M42 - taming an achromatic refractor.

cometcatcher (Kevin) · #1 16-09-2013, 10:51 PM

I've been experimenting with shooting deep sky through achro refractors, for curiosity to see how much can be squeezed out of one and how well the CA can be managed. Now days with fairly cheap semi-apo's, achros have become redundant for astrophotography.

In the raw, achros produce Chromatic aberration as a result of not being able to focus red, green and blue wavelengths of light at the same focal plane. One of the colours will be in focus, the others will not. Usually this results in blue or red halos around bright stars. See attached examples.

There's a few ways of fixing or reducing the problem without buying another scope. Shoot narrowband, use filters and / or process out the artifacts.

The last few nights I've been collecting data to experiment on the Orion nebula to see if I can outsmart the CA with filters. The Astronomik CLS filter reduces a lot of CA, reduces light pollution and also enhances nebula.

The Baader Semi-apo filter does much the same thing but to a lesser extent. The advantage is it lets through more light.

A simple yellow filter like the Hoya K2 almost eliminates the blue fringe by itself. Colour balance correction can be done in the camera itself.

GSO make a bunch of 48mm filters and I've been experimenting with those also. Their yellow is much deeper than the Hoya K2. It completely eliminates all blue fringe around bright stars but at the expense of a stronger yellow colour shift.

Anyway here's a couple of pics. More subs data would have been nice but the weather and moon had different ideas. There is a plain frame with no filters or retouching for comparison. The camera was an unmodified Pentax K-x.

The deeper frame used all the above mentioned filters and also an orange filter. I would like to have added some red or Ha but now the moon is interfering.

multiweb (Marc) · #2 17-09-2013, 06:46 AM

Cool pics. If you use an OSC camera there is no getting around an APO or you'll lose your star colors like in this two shots. Baader does a Fringe Killer for approx. $50.00 that will get rid of purple halos. I use it all the time when using lenses.

cometcatcher (Kevin) · #3 17-09-2013, 08:57 AM

The Baader fringe killer is built in to the semi-apo filter as it's a combo of moon and skyglow + fringe killer. It reduces fringe a little in achro refractors but a lot remains still unfortunately requiring either further filtering or software management.

I'm experimenting on star colour by adding further layers of filtered subs. A bit like RGB in a way which departs from OSC.

Jon (Jonathan) · #4 17-09-2013, 02:34 PM

So forgive a novice question - why can't you use a monochrome camera with RGB filters with an achromatic refractor and simply refocus with each filter?

cometcatcher (Kevin) · #5 17-09-2013, 03:22 PM

Quote:

Originally Posted by Jon

...why can't you use a monochrome camera with RGB filters with an achromatic refractor and simply refocus with each filter?

I don't have a deep sky capable monochrome camera to try it. Well I do, but it's a low res video camera.

My DSLR has a digital monochrome filter, but it clips the histogram if I try to use it for some reason.

The other option is to leave the camera in colour mode. I've tried with cheap filters and still have CA getting through. I may need a stronger blue filter. I don't have a set of 'real' 48mm RGB filters. They are quite expensive.

The whole point of this is to do it on a budget using pieces of string, wood, duct tape and glue.

My orange filter is held in place with a blu tack like glop!

Jon (Jonathan) · #6 17-09-2013, 07:53 PM

Quote:

Originally Posted by cometcatcher

I don't have a deep sky capable monochrome camera to try it. Well I do, but it's a low res video camera.

Sorry, sorry - imprecise use by me of the word "you". What I meant to ask was "why can't one/somebody just use filters with an achromat, and adjust focus?"

I thought one of you guys might have the answer. If one could, why are all the big guys imaging with monochrome cameras through apo refractors?

cometcatcher (Kevin) · #7 17-09-2013, 08:42 PM

Quote:

Originally Posted by Jon

Sorry, sorry - imprecise use by me of the word "you". What I meant to ask was "why can't one/somebody just use filters with an achromat, and adjust focus?"

I thought one of you guys might have the answer. If one could, why are all the big guys imaging with monochrome cameras through apo refractors?

The question is a bit beyond my experience, I'm an RGB newbie but I'll have a go at answering it. From what I've read, narrow band imaging is fine through an achromatic refractor. RGB filters for imaging however are wider bandwidth than the narrow band Ha / S2 / OIII etc. I think (someone correct me if I'm wrong) that with RGB some CA still gets through with the wider bandwidth. It certainly does with my primitive GSO green and blue filters. I'll upload a pic to demonstrate.

This is the Crux star Mimosa taken with GSO red, green and blue filters. As you can see there's still blue and magenta CA on the green and blue filters even though I've focused on each filter separately. Does this still happen with better filters like Baader etc? I'm not sure and would like to know myself. Need to get one of the pro shooters to answer this for us.

LightningNZ (Cam) · #8 17-09-2013, 09:00 PM

Your image made with the filters is quite good. There's still a fair amount of noise in the background so you could stand to take more subs. The colour looks very good though I can see that you've desaturated the blue still.

I thought long and hard about doing something similar but in the end bought an AT65EDQ 2nd hand and saved the few hairs I have left on my head from being pulled out in frustration. There are enough battles to fight in astrophotography without trying to get amazing images out of an achromat. I've kept mine and am going to use it as a guidescope (perhaps with an O3 filter).

In response Jon's question about refocussing - seriously, you don't want to. You'd spend more on a robo focus system than you would on buying a decent small APO or a medium sized reflector.

Cheers,
Cam

cometcatcher (Kevin) · #9 17-09-2013, 09:08 PM

More subs are definitely needed. But I wasn't born with any patience (I'd never make a doctor)

and want to move on to something else before it starts raining for 6 months again.

doppler (Rick) · #10 17-09-2013, 10:27 PM

I add a layer in photoshop, clone out the coma and use the eraser to put the stars back in with out coma. When I took a shot of a really bright star there was no CA, I can only asume that coma gets worse with exposure length.

cometcatcher (Kevin) · #11 17-09-2013, 11:23 PM

G'day Rick. I've found the eraser to be effective on CA also. Which scope did you use in the pics?

doppler (Rick) · #12 18-09-2013, 07:43 AM

I use a 102mm f7 bosma achro refractor hard mounted to the tube rings on a 200mm f6 eq newt (its a 1970's cave optical scope) The CA is not noticable visually on stars and just a bit of blue fringing on the moon.
The pics are 20s at iso 3200 canon 1100d not moded.