red fringing on bright stars after DSLR mod

[mindburner]

To try out some widefield, I have removed the second filter, the light blue one from my Canon 1100d camera. The top filter is still in place.

The lens is a 50mm 1.8 stopped down to f4.5

I now find that brighter stars exhibit red fringing. I assume this is chromatic aberration. Focus seems pretty good

I have a IDAS P2 Light pollution filter on the front of the lens and as it has a built in IR block I assumed I didn't need a replacement filter for the Canon one I removed

Is there any way of preventing this fringing pre processing? I understand ir can be done post but I prefer to correct it before. Perhaps a second filter?

Perhaps it's UV?

Is this just one of the trials of a modded camera?

This must be something that is encountered by those folk modding cameras, so any insights would be welcome

thanks

[todd8137]

Pic any chance ?

Pat

[mindburner]

Pic any chance ?

Pat

Hi just a screen grab as I'm out with the mount but gives an idea of the fringing

[wxsatuser]

Is your focus spot on ?

The intrafocal point will give stars a red fringe with the centre tinged blue.

The extrafocal point will give a blue/violet fringe with a centre tinged red.

[ronin]

If the mod is the removal of the IR filter only then it would in some respects make sense.

The lens will have been designed to operate from 650nm to 400nm, the optical window allowed by the filters in the camera, you have removed the filter so the sensor is now receiving 650-750 (and others) that the lens was not designed for.

So that bit, say 650-750nm, of the NIR and IR is "incorrectly" focused and you get CA.

The sensor picks it up (as you wanted) and it appears as red, but the focal plane is different to the original criteria that the lens was built to.

[todd8137]

Looks like ronin beat me to it

Pat

[mindburner]

Is your focus spot on ?

The intrafocal point will give stars a red fringe with the centre tinged blue.

The extrafocal point will give a blue/violet fringe with a centre tinged red.

hi 

yea I think so. I spent quite a bit of time with Backyard EOS and also a mini bahtinov mask.

I was thinking it was the filer removal as I did not seem to have this issue pre mod with the same focusing regime.

I also changed my LP filter from a Baader moon and skyglow to a IDAS LP filter although I don't think this should affect CA too much

[mindburner]

If the mod is the removal of the IR filter only then it would in some respects make sense.

The lens will have been designed to operate from 650nm to 400nm, the optical window allowed by the filters in the camera, you have removed the filter so the sensor is now receiving 650-750 (and others) that the lens was not designed for.

So that bit, say 650-750nm, of the NIR and IR is "incorrectly" focused and you get CA.

The sensor picks it up (as you wanted) and it appears as red, but the focal plane is different to the original criteria that the lens was built to.

hi many thanks for the explanation. That does indeed make sense. The removal of the filter has upset the focal plane, so I guess something must be in it's place and this may restore the distance.

Is there a I suppose a 'better' at least in astro term IR filter that could replace the original Canon one?

I have been looking at lost of lovely wide field images of stars taken with modded DSLR's and they don't seem to suffer from this CA very much. 

[StuartJPP]

hi many thanks for the explanation. That does indeed make sense. The removal of the filter has upset the focal plane, so I guess something must be in it's place and this may restore the distance.

I don't think that will matter as long as you can still reach infinity focus.

Is there a I suppose a 'better' at least in astro term IR filter that could replace the original Canon one?

A Baader replacement filter or an Astronomik CLS-CCD clip filter will cut a lot sharper.

The front filter does filter IR and UV but I don't think it is good enough when using refractors (and lenses). It is ok for reflectors of course but I noticed quite large halos around red stars that weren't there before removal of the "colour correction" filter. This isn't to say that a Baader filter will remove bloating, just that it will probably be less.

I have been looking at lost of lovely wide field images of stars taken with modded DSLR's and they don't seem to suffer from this CA very much. 

The stars are so small in widefield images that you probably won't notice any CA.

[james_screech]

I had a similar problem with my 500D when I used a refractor. 

I found that it could be corrected in post processing using the instructions here: http://budgetastro.com/micro/articals/red_halos/red_halos.html

James

[mindburner]

I don't think that will matter as long as you can still reach infinity focus.

A Baader replacement filter or an Astronomik CLS-CCD clip filter will cut a lot sharper.

The front filter does filter IR and UV but I don't think it is good enough when using refractors (and lenses). It is ok for reflectors of course but I noticed quite large halos around red stars that weren't there before removal of the "colour correction" filter. This isn't to say that a Baader filter will remove bloating, just that it will probably be less.

The stars are so small in widefield images that you probably won't notice any CA.

hi there

Yes I can just about reach focus with the mod now and I have a 2 inch Astronomic filter which may be worth trying to see the difference with the IDAS. I was hoping by leaving the top filter in the camera that this issue would not be so pronounced.

[mindburner]

I had a similar problem with my 500D when I used a refractor. 

I found that it could be corrected in post processing using the instructions here: http://budgetastro.com/micro/articals/red_halos/red_halos.html

James

That's great! a very interesting read and should be easy enough to do. I just have to get a decent run of weather to try it out.

Here's a better image of the issue, but it seem to be just the same as what's described in the link

[mindburner]

very poor cloudy 10 mins of the elephant's trunk nebula and after processing.....

[michaelmorris]

I had a similar problem with my 500D when I used a refractor. 

I found that it could be corrected in post processing using the instructions here: http://budgetastro.com/micro/articals/red_halos/red_halos.html

James

Nice tutorial, thanks for the link.

[Guest]

Hi,

Sorry to be a killjoy, but I am sure on that camera it is the front filter that should be removed and not the rear one, the rear one is left in to stop star bloating and fringing, the front one kills all the Ha so that is removed, the rear one is left also because it has IR cut which you need for digital imaging....just check that first

MM

[mindburner]

Hi. Just checked on astronomiser. And they state that it is the front filter that should be removed?

So getting a bit confused.

this (the front) filter is effectively transparent to the visible spectrum and is suitable for use for astrophotography and allows virtually a full pass for the OIII, SII and Ha lines as well as all other visible light frequencies with a sharp cut off towards the IR and UV ends of the visible spectra (IR and UV cause star-bloating in refractors so for astrophotography it is normally best to remove IR and UV light), meaning that it blocks these frequencies so no other filtering is required when using the camera with a refractor, if this filter alone remains in the camera.

So according to them, this filter should remain to provide uv/ir cut?

[Guest]

I can pretty much assure you that it is the front filter that is removed and not the rear one, as stated on the Astronomiser site, I had mine done by them, as I said before the rear filter had very good IR cut and that is why it is left in the camera, the front filter also has IR cut but it also blocks a lot of the Ha signal too so it is removed, so I think you have removed the wrong filter.....sorry

MM

[Guest]

Hi. Just checked on astronomiser. And they state that it is the front filter that should be removed?

So getting a bit confused.

this (the front) filter is effectively transparent to the visible spectrum and is suitable for use for astrophotography and allows virtually a full pass for the OIII, SII and Ha lines as well as all other visible light frequencies with a sharp cut off towards the IR and UV ends of the visible spectra (IR and UV cause star-bloating in refractors so for astrophotography it is normally best to remove IR and UV light), meaning that it blocks these frequencies so no other filtering is required when using the camera with a refractor, if this filter alone remains in the camera.

So according to them, this filter should remain to provide uv/ir cut?

Astronomiser state that the front filter SHOULD be removed.....

But you are saying you have removed the rear filter in your opening post.....

So are you saying you have removed the wrong one...or not...I am confused

As I think you have removed the wrong one....

[mindburner]

I think there may be some confusion on the definition of front and rear in this context.

Astronomiser state on their web site:

that the front filter as they call it, the clearish, pinkish filter stays on the camera as 

" this filter is effectively transparent to the visible spectrum and is suitable for use for astrophotography"

it can be removed also for full spectrum

----------------------------------------------------------------------------------------------------------------------------------------------

the rear bluish tint filter is removed as this blocks HA and reduces it to 25%

"(this) filter that is removed or replaced during modification - it has a blue cast and can be seen bottom left"

(print attached with annotation) 

So the rear blue filter is removed by Astronomiser, this is exactly what I said I had done in my original post. Maybe their web site is wrong

Again from astronomiser:

"Filter Removal: This involves removing the rear colour correcting filter..."

so it's not the front unless you have a different mod done. Easy way to check, have a look in the mirror box of the camera

[Merlin66]

The "front" filter (pink) is an anti-alias/ dust shake filter which also acts as a UV-IR cut (approx.400nm - 700nm)

The "rear" filter (blueish) is the colour correction filter which restricts the red response. This is the one removed for improved Ha results.

Removing both filters gives a FULL full spectral response - 360nm>NIR. This is the mod I use for spectroscopy.

I've previously posted the transmission curves of both filters....

Adding any of the Astronomik clip in filters (I use the clear version) will fix the backfocus distance issues and allow the ef lenses to be used.

Let me know if I need to repost the transmission curves......

[astromerlin]

I've been wondering if the anti-alias filter does do a full ir cut as I modded my 1100d (removed the blue filter only) and I get red rings around most bright stars. I am using a reflector with a coma corrector, At first I didn't bother with changing the white balance and pulled back the red tinted images in processing. All my stars had red rings until I setup the camera with a custom white balance so that the red in the histogram was almost the same as the green and blue.

This had the effect of reducing the red rings somewhat and making images easier to process, My train of thought is that without the custom white balance setup the stars were possibly bloating in the red channel. I set DSS to use the camera's custom white balance and now the stars in my processed images look much better.

My stars still have all the colour around the edges than the middle though.

[mindburner]

I've been wondering if the anti-alias filter does do a full ir cut as I modded my 1100d (removed the blue filter only) and I get red rings around most bright stars. I am using a reflector with a coma corrector, At first I didn't bother with changing the white balance and pulled back the red tinted images in processing. All my stars had red rings until I setup the camera with a custom white balance so that the red in the histogram was almost the same as the green and blue.

This had the effect of reducing the red rings somewhat and making images easier to process, My train of thought is that without the custom white balance setup the stars were possibly bloating in the red channel. I set DSS to use the camera's custom white balance and now the stars in my processed images look much better.

My stars still have all the colour around the edges than the middle though.

hi thanks for that/ I set up up the camera with a custom white balance taken with the filter removed. This brought back a fairly decent balance and my think was that it may help with post processing. It didn't really help with the star halo's though. 

I have used DSS but found it was sucking all the colour out of my images. I must give it another go though with custom WB setting

It seems from the research I have done that the front filter alone is not sufficient to remove IR/UV. 

I may try my Baader filter again as it may have a better cut than the IDAS

[mindburner]

The "front" filter (pink) is an anti-alias/ dust shake filter which also acts as a UV-IR cut (approx.400nm - 700nm)

The "rear" filter (blueish) is the colour correction filter which restricts the red response. This is the one removed for improved Ha results.

Removing both filters gives a FULL full spectral response - 360nm>NIR. This is the mod I use for spectroscopy.

I've previously posted the transmission curves of both filters....

Adding any of the Astronomik clip in filters (I use the clear version) will fix the backfocus distance issues and allow the ef lenses to be used.

Let me know if I need to repost the transmission curves......

hi thanks for the info. I had used a full spectrum T3 before and the star bloating was there as expected. I think I was under the impression that it would be much reduced in this 1100d conversion as I decided to keep the front filter in place. I though if it's good enough for Astronomiser then that must be the way to do it. But the results are less than satisfactory. 

I will maybe contact them to see if they have any suggestions for an aftermarket filter to reduce this effect.

I read on cloudynights that folk that recently converted their cameras using the baader IR cut in place of the Canon one still had some issue with red halos.

[Guest]

If DSS is taking all the colour from your images, then tick the box that says "embed changes but do not apply them" then all the colour and data will be there when you transfer the final image to your chosen software for final manipulation. This is a must or loads of data will be lost including most of the colour, I had the same problem with DSS as it try's to do the colour balancing itself but is not very good at it, you just need DSS to stack your subs and nothing else.

Hope that helps

On a side note my modded canon does not suffer from any colour fringing at all with the single filter left in the camera, but I use it on and 8" SCT and not a refractor, maybe that's the difference as I have heard of others with that problem with refracting scopes, unless they are of the very best quality glass.

MM

[mindburner]

If DSS is taking all the colour from your images, then tick the box that says "embed changes but do not apply them" then all the colour and data will be there when you transfer the final image to your chosen software for final manipulation. This is a must or loads of data will be lost including most of the colour, I had the same problem with DSS as it try's to do the colour balancing itself but is not very good at it, you just need DSS to stack your subs and nothing else.

Hope that helps

On a side note my modded canon does not suffer from any colour fringing at all with the single filter left in the camera, but I use it on and 8" SCT and not a refractor, maybe that's the difference as I have heard of others with that problem with refracting scopes, unless they are of the very best quality glass.

MM

hi I shall give that a try. It was doing the same with my CCD subs too so I guessed it must be software but knowing exactly what to change, so that helps a lot.

I found the colour balancing better with the new IDAS filter. It gives a green cast but the actual relationship between the colous has not been messed up.

That's very interesting about the reflector not suffering from the dreaded star halos. I have my eye on a Quattro so I may keep the camera for that. I managed to get my CCD connected to the Canon 50mm lens and results were good, so that is an option for me too.

I do like the  FOV that the APS C gives so I will try it on my Meade APO. It's good but not top end when it comes to fringing at least in imaging terms. So it will be interesting to see if the stars are better or worse than with the camera lens

cheers