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Advice needed on Coma Corrector / Light pollution filter


Hawkeye
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So I finally got around to getting a Baader MPCIII Coma corrector and an Astronomik CLS CCD Filter (IR blocked) to use with a Skywatcher 200P and a canon 1000D (Not modified).

Questions :-

1. Is it normal for the focus point to be further out with a Coma corrector fitted?

2. Will either or both of these effect the amount of data collected by the camera? 

I ask as having fitted both of these I have found the focal point is now further out by about a third of the original focus point, and when collecting data even on 5 minute exposures @ ISO400 the histogram remains right over the left hand side and there is very little detail in the subs when stretched.

Attached is 1*300 second sub of M81

post-20946-0-82930800-1428958458_thumb.p

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1. Probably, as you are altering the path the light takes.

2. Yes. Coma corrector to a minor extent as any glass will reduce how much light is transmitted, but mostly the CLS filter filters out some good photons aswell as the bad ones. I was finding i was needing maybe three times as long exposures to mimic the setup without the filter in place, but remember lots of the histogram without the filter will be unwanted photons, so hopefully your target signal to [light pollution] noise will be far superior using the filter.

I also used the CLS CCD filter and think it is maybe more intrusive at blocking good photons than the non-CCD version as others with the non-CCD version disn't find as much an impact on exposures times as me.

But it's an impressive filter.

James

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Yes, the CLS cuts a lot of useful light along with light pollution. There are more expensive filters that let some more useful segments of the spectrum pass, like the IDAS-LPS and the Orion Imaging Filter, but of course no filter can very effectively remove only the light pollution.

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So I finally got around to getting a Baader MPCIII Coma corrector and an Astronomik CLS CCD Filter (IR blocked) to use with a Skywatcher 200P and a canon 1000D (Not modified).

Questions :-

1. Is it normal for the focus point to be further out with a Coma corrector fitted?

2. Will either or both of these effect the amount of data collected by the camera? 

I ask as having fitted both of these I have found the focal point is now further out by about a third of the original focus point, and when collecting data even on 5 minute exposures @ ISO400 the histogram remains right over the left hand side and there is very little detail in the subs when stretched.

Attached is 1*300 second sub of M81

Thats the whole point of an LP filter.

It will increase the contrast and make the exposures longer, more like imaging under a dark sky.

Which is what you want, longer exposures.

If you find the exposures are to long for your tracking or guiding bump up your ISO, for Canons

800 to 1600 should be ok, just expose so you get the histogram in the 25>40% range.

The CLS CCD passes 97% of the good stuff, I don't believe you would notice the difference

of a few missing photons, in any case just get more subs, the more you get the better.

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Yes, the CLS cuts a lot of useful light along with light pollution. There are more expensive filters that let some more useful segments of the spectrum pass, like the IDAS-LPS and the Orion Imaging Filter, but of course no filter can very effectively remove only the light pollution.

I find this is not the case with both filters.

I have CLS and IDAS both do about the same on LP.

With my Canon 60Da the only difference is the colour balance.

The IDAS is near perfect the CLS is not balanced, blue shifted.

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One of my uses for this filter when i first got it was star trails from my light polluted back garden.

I'm surprised it passes 97% of the good stuff. For the same ISO settings and exposure times, I was finding many stars which were quite bright without the filter were just not even bisible with the filter. Does "star light", as such, contain a large amount of wavelengths which is also present, and desirable to be blocked, in light pollution?

Interesting.

James

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I also find the IDAS filter results in a more normal colour balance than the CLS one. Personally i don't think the IDAS filter is as "strong" as the CLS-CCD one and it also seems to block vastly less good photons.

James

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I have'nt noticed the effect with stars.

My subs are normally in the range of 5 to 10 minutes, may be at shorter sub lengths it might be the case.

The CLS CCD blocks the area around 530 to 640nm, thats the area around green, yellow and orange.

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I'm surprised it passes 97% of the good stuff. For the same ISO settings and exposure times, I was finding many stars which were quite bright without the filter were just not even bisible with the filter. Does "star light", as such, contain a large amount of wavelengths which is also present, and desirable to be blocked, in light pollution?

That 97% was an exaggeration. Unless you mean that it gets 97% of specific nebulae that shine mostly on specific emission lines. If you image stars, galaxies, clusters etc, you lose over 50% of "good" light, since the visible spectrum transmission of the CLS filter is under 50%.

That's why I said with an Orion Imaging or an Idas LPS you get more of the "good light" through. if you look at the transmission graph, they are like the CLS, but they also add about 10% of "good" spectrum back by fine-tuning around the Hg/Na emission lines. I have a CLS and an Orion Imaging and that extra 10% of spectrum when the CLS already blocks more than half, makes it for an obvious difference. I assume the Idas is not worse, but the Orion was less expensive.

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