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Astronomik LRGB Type IIc Filters


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Introduction

I have been using an Astronomik H-a filter for narrowband imaging in my Meade DSI-Pro cameras for over a year. I recently added their O-III filter for bi-color narrowband imaging. I have been very impressed with the quality of the data I have been able to acquire using those filters. The images are always smoother and cleaner than the ones I make using the LRGB filters that came with the camera. I have seen many wonderful images made by my peers using filters from alternate manufacturers, but never with the Astronomik LRGB set. Therefore, when the opportunity to evaluate their latest LRGB filters presented itself, I jumped at the chance.

It turns out that Astronomik is not a company. Rather, it is a brand name reserved for filters produced through a joint venture between the Astro-Shop (www.astro-shop.de) and Gerd Neumann Jr. (www.gerd.neumann.net). Gerd is a mechanical engineer specializing in astronomy accessories. The Astro-Shop is owned and operated by a long-time friend of Gerd. A visit to Gerd’s website reveals the breadth of accessories he produces. You will find Astronomik filters (obviously), adapters for various Webcams and cameras, telescope making materials, and imaging accessories. Make sure you see “The Elephant” tripod he produces. It is a remarkable sight.

Shipping/packaging

My set came directly from Gerd, so my packaging was probably a little different from what is typical for third party vendors. The filters come in one well-padded plastic case. That case was enclosed in a well-padded envelope-like box. The outer box was a little roughed up during the journey from Germany to my home in New England, but the contents were completely undamaged.

Since my set came directly from Gerd all markings are in German. I have family in Germany so I took a special liking to them immediately.

Installation

I used these filters in both my Meade DSI-Pro II and an Orion StarShoot Deep Space Monochrome Imager during this evaluation. I wanted to compare the filters in both of the imagers since they use the same chip to see if there was a similar effect on the image quality in both imagers.

I installed the filters in the filter bar for the Meade with almost no effort. The threads on the filters match the filter bar smoothly and the filters seated at the same level with little effort. I am not a fan of the design of the filter bar, but at least the filters went in with no trouble.

When it came time to install the filters in the filter wheel that came with the Orion imager I found that the threads did not match quite as easily. However, the filters did install completely and with careful effort, I was able to seat them all to the same level in the wheel. When I was done using the filters in the wheel, I tested the Orion filters in the Meade bar and found that they did not match there as well as the Astronomik filters. I was hoping that would be the case as otherwise it would have meant I had damaged the threads in the Astronomik filters when using them in the bar. Thankfully, the filters still matched easily and smoothly with the filter bar, so there was no damage. That would have been very disappointing.

First light

My first light with these filters was a quick trip to two of Leo’s Trio of galaxies, M65 and M66. I chose this target because I had just started working on it using the filters that came with my Meade DSI-Pro II. Image 1 shows the results with my Meade filters. It is poorly framed but it was the first attempt and I was just experimenting to determine appropriate exposures, so I did not really care about the framing.

Image 1 – M65 & M66 with Meade RGB filters

The day after I took that image the Astronomik filters arrived, complete with as many clouds as Gerd could fit in the package. Thankfully, it was a small package and I only had clouds for three nights. I unpacked the filters, removed the Meade filters from the filter bar and installed the Astronomik filters. When the clouds cleared, I took a set of images of the same subject using the exact same exposures and settings.

Image 2 shows the results. I was amazed at the difference. The detail in the galaxies is significantly improved with these filters. The stars are much tighter and crisper. The color balance is extraordinary. The data from the Astronomik filters required less processing to produce a far superior image. Bear in mind that both these images are the result of only 28 minutes in each channel and there is no Luminance data in either of them.

Unfortunately, that first night was followed by many nights of rain, snow, clouds, high winds and generally awful weather for astronomy. Great reading weather, but lousy for astronomy.

Image 2 – M65 & M66 with Astronomik RGB filters

A little examination of the first image shows that the color balance is a little off, as expected. I used the same number of two-minute exposures with each filter. The camera is not as responsive to green and blue, so I had to work harder to balance the colors in post processing than I would have liked. To address that issue, I did some research on how to determine the appropriate method for calculating the proper ratios for each channel.

I found many references to the G2V star calibration method which basically comes down to picking a star that is similar to our own Sun in color and then adjusting your color ratios to make it look white. It sounds simple enough in theory, but I found it nearly impossible to identify appropriate stars without purchasing some software.

Fortunately, I found another method that is quite simple. It requires an 18% grey card, available at most camera shops, and a full or nearly full moon. You make images of the card of equal exposures through each filter using the moon for illumination. You import each of them into Photoshop or whatever post processing software you use and merge them to form and RGB image. It should look pretty much like the grey card although it might be a little lighter or darker. Then you measure the values of each channel in one spot. The Color Sampler tool in Photoshop is great for this. A little simple math with those values will determine the correct ratios for each filter in a given camera. I did this for an Orion StarShoot DSMI-II and calculated a ratio 1:1.5:1.6 for the Orion. So now, I just use the red filter as the basis and multiply the number of exposures there by the calculated value for the green and blue filters and off I go.

Using those results, I installed the filters in the Orion camera and pointed it M3, a globular cluster. Image 3 shows the results of 40’, 26’, 40’, and 42’ of LRGB data respectively.

Image 3 – M3 through Orion StarShoot

Image 4 is of a smaller globular and shows the results of 60’ each of LRGB taken through my Meade DSI-Pro II. I have not performed the calibration test on my Meade imager yet, so I just used equal amounts of data in all channels and adjusted the balance in Photoshop. Both imagers use the same imaging chip, so the field of view and sensitivity of the chip is nearly identical.

Image 4 – NGC 5024 through Meade

Overall Impressions

These filters are very well made. Astronomik filters are dichriotic interference filters made to extremely precise tolerance. The attention to quality is clear at first glance. The surfaces are smooth and hard. Gerd Neumann tells me that you would need something like sand paper to damage them. I do not know that I would go that far, but they do seem pretty tough. I cleaned them a few times while using them in the Meade filter bar and they show no ill effects.

The quality of these filters becomes apparent at first use. The background of images is much smoother and easier to process than has been the case with other filters I have used. Gradients are a fraction of the problem they were and are relatively easy to deal with. Perhaps the most striking improvement provided by these filters is that the stars are the same size in all color channels. Until now, I have had to deal with the stars in my blue data being significantly larger than the other channels. Correcting that is an exercise in compromise, where I have had to balance the processing such that there is little or now blue halo while maintaining enough blue in other areas to reveal accurate details.

One more detail of these filters that I find very helpful is that they are par focal. I cannot tell you how many hours I have spent adjusting focus when changing from one filter to the next. The most I have had to do with these is a minor adjustment to the focus. Once I seated the filters properly in, whichever holder I was using (bar or wheel) I no longer needed to adjust between filter changes. I still check, but I have not had to adjust when using it in my refractor and only a little bit when using a Newtonian. I suspect that is due more to the nature of the optics in the telescopes than the filters.

I feel these filters are among the best available on the market. However, they come at a significantly lower cost than their competition, currently $280 for the 1.25” LRGB set. In my opinion, that makes these filters a superior choice for your astronomical imaging requirements.



  • jimsmith and Jeff Struve like this


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