12 replies to this topic

[Samir Kharusi]

Here are my views on the choices to be made between these two popular filters.

In brief, the LPS-P2 remains an excellent all rounder "white-light" choice that attempts to filter out the major emission lines in older fluorescent lighting. But the lamp manufacturers are getting absurdly sophisticated in generating ever more spectral lines in their endeavours to mimic broadband white light. This could make the LPS-P2 less effective than it was when your street lights were still at an ancient technology level. At the extreme, we wish to narrow down to single spectral lines. The most cost-effective way of capturing OIII and Ha simultaneously today seems to be the Astronomik UHC filter combined with a UV/IR Blocker. It does not block IR by itself. The broader such a duo-tone filter is, the less effective it is in blocking generalised light pollution, but more "white-light"-like it presents its image captures. Filters available in this category of half-hearted duo-tone, near-white-light, are the CLS and the LPS-V3. There are of course many other vendors that I have not examined. So, to mimic white light captures, IMHO the LPS-P2 remains king. To mimic narrowband captures, the Astronomik UHC (not necessarily other manufacturers' UHC) + a UV/IR Blocker remains current king. The CLS and the LPS-V3 are in-between... Click the referenced URL for more bumpf

[HaleBopper]

I've noticed in my mag 4 - 4.5 skies in my yard, that the IDAS filter works reasonably well if I keep the exposures at about 3 min. I even had a gibbous moon to content with.I have tried 5 min, but it just seems to be a wash out. I corrected the vignetting on the 3 min exposues with flats but my flats did not work so well at the 5 min exposures. The biggest reason is due to my not taking proper flats I'll concede. I'm still learning how to take those.However, I did expect a better performance from the IDAS. Perhaps I'm expecting too much? I tried the IDAS on M1 at a pretty dark sky, ~21.5 mag/sqarcsec, and liked it's performance. I was using an 8" SCT at f/6.3, ISO 800 for all my above examples.

I have gone ahead and ordered an Astronimik CLS for my DSLR. Looking at its' tranmission curves, it filters out Na and Hg emission lines better than the IDAS. I'll try it from my yard on an emission nebula and see how long of an exposure I get before it becomes pointless.

[mclewis1]

Samir,
Thank you for the analysis ... after reading your website there were a lot of ahaaas.

[Nils_Lars]

Excellent work once again Samir , I wish I had known about your sight before I picked some of the gear I did.

[Samir Kharusi]

Excellent work once again Samir , I wish I had known about your site before I picked some of the gear I did.

Thanks for the kind words. I wish I had known what I know now before I bought the gear that I did buy over the years and "upgraded", endlessly. The journey would have cost half what it did; but as they say, sometimes it's the trip itself that is most fun, rather than the destination Enjoy the trip.

[Doubleglaze]

Thanks Samir, easy to see the differences between the filters visually by looking at your spectra - more intuitively satisfying than looking at spectral plots.

Any thoughts on the differences between putting the filter in the camera body (clip in style) vs upstream in the optical path (2 inch filter thread style)?

For me, I have an unmodified Canon 40D with built in filter window in place in the camera body. Main concern I have is picking up and imaging back reflections leading to halos on brighter stars. I'd suspect the clip in would be more susceptible to this being closer to the image plane and built in filter, but looking for experience / opinions.

Thanks

Mark

[s58y]

One advantage of the clip-in filter is that the same filter can be used with almost any telescope or camera lens. Many scopes can use the 2-inch (48mm threaded filter), but with camera lenses, you may need different sizes to avoid vignetting.

The big disadvantage of the clip-in filter is what happens when you graduate to a full-frame DSLR (Canon 5D2, for example), or switch to Nikon, once Nikon "Mode 3" is a thing of the past?

[Samir Kharusi]

Thanks Samir, easy to see the differences between the filters visually by looking at your spectra - more intuitively satisfying than looking at spectral plots.

Any thoughts on the differences between putting the filter in the camera body (clip in style) vs upstream in the optical path (2 inch filter thread style)?

For me, I have an unmodified Canon 40D with built in filter window in place in the camera body. Main concern I have is picking up and imaging back reflections leading to halos on brighter stars. I'd suspect the clip in would be more susceptible to this being closer to the image plane and built in filter, but looking for experience / opinions.

Thanks

Mark

I believe that experience with astroCCDs has shown that the most problematic halos are caused by reflections from glass that is very close to the sensor (a mm to a few mm) like the cover glass of an astroCCD, much more so than glass that is at a cm or further away (like in a filter wheel). For DSLRs the pop-in filters are still more than a couple of cm from the sensor. So I do not "think" they will cause problems. Halos are very difficult to trace the cause of, so that's why I say "think".

[KevinUK]

I agree with your verdict on the UHC, unless I'm specifically trying to capture a broad spectrum of white light in galaxies and LP isnt too much of a problem I use my my Astronomik UHC filter as first choice.

I'm continually surprised as to how little commendation is made regarding its efficacy as a imaging/LP filter. Perhaps its more popular in Europe than with our fellow US/CAN cousins.

At any rate, I recommend it wholeheartedly as one of the best all round imaging filters for its price and what it achieves.

[Jason Freeland]

So a UHC filter with an unmodified camera would not need any aditional filters (UV/IR)?

[Samir Kharusi]

Correct, all unmodded cameras block UV and IR.

An unmodded Canon DSLR is very insensitive to Ha, so the UHC will, effectively behave like a narrowband OIII filter (43nm bandwidth) and give you a "bit" of Ha, better Signal to Noise Ratio for the Ha than no filter at all, but still at a low level. The Ha in an unmodded camera with no UHC filter is swamped by a lot of skyfog, hence should have a worse SNR.

Not all unmodded cameras are created equal. I suspect that the cheaper ones let through more Ha (to help boost sensitivity) than possibly the One series (for which colour accuracy is more important and the pixels are often larger anyway). The URL I gave in the first post delves further into this but here is an example from that URL and high light pollution; left to right, Unmodded 1Ds no filter, 1Ds+UHC, Modded 20D+UHC:

Much of the signal in the unmodded 1Ds is from reflection nebulae, hence its much bluer colours. The UHC blocks out the reflection nebulae. The middle image clearly shows regions of strong Teal OIII. The modded camera is very sensitive to deep Red and the right image, without some cutting back on the Red in post processing will show it as overwhelmingly an Ha nebula, with the Teal submerged.

[Jason Freeland]

Thanks. I have an unmodded 300D and will not be purchasing anything else for quite a while. I just want to make sure that the accessories I add will give me the most bang for the buck. I appreciate your work on this.

[WarrenS]

FWIW, here's M42 with my unmodded 40D and the Astronomik CLS clip in filter. More red than I thought possible, but also more red noise than I would like.

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