Filter Experiments

ZeroID (Brent) · #1 01-08-2014, 12:13 PM

Like many of you guys I suffer from LP problems, sodium street light glow mainly. I have used an Astronomik CLS filter quite effectively but it's only a 1.25 ", it extends the exposure time considerably and you have to seriously counter the blue effect in processing.
I was up at our local Photo retailer and he had a bin of filters on sale, 5 for $10, digital does not need them so out they go.

Grabbed $10 worth and was playing around with the SONY daylight just for fun. Noted clouds and sky got better seperation with various filters so I wondered how they would do astronomically. Did a couple of 10 second tests widefeild with the camera and with one particular filter got a distinct improvement with the pesky background red glow being almost eliminated. More testing required.

Finally got some clear sky and the Ob working again so did some tests.
I pushed the ISO and time to see what difference it made. These are single frames, unprocessed other than being reduced to meet size restrictions.
Exposure is 30 secs at ISO 3200 for both frames taken within a minute or so of each other. First frame without the 80A, second with it obviously.

The filter was a standard Wratten C80A, light blue. $2.00
I'm going to do more testing but one area it may also help is in guiding. I think I'm having guiding issues because of lack of contrast between the target and the background. First tests with AmCap and an SPC900 seem to support this.

ZeroID (Brent) · #2 01-08-2014, 12:31 PM

More info.
Here are 2 @ 30 sec @ 1600 ISO, a more normal exposure value.

I had previously found an article on someone else's filter tests with graphs on a 'standard' light source. I'd credit the author if I could remember where I found it but I grabbed two of his images, one for the Astronomik CLS filter which I have ( not the CCD one ) and for the C80A. Hope he doesn't mind.
But I was interested in the similarity of the profiles with the 80A being a much less pronounced variation of the CLS. My thinking is that the 80A may have enough LP blocking to be useful yet not filter out too much of the rest of the spectrum. And help my guiding issue as well.

LP has virtually limted my exposures to max of 30 secs so far, sometimes less, 20 -25. So being able to extend exposures to more than a minute or so in my books is useful.

Testing will continue ..

P.S. Ignore the trailing stars, no guiding. Of the graphs, first one is the CLS with yellow lines for the sodium and mercury vapour lights, second the 80A. Purple line is the light source.

2" LP filters are EXPENSIVE !!

cometcatcher (Kevin) · #3 13-08-2014, 04:08 PM

Hi Brent, I've been doing the same sort of thing over the last couple of years with plain photographic filters. Mostly because it's just fun to experiment, but I also wanted to get rid of the blue CA on achro refractors, be able to shoot in moonlight, enhance comets, suppress LP and even do RGB with them on the cheap.

I found that under certain circumstances, for certain objects, certain filters can be useful. It depends what you want to shoot. For instance you will notice that your blue filter certainly has reduced the orange sodium street lights. But it has done a couple of other undesirable things as well. Emission nebula are Ha rich and the blue filter has taken a big bite out of the Ha line at 656nm as you can see by your graph. The blue might be okay for non Ha objects like star clusters, Pleiades, reflection nebula etc but it's no good for Ha rich nebula. It will also likely make CA more noticeable on some ED or achro refractors.

Here's the problem with photographic filters, they are monoband. That's why we pay big bucks for those expensive 2" LP filters, they reduce the LP without reducing nebula emissions, usually in multiple bands. It's like being able to have the cake and eat it too.

Monoband filters are not without use however. A light yellow or yellow green filter can reduce the CA of refractors, as will warming filters like the Vivitar 81B. The yellow and green filters also cut through haze, just like they do in daylight with black and white film. There is a slight colour shift but it can be compensated for, either in camera or post processing.

If you want to kill the light pollution and have your nebula too it gets tricky in monoband. A 25A red filter will cut a lot of LP and still let through Ha, kind of like a Ha filter on the cheap. On it's own it colour shifts too strongly to use as One Shot Colour (OSC). But it can be used together with a regular shot to enhance Ha in a photo. Or it can be used in a bi-colour or tri-colour sequence for RGB. In fact doing RGB on the cheap with red, green and blue photographic filters is possible. It doesn't work as well as the genuine astro ones that have sharp cutoffs, but they do work and they do reduce LP and CA in the process. They also increase your workload.

Dusty comets don't mind a yellow, green or yellow green filter, especially if there is moonlight around. I have a pic in the solar system section at the moment taken through a Telesar YG filter. http://www.iceinspace.com.au/forum/s...d.php?t=124779

I used it for several reasons, 1 to reduce the impact of moonlight, 2 to reduce the CA of the 120mm F5 achro refractor and 3 it was good for the dusty comet. Colour shift was compensated for in camera for the most part.

Have fun with those photographic filters. I know I do!

ZeroID (Brent) · #4 14-08-2014, 07:07 AM

Hi Kevin, thanks for the info. Yep, fun to play with alternative ideas for sure. I was using the Astronomik CLS as I've mentioned but like others have posted it sometimes is a bit too harsh for the situation. I was trying to acheive a more level response across the spectrum with enough red cut to allow longer exposures. I have a range of other colours and might pop up to the photo shop for some more. At $10 for 5 it's cheap fun.

Also it may resolve my guiding problem just by defining the guide star target better so if that is the only useful outcome I'll be more than happy.

Unfortunately the clouds have been very onco-operative over the last few weeks and my health is a bit fragile so it is all a matter of conjecture at present. But fun to plan and gives me something to work on while the clouds pass over ..

ZeroID (Brent) · #5 02-10-2014, 04:59 AM

Finally got some weather that didn't fall out of the sky .. so another test.
This is a single frame in the Triffid\Lagoon region of 30 secs ISO 1600 on the SONY taken through the C80A filter about 5* from the moon, my finder was actually full of glare. Moon about 45% full.
The only processing for the second image was to correct the blue effect of the filter with a bit of curve and reduce size for IIS loading.
I think for those working against LP a simple C80A filter is worth experimenting with.

So, before and after.

cometcatcher (Kevin) · #6 02-10-2014, 12:56 PM

Hi Brent, the effect may have been more dramatic without a nearby moon. Moonlight produces a blue sky like sunlight does.

The Triffid nebula is extra tricky because it contains reflection and emission nebula from both ends of the spectrum (blue and red). Enhancing one side attenuates the other with a single bandpass filter.

A blue filter on the Pleiades might be the way to go in heavy LP skies.

ZeroID (Brent) · #7 03-10-2014, 05:35 AM

Hi Kevin, unfortunately the Pleiades hides behind our local volcano from my OB, too low to attempt. But I imaged a few other objects and have been playing with them. Obviously longer and more exposures are required to get more photons but now I have sorted my SCP and guiding I'll be happy to spend time on it.

LightningNZ (Cam) · #8 03-10-2014, 11:03 AM

I didn't get the CLS for 2 reasons:
1) It is harsh and you lose reflection nebulosity in a big way
2) I've been planning to do my photography from a dark sky site so something that dimmed stars a bit relative to nebulosity would still be kinda nice.

In practice I haven't escaped my back yard yet, but the Hutech IDAS filter I have and use in all my shots seems to do a fairly good job. I still get gradients from light pollution but I've gotten fairly good at processing those out now. As always your mileage may vary with usage.

EDIT: I should add this this thread of mine has image of the Lagoon and Trifid with 24 mins and 1 1/2 hour respectively exposure time with the IDAS filter. I will do a side-by-side, with and without filter, of the M42/Running Man area when I get a chance. http://www.iceinspace.com.au/forum/s...d.php?t=126663

cometcatcher (Kevin) · #9 03-10-2014, 12:23 PM

It's funny how the CLS produces such a strong blue colour cast but isn't very sensitive to blue reflection nebula.

It is a tricky filter to tame.

I want to try the UHC-E as well as the IDAS filter, and get a fast newt, and an electric bike... But Bill has moved in for a while so I'm stuffed.

I was wondering about those cheap light pollution filters and how well they work, like the $50 ones on ebay.

ZeroID (Brent) · #10 06-10-2014, 08:09 AM

I'm still waiting for some decent cloudless skies and a suitable moonage to give the 80A a proper test. The proof will be in the pudding.

cometcatcher (Kevin) · #11 07-10-2014, 12:49 AM

I thought I'd deliberately try some experiments in bright moonlight to see if any of the filters help with a star cluster, M41. I wanted to see which filter blocked the most moonlight and showed the most stars. Note that it would be a different result for nebula. I also used an achro refractor for the experiments to see which filter had any effect on chromatic aberration. All pics with each filter are 1 minute each at ISO 400. I didn't test blue as that would be no good in moonlight, but then again it is a test so I should have. Oh well next time... I'll do some nebula test next time as well.

The pics I've uploaded are clear (no filter), 81B light yellow, Baader Semi-Apo filter, K2 yellow and YG yellow green. I used in camera colour balance and a little post processing to make the colour balance neutral.

Moonlight has a blue colour cast and most of these filters reduced that. I was surprised that out of all the filters only the yellow showed fainter stars and the effect was minimal. But this was with the same exposure. Given that filters reduce the amount of total light, I could have exposed longer with the filters before the Moon fogged the frame. In effect this means that most of the filters I tested would go deeper in moonlight for the same amount of sky fog making them worthwhile using for this application.

The next obvious advantage is the amount of CA some of them reduce. The 81B and Semi-Apo had the least effect on CA, while the yellow and yellow / green has the greatest effect. In fact the yellow had the best result of reducing CA while cutting through the moonlight and showing faintest stars, nearly making this achro refractor semi apo like, far better than the much more expensive Baader semi-apo filter!

cometcatcher (Kevin) · #12 07-10-2014, 01:21 AM

This is a strip of the above filters. In this comparison the filters have been colour balanced but not the exposure. This shows how much they darken the moon lit sky by, compared to each other and to no filter. No filter (clear) is clearly washed out with the Baader Semi-Apo producing the most contrast.

ZeroID (Brent) · #13 07-10-2014, 12:28 PM

Whoh!! Looks like another trip to the camera shop is in order although I know I've got a Yellow\Green. K2 eh ? Lemme see now, $10 = 5 filters. $20 = 10 filters. Obviously more experiments are in order.
There is of course the other option of multiple exposures with several filters then combining after to get the best of both worlds.
I always see a lot of red in my LP background hence what appears to be best result with the 80A light blue.
The yellow I definitely have to try through the ED. I also now have the SW 80mm f11.3 to experiment with. Been quite impressed with that for low CA in an Achro although I haven't pushed it .... yet.

Excellent ...