advise on filters

[jeelan (Jeelan)]

hey all,

I'm looking at getting a couple of filters to enhance my DSO viewing experience.

Looking at an OIII and H band filter (narrow band, I think), and PERHAPS one Broadband filter.

My question is this - Where is the best place to attach a filter? I know general consensus is on the eyepiece itself, however I'd like to know if filters can be attached to the nosepiece of a star diagonal?

the visual back on my SCT is a Baader Click lock, so my star diagonal has a slide in nosepiece. the inside of this tube is threaded to allow for 48mm filters.

Can i screw a filter onto this?

what are the disadvantages of attaching a filter this way?

Just wondering as I've not seen it discussed.

cheers

Jeelan

[Atmos (Colin)]

If your diagonal has a thread you can put a 2" filter onto there but most I believe just screw it into the eye piece.
It looks like all but one of your eye pieces are 1.25" so you could just get those and save yourself the extra expense of buying the 2" threaded filters.

[ausastronomer (John Bambury)]

Hi Jeelan,

In your situation screwing a 2" filter onto the nosepiece of your diagonal should be a good option. In a small number of cases this isn't a great option as filter performance can suffer, but in your case it should work well, without any negatives other than cost and a few positives.

Filters generally work best when the light cone passes through the centre of the filter and the light cone itself is narrow. Some filter manufacturers publish recommended F-ratios of telescope for use with their filters. This might sometimes be in the range of F4 to F15. The reason for this is that the light cone widens as you get further away from the field lens of the eyepiece and the rate the light cone widens increases with a decreasing F-ratio.

Consequently, the light cone will be at its narrowest when the filter is used on the bottom of the eyepiece. The bottom of the star diagonal is a couple of inches further away and the light cone will be wider than it would be if the filter was used on the bottom of the eyepiece. However, with your scope being an F10 SCT it shouldn't make too much difference. I have used my filters on the bottom of my focuser drawtube on both my F4.5 newtonians for over 10 years and I haven't seen any drop in filter performance. The filter in my situation is closer to the bottom of the eyepiece than it would be on the bottom of a star diagonal but the scope is much faster. I can only imagine this being an issue if you used the filter on the end of a star diagonal with an F4-F5 refractor and even then it may still work ok, depending on the filter.

I would recommend the DGM Optics NPB filter as being the best choice for all round Deep Sky use. This is a UHC filter (narrowband) with a very tight band pass and it works very well on most targets which benefit from a UHC filter or an OIII filter. That one filter would cover 98% of your deep sky observing needs.

The big advantage of using a 2" filter on your diagonal is that it will work equally well with 1.25" or 2" eyepieces and you can change eyepieces without having to unscrew and rescrew the filter.

Cheers,
John B

[wavelandscott (Scott)]

I would also speak highly of the DGM Optics NPB filter.

[ab1963 (Andrew)]

I have the DGM nebula and oiii filters and IMO are better than the baader filters I had

[dreamstation (James)]

Best/cheapest place to buy DGM filters?

For you blokes who have had the pleasure of using various different brands of filters, which would you suggest has the best price/performance? DGM, Astronomik, Lumicon, etc?

[jeelan (Jeelan)]

Hi guys,

thanks very much for the feedback.

John your comments below nail the reason I've been thinking about screwing onto the diagonal. If I'm doing a deep sky observing session, its much easier to keep the filter on the diagonal rather than each eyepiece.

Originally Posted by ausastronomer

The big advantage of using a 2" filter on your diagonal is that it will work equally well with 1.25" or 2" eyepieces and you can change eyepieces without having to unscrew and rescrew the filter.

That being said, thank you for explaining the science behind this - I'm aware now that screwing the filter onto the diagonal isn't always going to work. I've been doing a fair bit of visual observation with the 80mm APO - i'm guessing that being a faster scope Im better of getting a 1.25" filter for this and screwing into the eyepieces? I'm using the Pentax XW with this scope.

cheers
Jeelan

[ab1963 (Andrew)]

it's the DGM for me,never tried Astronomik nebula filters but have the cls light pollution filter and is ok, the DGM nebula and oiii filters just make nebula stand out and with great detail ,have bought and sold a few but these are keepers at a very reasonable price point ,don't profess to be no expert just going by what i see in the eyepiece and are the only 2 i have kept ,bought mine used on iis and the only reason he sold them was he went solely into imaging,I would think direct to DGM optics would be the way to go for the purchase and sometimes they offer slight seconds at a good saving

Originally Posted by dreamstation

Best/cheapest place to buy DGM filters?

For you blokes who have had the pleasure of using various different brands of filters, which would you suggest has the best price/performance? DGM, Astronomik, Lumicon, etc?

[jeelan (Jeelan)]

Just been on DGM's website and they're running a combo promotion.

the 1.25" and 2" NPB filters for US$205. I'm guessing that includes shipping as price is different for US customers, Canadian customers and international customers.

cheers
Jeelan

[inertia8 (Australia)]

I was about to post the same thing, I have been researching this very topic and following a thread on cloudynights and this forum, along with John's thoughts decided to pull the trigger on the DGM NPB filter combo, reasoning that for the little extra I can use the 1.25" with my grab and go camping scope which only takes 1.25".

I can confirm that it is $205 including shipping, the PayPal link brought up the invoice as $190 + $15 for shipping.

[Allan]

Narrow band and line filters work particularly well at large exit pupils, which generally means 2" eyepieces. My biggest suggestion is to get a 2" filter from the get go, it will save you money when you realise you need to upgrade from a 1.25" filter.

DGM, Lumicon, Astronomic, Baader, they all make good filters. The pass band on Astronomic filters is wider than the others, so contrast is a little less, but the extra light may be more appealing to some people with smaller scopes. Baader updated their OIII filter to 10nm pass band, and in performance is almost identical to the Lumicon OIII. The Lumicon is still the best OIII (my opinion), but as a value proposition the Baader OIII is hard to beat.

The Lumicon UHC and DGM NPB are the top narrow band filters going, and one of those should be on everyone's buy list. I owned both for quite a while and doing comparisons was a lot of fun. They are more similar than dis-similar so I eventually sold one off. You could buy either with confidence, knowing you have a great filter.

[ausastronomer (John Bambury)]

Originally Posted by jeelan

I've been doing a fair bit of visual observation with the 80mm APO - i'm guessing that being a faster scope Im better of getting a 1.25" filter for this and screwing into the eyepieces? I'm using the Pentax XW with this scope.

cheers
Jeelan

Hi Jeelan,

The ED80T is an F6 refractor and it more than likely will work fine with the filters screwed onto the diagonal. I would certainly be trying that option first, before discarding it.

Cheers,
John B

[ausastronomer (John Bambury)]

Hi Allan,

Originally Posted by Allan

Narrow band and line filters work particularly well at large exit pupils

That depends on what you define as a large exit pupil. IME Narrowband and Line filters work best with an exit pupil from 2mm to 5mm. I would call that medium sized not large. Many people (and several filter manufacturers) regard a 4mm exit pupil as the optimum with these filters, although my preference is sometimes to go a little smaller than this.

Originally Posted by Allan

My biggest suggestion is to get a 2" filter from the get go, it will save you money when you realise you need to upgrade from a 1.25" filter.

Agree 100%. 2" filters give you a lot more versatility than 1.25" filters and importantly the light cone will always pass through the filter further away from the edges of the filter where the substrate may not be quite as well polished and flat.

Originally Posted by Allan

DGM, Lumicon, Astronomic, Baader, they all make good filters. The pass band on Astronomic filters is wider than the others, so contrast is a little less, but the extra light may be more appealing to some people with smaller scopes.

I currently own the Astromics UHC, DGM NPB and the Lumicon UHC (all 2"). I have also used the Baader UHC-S, Thousand Oaks LP-2, Meade 908N, Orion Ultrablock and the Televue Nebustar. The Astronomics is German made and IMO has the best construction quality of all the filters I have owned and have used. It also has the highest price tag. I previously owned a late 1980's model Lumicon UHC which delaminated at about 15 years of age. It also had a wider bandpass than the more recent Lumicon UHC.

You're correct in your comment about the Astronomics UHC having a wider bandpass than the DGM NPB and the Lumicon UHC, but it's suitability doesn't necessarily relate to smaller scopes. It relates to all apertures and it works better on some targets regardless of aperture. Because the Astronomics has a wider bandpass than the others it allows a lot more of the background stars to shine through and on some targets provides an aesthetically nicer image than either the Lumicon or DGM NPB. This is noticeable on emission nebula with young embedded star clusters. Like for instance M42 (Orion Nebula), M8 (Lagoon), IC2944 (Running Chicken nebula). It also allows the central star to remain visible on a couple of planetaries, whereas the Lumicon UHC and DGM NPB knock the central star out. The Astronomics UHC filter (and the DGM NPB) as well as having a bandpass straddling the H-Beta line at 486nm and both OIII lines at 496nm and 501nm also have a very wide bandpass straddling the H-Alpha line around 656nm. While this isn't a lot of benefit on planetary nebula and a lot of other targets it is a benefit on some emission nebula which emit in the high red end of the spectrum. For instance the Astromic UHC and the DGM NPB filter work much better on the Horsehead Nebula (B33) because of their H-Alpha bandpass, than the Lumicon UHC, as IC434 (the illuminating emission nebula) emits in the high red end of the spectrum. On many other targets the DGM NPB and the Lumicon UHC provide more detail of the nebula itself than the Astronomiks. This is the reason I own and use all 3 of my narrowband filters, as well as my OIII's and H-Beta filters.

Originally Posted by Allan

The Lumicon UHC and DGM NPB are the top narrow band filters going, and one of those should be on everyone's buy list. I owned both for quite a while and doing comparisons was a lot of fun. They are more similar than dis-similar so I eventually sold one off. You could buy either with confidence, knowing you have a great filter

Notwithstanding that on over 90% of targets suited to a narrowband filter the DGM NPB and Lumicon UHC perform similarly, if I could only keep one filter of the 3 narrowband filters I own it would be the DGM NPB due to its greater versatility having the H-alpha bandpass which the Lumicon doesn't have. If I had to get rid of one of my 3 filters it would be the Lumicon every time because between the DGM NPB and the Astromics UHC those 2 cover everything the Lumicon UHC covers.

Cheers,
John B

[ausastronomer (John Bambury)]

For those interested in reading a little more about the different types of filters I gave a presentation on "Visual Astronomy and the Use of Filters" at IISAC 2011.

A copy of my IISAC presentation can be found on the IIS website here.

Cheers,
John B

[Allan]

John, you've got a lot of nice filters and certainly have all the observing options covered.

[Don Pensack]

In terms of filters for nebulae, there are several types:
1) the broadband. Sometimes called a CLS or an LPR filter and often sold as a light pollution filter. These work best in dark skies, where the contrast pick up is small but can be useful on some objects. They don't work well in heavy light pollution because scattering in the filter can make light in the field worse.
2) the narrowband. These transmit the H-Beta line and the O-III lines in the spectrum where almost all emission nebulae transmit the vast majority of their energies. Actually, nebulae emit more energy in H-alpha wavelengths, but our eyes are insensitive to light that deep in the red, so having an h-alpha transmission in a filter doesn't help us see them at all. If you get just one filter, the narrowband is what you get. They go by names like UHC, Ultrablock, NPB, LP-2, etc. The best ones transmit higher than 90% at all 3 lines and have bandwidths of 23-28nm.
3) O-III filter. These restrict the bandwidth to only the O-III lines and enhance better the nebulae with predominantly O-III emission, like most planetary nebulae, some supernova remnants, or Wolf-Rayet nebulae.
Good ones transmit >90% at both relevant O-III lines and have bandwidths of 12-15nm. Many O-III filters transmit primarily the 501nm line and are oriented to the astrophotographer.
4) H-Beta filter. This filter passes the hydrogen beta line in the blue only. There are a handful of nebulae where this filter yields the highest contrast, like the Horsehead. This would be the very last nebula filter you'd buy since it has the smallest number of objects it works on better than the other nebula filters. Good ones transmit >90% at the 486nm H-Beta line and have bandwidths from 7-10nm.

Recently, there have appeared a lot of nebula filters that have wider bandwidths. They still work, but yield lower degrees of contrast. They may be preferred if the observer wishes to see more stars in the field, and/or they may be preferred if the scope is small (under 150mm) because they dim the overall field less.

There are a lot of filter response graphs now posted on-line (many different sites). I encourage observers to look at the graphs, look for high transmissions, narrow bandwidths, and coverage of all the necessary spectral lines.

[ausastronomer (John Bambury)]

Originally Posted by Don Pensack

Actually, nebulae emit more energy in H-alpha wavelengths, but out eyes are insensitive to light that deep in the red, so having an h-alpha transmission in a filter doesn't help us see them at all.

Hi Don,

You read this comment repeatedly on astronomy forums but unfortunately it's incorrect. The human eye will generally have a sensitivity range from 400nm to 700nm which includes the H-Alpha line at 656.3 nm. Some individuals have a sensitivity up to 730nm to 750nm. Whilst red lasers usually emit at 632nm (the cheap ones emit up to 650nm), visible red light has a wavelength of around 650nm, which is just under the H-alpha line.

Here is a link to some commentary on this on the NASA website.

http://science-edu.larc.nasa.gov/EDD...or_Colors.html

Of course this varies from individual to individual and there is a large percentage of the population that will in fact not be able to see as deep into the red as others. I have seen this situation manifest repeatedly on the observing field. Andrew Murrell and I can both see quite deep into the red. There have been quite a few targets that we have been able to see reasonably easily because of our deep red sensitivity yet other skilled observers have been unable to see these targets, or parts of them, because of their insensitivity at the high red end of the spectrum.

Cheers,
John B

[Don Pensack]

Photopic vision ranges from as low as 425nm to as high as 750nm.
But for most people at night, peak scotopic sensitivity is 507nm +/-50nm with response at 400 and 600 as low as 1 or 2%.
What changes this is when light is bright enough to start activating cones, and vision becomes mesopic (especially the L cones for red sensitivity).
Most nebulae, however, are not bright enough to activate the cones. I think M42 is and a fairly good handful of planetary nebulae, but generally, if an H-alpha filter is used on even as bright a nebula as M42, most people *barely* see any light except in the Regio Centrale area and even then it is exceedingly dim.
When people see red in nebula filters with a red response, the red they are seeing is often a peak about 625nm that is common in nebula filters that have a two-peaked response in the red. They see red shadows in the star images. In the nebula filters with a deeper response, say 650nm and below, people do not report any red in the star images and do not not reds being seen.

In general. But not necessarily in specific. There are individuals, usually those using very large scopes, who report a lot of red visibility in nebulae without using filters. For those individuals, having a red transmission at the H-alpha wavelength can be useful in picking up the reds. I am one of them. But very few other people have any scotopic sensitivity at that wavelength, so it is fair and reasonable to say that having a H-Beta transmission will help see nebulae better than H-alpha transmission.
In fact, the H-Beta wavelength is near the peak sensitivity for scotopic vision.

Every nebula that emits energy at H-alpha also emits energy at H-Beta, just at a substantially reduced level.

More information on the human eye response can be found here:
http://hyperphysics.phy-astr.gsu.edu...on/bright.html
It's an easier site to get to the data charts easily than most, which is why I list it.

[ausastronomer (John Bambury)]

Originally Posted by Don Pensack

Photopic vision ranges from as low as 425nm to as high as 750nm.
But for most people at night, peak scotopic sensitivity is 507nm +/-50nm with response at 400 and 600 as low as 1 or 2%.
What changes this is when light is bright enough to start activating cones, and vision becomes mesopic (especially the L cones for red sensitivity).
Most nebulae, however, are not bright enough to activate the cones. I think M42 is and a fairly good handful of planetary nebulae, but generally, if an H-alpha filter is used on even as bright a nebula as M42, most people *barely* see any light except in the Regio Centrale area and even then it is exceedingly dim.
When people see red in nebula filters with a red response, the red they are seeing is often a peak about 625nm that is common in nebula filters that have a two-peaked response in the red. They see red shadows in the star images. In the nebula filters with a deeper response, say 650nm and below, people do not report any red in the star images and do not not reds being seen.

In general. But not necessarily in specific. There are individuals, usually those using very large scopes, who report a lot of red visibility in nebulae without using filters. For those individuals, having a red transmission at the H-alpha wavelength can be useful in picking up the reds. I am one of them. But very few other people have any scotopic sensitivity at that wavelength, so it is fair and reasonable to say that having a H-Beta transmission will help see nebulae better than H-alpha transmission.
In fact, the H-Beta wavelength is near the peak sensitivity for scotopic vision.

Every nebula that emits energy at H-alpha also emits energy at H-Beta, just at a substantially reduced level.

More information on the human eye response can be found here:
http://hyperphysics.phy-astr.gsu.edu...on/bright.html
It's an easier site to get to the data charts easily than most, which is why I list it.

Hi Don,

That all makes perfect sense. I would suggest that Andrew Murrell and I also have extended red wavelength visibility response. Andrew probably a bit more than me in fact. This would explain why we sometimes see some nebula and features in some nebula more clearly than a lot of other observers. It probably also explains why I see features in some targets with a UHC filter having a H-alpha bandpass, that I cannot see in UHC filters that don't have the High red end bandpass. Sometimes I think I just take it for granted that everyone else is seeing exactly what I see at the eyepiece and obviously this isn't always the case.

Cheers,
John B

[jeelan (Jeelan)]

thanks for all the info on this gents,

I've just ordered the 2" NPB and OIII filters from Dan Mcshane at DGM

cheers
Jeelan