. stopping down the aperture will most likely help here.

And, you don't really need filters if you want to try with RGB (because the camera sensor already have them anyway): You only need to take 3 shots, focussed for R, G, B.
Then, you can separate channels (using IRIS software) and recombine the focussed frames only, discarding the rest.

[mswhin63]

Hi Bojan,

I was considering the alternative of purchasing cheap RGB filter to cover the monitor and re-focus on each. This would save on buying filters that cost at least $99.00 each.

Without the filters it is only an approximation.

[bojan]

Fair enough, sounds reasonable :thumbsup:.
Let us know the outcome