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Light pollution filter- where in the imaging path?


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I'm fitting an adapter which allows me to put a light pollution in my imaging train so I can us this in combination with RGB filters if desired or remove for narrow-band filters at my option.

A question- should the filter be place before the filter wheel or after-or does it matter?

Has anybody tested to see if the placement of the filter makes any difference?

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I'm fitting an adapter which allows me to put a light pollution in my imaging train so I can us this in combination with RGB filters if desired or remove for narrow-band filters at my option.

A question- should the filter be place before the filter wheel or after-or does it matter?

Has anybody tested to see if the placement of the filter makes any difference?

Hi,

It sounds daft to most and would be  bar for the very rich, but before the whole telescope!

In practice. If possible it should be fitted before the focal reducer, this will reduce any reflections within the reducer and the rest of the light train after that of unwanted frequencies having any effect. Having said this it depends upon the angle of the light entering the reducer. For any filter the light should be as near orthogonal to the glass surface as is practically possible. Once away from the orthogonal the filter will change characteristics and allow slightly different frequencies though. In practice it may not be noticeable at all and is  dependant upon specifications of the filter used, see the manufacturers web site for details. Also you may find that in that position the filter is more awkward to remove simply. If placed just before the filter wheel you again reduce unwanted frequencies of light getting to the RGB filter surfaces, hence the RG and B filters may give slightly better results. No filter is perfect and even with the highly sophisticated methods of production these days there are differences between the same manufacturers filters in each run produced. In other words there are always statistical errors. In the end it may come down to the easiest method for removal in the field. The best way is just to try it and see what the outcome is.

Of course others may disagree.

Best of luck,

 Derek

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Thanks for this Derek.

My gut feeling is to place the filter before the filter wheel. This is fairly easy to do as I'm simply replacing a 24 mm T-extension tube with the filter holder. I can't wait to get the new system fully operational as the light pollution has been a real nuisance.

I have bought possibly the best LP filter the Hutech IDAS which is waiting for first light testing with the RGB Baader fiilter set.

I can't fit before the focal reducer as the edge spacing for the lens assembly is very critical so my only options are before or after the filter wheel. I'm certainly not removing the reducer and imaging deep sky at F10 and even if the filter could be fitted inside the reducer it would be very inconvenient to have to remove the whole camera assembly to enable or remove.

I will keep you all posted.

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Thanks for this Derek.

My gut feeling is to place the filter before the filter wheel. This is fairly easy to do as I'm simply replacing a 24 mm T-extension tube with the filter holder. I can't wait to get the new system fully operational as the light pollution has been a real nuisance.

I have bought possibly the best LP filter the Hutech IDAS which is waiting for first light testing with the RGB Baader fiilter set.

I can't fit before the focal reducer as the edge spacing for the lens assembly is very critical so my only options are before or after the filter wheel. I'm certainly not removing the reducer and imaging deep sky at F10 and even if the filter could be fitted inside the reducer it would be very inconvenient to have to remove the whole camera assembly to enable or remove.

I will keep you all posted.

Hi,

There only two things I would say. 

(1) Is just to make sure that you have a filter big enough not to introduce any vignetting in addition to any already there. That is, if it is possible. I don't do any RGB from home (South Shields) as the light pollution is awful.  I have tried in narrowband with good results. Just adds to the cost though! I have something of the same problem, as the field flattener is fitted to the rear of the scope and would be too problematic to remove and far too costly to buy if at all possible (large size).

(2) Be aware that by removing or adding the filter in line alters the focus point for the attached CCD. This is because of the either added or removed glass thickness in the light path, (added glass reduces the apparent light path).

If you already know this apologies! (Not trying to teach granny to suck eggs) :grin:

Regards,

Derek

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Thanks Derek,

I use focus max so any focus deviation is immediately taken care of. I'm planning to use a software solution called Sharplock on the on axis guider which will continuously refocus as images are taken to adjust for temperature changes in focus or other anomalies.  I have purchased the 2" IDAS filter so vignetting should not be an issue as the Atik CCD is quite small. The components to couple the filter holder are on order so I expect to be fully operational this week (weather permitting) with the new solution.

I produce reasonable results with RGB but the light pollution arises more with the L channel (UV-IR/Cut) so I'm hoping the filter will help in that respect. I'm also hoping to be able to use the Atik 490 OSC which I have to try some single colour exposures with the IDAS in place. I've left one of the filter slots empty for this purpose.

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I've done some initial testing. I have tested my RGB exposures to 30 mins with the IDAS filter & see no discernible gradients- so that's a good result. I've tested the UV-IR cut up to 16 minutes & again no gradients. I can't test properly until we get better weather which has simply been awful this month.

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  • 4 weeks later...

I accidentally posted in the wrong forum section- so excuse the double post. Here is my first light using the Hutech IDAS filter in front of the Baader LRGB filters. The picture is not technically accurate on the alignment front but is only a total of 4 exposures with the Atik 460 mono 2x2 bin. The RGB is 3 x 15 minutes & the UV/IR cut 8 minutes. I can use much longer subs now without gradients so the technique shows much promise. No real processing other than flats & highpass filtering. Let me know what you all think.

post-36426-0-42969900-1424125868_thumb.p

Edited by pyrasanth
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I accidentally posted in the wrong forum section- so excuse the double post. Here is my first light using the Hutech IDAS filter in front of the Baader LRGB filters. The picture is not technically accurate on the alignment front but is only a total of 4 exposures with the Atik 460 mono 2x2 bin. The RGB is 3 x 15 minutes & the UV/IR cut 8 minutes. I can use much longer subs now without gradients so the technique shows much promise. No real processing other than flats & highpass filtering. Let me know what you all think. attachicon.gifM82.png

Hi,

Nice image, bit of fringing but nice start. If that is the filter test it's good! Be realy good to see what is possible with more image time and processing.

Glad to see the update. Keep us informed.

Regards,

Derek

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Thanks- yes this is the first light test. The weather has gone back to normal- awful so not sure when I will be able to retest. The light pollution filter is expensive but it is looking like it's worth every penny as the images i am getting are so much brighter & vibrant. I'm looking forward to a more substantial test when the weather is better. I have to say that the new ONAG makes guiding so much easier in respect of guide star selection. I don't need to rotate the field as in the OAG solution I can keep pretty much the same flats. I am surprised as to how much light pollution destroys definition.

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I have a nice clear sky tonight (very rare!) so I'm using the new technique for M97 the owl nebula. I'm using LRGB & narrow band filters so I can study the effectiveness. I've just bought a copy of CCDStack 2 which seems to have the appropriate learning curve! (I've also ordered a tutorial DVD)

I will post the results.

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Hi,

Looks promising. Still a bit of fringing on the stars though. Its seems to show blue fringes now as opposed to the orange in your first effort. So I suspect there is something in your image train causing this. I cannot see it being the Celestron scope, but something is causing the colours to reach focus at different points!. You are using a focal reducer and a Hyperstar at the same time, just maybe the problem. The greater angles of incidence to the filters etc may be the problem. What 0.7 focal reducer is it that you are using?

Regardless they are nice images.

Everyone seems to have some problem or other in this hobby! Me I am struggling to get my A.O. guider to move/bump the mount properly. Works using the Starlight Xpress utility program but not from Maxim DL. Hair pulling exercise!!!!! Need more good nights to test it but clear skies dance not working as yet :mad: .

Regards,

Derek

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Hi Derek

I'm still very much in a testing phase with this technique. The fringing is probably caused mostly by poor image alignment-as I said I'm not really attending to detail just yet. I am now moving to use deep sky stacker having invested in the software & a good tutorial. The optics in the train are the Hutech IDAS light pollution filter, the imaging filter & the Celestron 0.7 focal reducer. These images don't use the Hyperstar as the only filter I can use with that is a manual insertion before the Hyperstar. I tend to use the OSC with the Hyperstar with just a light pollution filter as multiple filter changes on short exposures would drive me nuts although its possible if I want pain.

The second issue I find is that imaging is very moon sensitive & the image of M51 was shot with a quarter moon so perhaps not the best combination as the lights were a bit weak & washed out. I long for dark skies which ain't going to happen where I live.

I hope you get your dithering sorted- I've had a thought my dithering might be a bit aggressive at 4 pixels- hence the colour shift- I might knock it down to 1-2 & see if that makes a difference. 

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Hi ,

I expect you are right. I have never imaged with a Celestron Edge. In fact never with my Meade LX200 either. I moved on to a refractor as I wanted a image wider field.  It is always a slow process getting everything just as you want it before you move on to do exactly what you intended all along. I'm still attempting to get the Active Optics to work correctly. I live in the north east and am plagued with light pollution so I understand the frustration. Like an idiot I did not think about the Hyperstar properly so get it now. You seem to be doing a damn site better than me at present anyway, so best of luck for the future.

Regards,

Derek

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