Question about Deep Sky or UHC filters

[barx1963 (Malcolm)]

I have a number of Nebula filters. One is a Bintel Branded (GSO) UHC 1.25" filter, the other 2 are Lumicon Deep Sky 2" filters. I have 2 of the Lumicon ones as one came with the scope I just acquired so I am thinking of selling one.
One thing I have noticed is that I often see a little red spot next to stars in the field when I use these filters. Do any others see this also or is it a fault with the filters?
I had noticed it with the GSO one but sort of assumed as these are less expensive they may not be as good, but I have also noticed it with the Lumicons.

Malcolm

[mental4astro (Alexander)]

Malcom, what you are seeing has to do with the process used in producing the filters. The older method sees the red spot around stars, & if I remember correctly, how much is seen depends on your eye placement or how square the filter is, even the EP being used - I can't remember which. Nothing wrong with the filters. I have a Lumicon DSF 1.25", & I see it too.

Newer filter production techniques have this thin film phenomenon eliminated. John Bambury I'm sure will be able to shed more light on this

[barx1963 (Malcolm)]

Thanks Alex
Much appreciated.

Malcolm

[mental4astro (Alexander)]

I found an explanation for this in the blurb from DGM Optics that makes exceptional astronomy filters:

"Specifications


This DGM Optics custom designed filter is well suited for a variety of telescopes and objects. The filter design stresses maximum optical throughput utilizing state-of-the-art optical thin-film designs and materials.
This filter is a “first surface” hard oxide thin-film and is much more durable and long lived than laminated “soft film” designs used by several famous makers. They are also much less prone to the internal reflectance problems that plague laminated filters. Laminated filters can produce a very undesirable “doubling” of stellar images often with one of the doubles being red. Contrary to popular belief, this image doubling is not due to the red sideband component that many nebula filter designs produce, but in many cases is due to laminated glass not being exactly parallel to the protective cover plate after lamination. Many laminated filters display this annoying characteristic.
This filter averages around 1% transmission (optical density of 2) through the rejection region from 540 to 590nm, and average transmission greater than 90% in the passband, and greater than 75% at the 656.3nm H-Alpha line. The design yields a filter with a very symmetrical shape and high optical throughput, for maximum enhancement of nebula with a minimum of loss of stars in the field of view.
The substrate is B-270 and has measured total wavefront values of .25 waves. The combination of first surface optical thin-film technology and a polished, flat substrate enables this filter to be used with high magnification, a real plus for small planetary nebula."

The parent compay of DGM, Omega Optical, makes astronomical filters for other big brands. This manufacturing capacity allows them to experiment and produce niche astronomical filters, including the famous "NPB" ultra high contrast filter.

[ausastronomer (John Bambury)]

Hi Malcolm,

The information Alex has quoted from the DGM website pretty much explains things. Generally this occurs with "sandwich" or "laminated" filters, where the filter substrate is sandwiched between 2 flat highly polished pieces of glass. If all surfaces are not perfect, internal reflections occur within the filter. The reasons why newtonian telescope mirrors are "first surface coated" and not coated on the back with light passing through the glass, are similar. Some years ago this was considered the best way to manufacture these filters as it was felt this would make them more durable by sandwiching the filter substrate between 2 layers of glass and cementing it all together. At this time the filters which were externally coated were not that durable as the coatings in some cases could be prone to scratching. Consequently, most of the better filters were "sandwich" filters a decade or more ago. Unfortunately these sandwich filters did not last forever as they often delaminated after several years use being subjected to dew, moisture and dramatic temperature fluctuations. In addition they could sometimes exhibit minor internal reflections.

Turn the clock forward 10 years. Most of the better quality filters made today are externally coated using hard oxide dielectric coatings. These will last forever if treated carefully as the oxide coatings are very hard and scratch resistant. They also perform better optically than the older sandwich filters as the high quality ones eliminate the possibility of ghosting and other aberrations

I am not sure how the GSO filters are made, or how old your Lumicon filters are, but I would guess " a good few years".

What I can tell you is the newer Lumicon filters, the DGM Optics (made by Omega Optical) and the Astronomiks filters, all of which I own, are all dielectric surface coated filters of extremely high quality. None of them suffer internal reflections and they perform flawlessly.

I have the highest regard for the DGM filters manufactured by Omega Optical. They are superb mechanically and their passband characteristics are perfectly matched for their intended uses. I rate the DGM NPB filter as the best UHC filter currently available anywhere. The upside is they are very competitively priced with the 1.25" NPB costing about $85 landed in Australia and the 1.25" OIII costing about $80 landed in Australia. This is about the same price as what a Taiwanese made GSO UHC filter, of unknown quality and construction will cost you. Why delve into the unknown when you can buy the best for the same price? That's not downgrading GSO products BTW, I own a 10" GSO newt which is excellent after I made a lot of mods to it. GSO have put some very good quality products into the hands of amateur astronomers at very competitive prices but in this case you can buy the best filters (DGM) for the same price as a GSO filter. I am sure if you could buy a 12" SDM scope for the same price as a 12" GSO scope, GSO wouldn't sell a lot of 12" scopes.

Cheers,
John B

[barx1963 (Malcolm)]

Thank you John for taking the time for such a comprehensive reply.
I agree with you about GSO gear, I have had a 12" GSO dob for over 3 years now and think they are the greatest thing for the amateur observer, putting good optics and aperture in packages that almost anyone can afford.
With the images in the filters, your explanation was much as I suspected, and I was considering upgrading to better filters but was concerned that I would end up with the same problem. When I was only using the 12" I wasn't that concerned as the filter didn't make that much of a difference, I mainly only used an OIII to confirm PNs. Now with the 20" I notice that the DS filter makes a considerable difference to nebulosity. I tried out the filter on the Flame nebula the other night and it really popped!

Anyway, thanks again and I will have to think about my next step!

Malcolm