Friends,
As the 2016 AAVSO Transform Campaign is underway (please see relevant forum post https://www.aavso.org/aavso-transform-campaign?page=3 and https://www.aavso.org/transform), one of our colleagues suggested that perhaps this is a good time for those who are interested in purchasing photometric filters to do so. He also suggested that HQ could consider helping with such purchases, in a manner similar to the help we provided with gratings (in other words, contact vendors to explore group pricing of appropriate filters and ensure shipping/handling through HQ). If we manage to get a good deal, this could reduce the purchase cost. I have two questions for you:
1) I would like to get a sense on how many people are interested in purchasing filters. As always, more people (bigger order) would help with pricing negotiations
2) Are you interested in one filter (V or I) or multiple ones? If the latter, would they be the classic Johnson/Cousins filters, or a couple of Sloan ones (say, B,V, g’, r’, i’ … “other”)?
I am looking forward to your responses.
Best wishes – clear skies,
Stella.
Hola, Estella:
Me parece Una estupenda idea.
Yo en la Actualidad solo trabajo con el filtro V de Johnson.
Pero sí se puede CONSEGUIR Una buena oferta estaria Interesado EL Un Juego completo de B, V, R, I de laclase Que mejor se comporte con la fotometría de la AAVSO.
Un cordial saludo.
Teófilo.
Bonjour Stella,
Yes, I would like to use this opportunity to upgrade my set up. Presently I only have the the V filter, looking for adding B, R and I (diameter 2 inch).
It's a good thing and good news!
I have just completed the acquisition of five filters Bessel '(clear for focus, B, V, R, I) 1.25 inch. of the company
"Omega Optical ".
Much cheaper than the other usual companies but specs are okay.
(I had submitted to Mr. Henden before buying.)
I still have several tests to make yet but for now, I am very satisfied with my purchase.
And they can make custom filters!
It might be a good supplier for AAVO?
JBD
Hi Stella,
I've got two comments here. It seems to have become accepted in the amateur area that it's not possible to make U measures with CCDs or, alternatively, that these measures have little value. Both these ideas are incorrect.
Variable Stars South has several members who are working with professionals, largely with hot massive stars, but also on objects like L2 Puppis and other SR stars where the U filter reveals things like the presence of a hotter star in the system or strong emission features. The response is not ideal and is probably shifted somewhat to the red but this applies to most other filters, which is why correct transformation is essential. At present we're promoting observations of the imminent eclipse of V777 Sagittarii where the U, B and V depths are about 1.3, 0.3 and 0.08 respectively. Many of the stars with widely spaced eclipses like zeta Aurigae are similar.
The second comment relates to the UBVRI system and the Sloan system. In 50 years of photometry I've seen a number of systems, enthusiastically promoted by various people, which seem to have almost disappeared. Arne commented about this some time ago but I wonder what your thoughts are - will UBVRI JHKL etc, be still around in a few decades, or will Sloan replace it, or will both be in use?
I like the idea of helping observers to make more use of filters but which are the best in any one system? Various manufacturers seem to have different interpretations of the bandpasses and the quality seems somewhat variable. So an impartial review of many aspects relating to filtered photometry would be of interest to many people.
Regards, Stan
Stan,
You wrote " So an impartial review of many aspects relating to filtered
photometry would be of interest to many people. " YES, this is something I am really interested in. I have heard that depending on the process used to make them, some filters might deteriorate with time. I would like to know more about this. In the end I expect that the more you pay the better they are, but it would be nice to know all the pros and cons before ordering.
Damien
I presently have a set of Astrodon BVI 1.25" (dichromatic) and I & R Schuller (glass) filters, but would like a cheaper alternative to equip my 2nd scope's ST-8XE with.
When I started in photometry about 12 years ago, I purchase all glass (dyed) schuller filters, but my 1.25" and 2" V and B filters deteriorated within 4 years because of cystallization and cloudness in the glass; my I & R schuller look good but may also suffer from this.
Dichromatic filters dont seem to have this problem but cost 1.5-2X more than glass filters. These filters offer better cut-off of unwanted light; ie. the old R schuller let allot of the I cousins light through. Despite increased costs for the dichromatics, I believe these are the best for photometry because of their stability and cut-off accuracy. It would be nice to find a cheaper alternative to the Astrodons ($200 for 1.25" and $400 for the 2"/50mm) but their quality seems hard to beat.
Sometimes, a used ones can be found on Astromart, but not allot of photometric filter are circulating in the market as much as "pretty picture" filters like H-Alpha, O-III, SII, or basic Red, Green, Blue, and clear filters filters.
James
James,
I have gone through several sets of BVRI glass filters for my old ST7. The V goes first then the B and the R & I seem to remain pretty good. However, when I changed out I changed them all out. There have been some changes in the glass formula in recent years because the original became hard, perhaps impossible to get for some of the filters. and I was concerned that If I didn't buy a complete set they might no longer be par focal. I always bought Custom Scientific filters because the quality was high but the hygroscopic nature of the glass, particularly the V and B causes fairly rapid deterioration in the humid weather we have SE of Austin. I even tried keeping the filters in an air conditioned environment when not in use. That was a pain. It did extend the life somewhat, but still, every 4 to 5 years the deterioration started to become evident.
When I bought the STXL 6303 camera that required 50 mm round filters I went with the Astrodons due to lower losses, particularly in the B filter, better cut of that with a CCD makes them more like the J-C set used with a PEP, particularly in the I but also the R, and because the cost of replacing the larger sized glass filters every few year was a significant expense.
I think you are better off with the Astrodon dichromatic types. There may be other good manufacturers as well but you have to be sure that the transmission curve is as good AND they have eliminated red leaks. You may recall that the original version of the Astrodon filters had a red leak problem.
Brad Walter, WBY
I have a set of Schuller filters (C,B,V,Rc and Ic) I use, although the C filter is used most often.
I have had to restore the haze on the V filter twice in the ~17 years I've had them. (I must admit that for ~6 years I did not use the V because of haze on it - until I found out HOW to remove the haze.) Haze removal is simple. Here's how I do it::
0. Wear old clothes!
1. I set up a clean area covered with white butcher paper - shiny side up.
2. Remove the filter from the filter wheel.
3. Rub the filter with optical rouge until all traces of haze have been polished away.
4. Clean off the filter THOROUGHLY.
5. Clean off the table/bench where you were working.
6. Wash your hands. Twice.
7. Reinstall the filter. Button up the camera.
8. Shoot a set of transformation images. Use all filters.
If you were careful with the rouge, you won't have any red spots on the walls, light switches, dining room table or chairs to explain. There may be tell-tale blotches on your clothes, though! That's why step 0 is there.
Lew
Lew,
I have never owned the Schuler brand. But we have tried this on Custom Scientific filters. In our case the crust not only gets on the surface of the glass it gets between the layers of glass as well. That may be a peculiar phenomena to high humidity areas such as mine. On one occasion we even sent them back to Custom Scientific to see if they could separate the layers and repair the filters. They said that was not possible and the filters were beyond repair. If the crust is only on the outer surfaces, I think your repair should work, but that was not the situation with ours. It certainly is worth trying before buying a new set.
Brad Walter, WBY
Ouch. Astrodon looks cheaper now. Anyway, it is in the "Neighborhood" - and Don is a great guy! His filters are the best, they say.
Lew
Hi Lew,
A friend of mine in my local Society restored a set of Bessell BVRI filters for me. He experimented gingerly with solvents, succeeded in separating the glass elements, repolished and recemented the lot. Obviously it will depend upon what cement as to what solvent you have to use. Arguably it took quite a bit of soaking in said solvent before the glass elements just slipped apart (his description). I don't think its a process that would not be commercially viable - time and effort wise a new set of filters would be cheaper, but if you have the time, and its just your own, it'll save money.
You can see the filters before and after here.
The wet humid spring and summer here have played havoc with the hygroscopic glass in the V filter and we are about to try sandwiching it between the non-hygroscopic element and a clear glass to see if we can't get better protection from reacting with water, but the point is you can with time, effort and the right solvent, separate and clean up the glass elements.
- Carl.
The only filter I've had issues with is my Schuler V, and only on the outside on one side. Wow! what a difference you made!
I'd be interested in learning which solvent he used, and how he got them glued back together.
Thanks for sharing your experience.
Lew
Hi Lew,
I've just got off the phone to my friend Peter. Everything he did will of course only apply to the cement that was originally used on the filters he restored for me.
Be warned, there was a little bit of the two of us trying to remember exactly what was done and in what order. Alas our past selves did not take the notes that our present selves need to be sure of everything our past selves did!
Firstly, the filters were soaked in white vinegar in case the cement was epoxy based. Secondly acetone. The white vinegar worked for a couple of the filters without any additional work.
The other two filters Peter used a safety razor blade to gently press into the edge at the cement layer between the glass elements. He did not lever or pry and was keen to emphasise that common sense must apply! He was simply endevoring to expose new cement to the white vinegar/and or acetone - sorry our respective memories are a bit vague on which exactly when and for which of the four filters.
Having separated the elements, Peter polished them to varying degrees. I seem to recall he used a coarse grind followed by fine polish so he could be sure to be rid of any of the old cement that was proving difficult to remove.
Then he recemented the glass elements into their original configurations and remounted them into their filter cells.
Cheers.
- Carl.
Hi Lew,
A well-known professional astronomer suggested with a similar problem that toothpaste worked well and it's easier to come by and washes off easily.
Regards, Stan
Sounds preferable, Stan. No red smears on the furniture that the wife could use as an excuse to buy another set!
Thanks for the tip.
Lew
Stan asked, "will UBVRIJHKL etc, be still around in a few decades, or will Sloan replace it, or will both be in use?" Sloan certainly is the front-runner for most professional photometry at this time - SDSS, PanSTARRS, LSST are all using Sloan passbands. APASS and others are providing calibrated Sloan magnitudes for stars across the sky. The AAVSO does not fully embrace the Sloan system yet - while such photometry can be submitted to the AID, it cannot be displayed with the Light Curve Generator.
The Johnson/Cousins system has been around for 60 years, and doesn't seem to be going away anytime soon. Much of the variable-star photometry is being done in this system, which has the advantage of matching visual photometry reasonably well for most variables for archival continuity, and also matches the "G" filter found on most DSLRs. B and V give more spectral resolution than does the Sloan g' filter that covers the same wavelengths.
So both are viable systems. Until the AAVSO fully supports the Sloan system, I don't see an urgent need for amateurs to switch. The exception would be if a researcher asks specifically for photometry using Sloan filters. If you have a big filter wheel with spare slots, then adding some Sloan filters would be great, but for the current moment in transient astronomy, I'd consider Johnson/Cousins filters as having higher priority. As for whether Johnson/Cousins will be around in a few decades - your crystal ball is as good as mine!
Arne
Arne said:
That is true, but just to be clear, the only AAVSO tool that does not currently support obervations made using Sloan filters is the Light Curve Generator. Observations made with Sloan filters can be plotted perfectly well using VStar (https://www.aavso.org/vstar-overview) and Zapper (https://www.aavso.org/zapper) and they are included in data obtained using the Data Download tool (https://www.aavso.org/data-download). As Arne said, these data can also be submitted to the AID through WebObs.
For an example of a star in our database containing Sloan filter data, check out BN Ari.
-Sara
Stan mentioned that his group was doing U-band photoelectric work several decades ago, and that there is still professional interest today. I fully agree with him on this. It is possible to do U-band photometry with a commercial CCD camera. It is harder than, say, BVRI photometry, because of the lower throughput, some atmospheric complications, and the care necessary to ensure no light leaks are occurring in your system.
APASS is currently doing Sloan u' photometry as part of its bright star extension. V-band exposures are 10 seconds for that extension, and u' exposures are 180 seconds. The u' exposures do not go as faint as the V-band exposures, which tells you right off that "ultraviolet" photometry can only be done on the brighter stars in your observing queue. It is certainly worth experimenting with if you have extra slots or are working with an astronomer who needs the far-blue passband.
We're using u' rather than Johnson U because we feel it is better defined; the u' filter is shortward of U and better measures the Balmer jump; and the professional community is in need of bright-star calibration in that passband. On the other hand, Johnson U is a wider bandpass and a little redder, so exposures are 2-3x shorter than for Sloan u'.
In order to compare with existing PEP photometry, there is a slight preference for the Johnson U if you were purchasing filters. Since you will be working with bright stars, there is a fair amount of existing UBV photometry in the literature for comparison stars. Either bandpass will give you some very interesting astrophysical information.
Arne
Hi Arne,
Thanks for all that - things are now somewhat clearer. And whilst my UBV photometry ceased about 2000 there are now some VSS people doing it. My photometer has a few problems which are proving difficult to fix but I'm now using an Optec SSP3 for bright stars - and I replaced the observatory opening roof this morning so maybe some measures once VSSS4 is over.
There is one thing with blue stars which helps a little - they're all much brighter in B and U than V. Maybe 0.05 to 0.15 in B and 0.5 to 1.0 in U (some are even brighter but they're rarer) So the exposure times aren't always 18 times longer and the information is often worth the extra patience required.
Regards, Stan
Hi Sara,
That is why I said "does not fully support". Actually, there are more things missing support than the LCG. For example, comparison stars do not yet have Sloan magnitudes. VPHOT/TG/TA don't work with Sloan filters right now, mostly related to the lack of Sloan magnitudes in the standard fields. The CCD manual doesn't deal with Sloan filters.
All of these things are straightforward to implement sometime in the future. At least observers can currently submit Sloan observations, retrieve them, and display them with some tool, which are great first steps.
Arne
RE: Lew says:"Haze removal is simple."
I concur with Brad's issue of trying to clean his glass (non-dichromatic) filters. I tried everything to get the V and the B Schuller filters cleanned. On closer observation I noticed the "haze" was in the sandwiched area of the filter and un-cleanable unless I took the filter out of its cell and tried to seperate the layers.
In the meantime, I've used the B&V Astrodons since 2011 and have observed no deterioration in this filters and very little change in the transform coefficeints derrived from TG.
James
There are basically three types of filters available on the amateur market.
The first are the colored glass filters, primarily from Schuler or very early Astrodon, that followed the Mike Bessell prescription of thickness and types of glass. These filters were 4mm thick, not AR coated, and the BG39 glass in the V filter was exposed to air on one side - it is hygroscopic and absorbs moisture, forming crystals and haze. They work fine in dry environments. For example, BSM_NM has Schuler filters, and has been running for 7 years in New Mexico without problem. These filters used to be very inexpensive ($60 each).
You can clean the filters that have hazed. A couple of the techniques are given in earlier posts. John Menke has cleaned four of our colored glass V filters with excellent success. I'm going to look at HQ the next time I'm down and try to form filter sets with at least a B or an Ic filter to be used with the V filter, and I'm hoping Stella will let these be loaned or given away to observers.
The second are again colored glass filters, but improved. They are polished flat, have AR coatings which help in moisture prevention, and the V filter is redone so that the BG39 glass is sandwiched between other glasses and not exposed to the air. These last a long time, even in more humid environments. I tested a 5mm thick set from Chroma Technologies on BSM HQ. The transformations were excellent, and other than a problem with holding the thicker filters down in filter cells (and that the thicker filters didn't work in SBIG filter wheels), they worked great. I don't know if they've solved that problem yet. Other vendors are available, such as Custom Scientific. Cost of these more professional filters is around $130-150.
Colored glass filters for the Cousins Ic bandpass are usually not bounded on the red edge; the sensor response forms the red limit of the bandpass. Therefore, every different type of sensor will have a different response in this filter. They have less throughput than the dielectric filters below. The U-band filter requires a redleak preventing glass, and the original glass type is no longer available. I don't know how vendors block their colored glass U-filter today, but they've probably found a solution.
The final filter type is the dielectric variety, where there is an interference coating on top of clear glass. These have no hygroscopic tendancies, and the dielectric coating can be custom tailored to enable any sensor to nearly perfectly match the standard system. One of their big advantages is transmission throughput. Compared with colored glass, the dielectric B filter from Astrodon has nearly twice the transmission. The Astrodon filters used to have red leaks, but Don Goldman has solved that problem in the filters he is currently providing. One of the problems of dielectric filters is that, because they are based on light interference, their bandpass changes depending on the angle of incidence of the light beam onto the filter. Ideally, they should be put into a plane-parallel collimated beam of light. In practice, typical f/8-f/15 telescopes have nearly parallel light near focus and the filters work fine. When you approach f/3, or have a very wide field of view, the filter spectral response changes, and that change is also a function of distance from the optic axis. The dielectric coating is very hard, but if it is scratched, unfiltered light will reach the detector. There used to be a problem of light leaking around the edges of the filter, but Astrodon is now painting the edges black to prevent this issue. If you want Sloan filters, they are only available using dielectric coatings; there is no colored glass equivalent. Astrodon filters are $200 in the 1.25" size, and similar filters are available from other vendors.
So what is available ranges from cheap to more expensive, and each has good points and deficiences. I think a group purchase would be great and may significantly decrease the cost, but that depends on how many people want what type and bandpass of filters.
Arne
Hi Arne,
That's a very good summary and I'll spread it around in our part of the world. It will get rid of some of the uncertainties of what to buy.
It's also reminded me of U measures in PEP with pm tubes where the red leak was a problem. Like you mention for I today both the U and V filters relied oncut-offs external to the filter to match the bandpass approximately. U is of course limited by the atmosphere but V was limited by the long wavelength cut-off of the S11 photocathode at around 6000A. This had the added advantage of nullifying the red leak in the U filter as well. I didn't think of that in my ealier comments. But it needs to be resolved for CCD work.
Regards, Stan
Cheers Arne,
I have a set of Bessell's BVRI filters (from Stan Walker (a long story)) restored and over the warm humid (read apalling and unusable) spring the hygroscopic glass in the V has found the perfect conditions to react. My first V images in some months appear like they were taken through smoke and the B and R I took at the same time are perfect. Eyeballing the filters, indeed just simply looking through them you can see the haze on the V whilst the other filters are clear. I was hoping to get years (naively) before having to polish the V fitler again.
My friend who does lots of optical glass work and restored the filters and I are going to try sandwiching the hygroscopic glass as you have indicated is used elsewhere as a solution. In addition I think I will get in the habit of shooting far more flats so I can remove the filter wheel (not its housing, just the wheel) store it airtight with descicant and see how we go.
One obvious problem is that the extra clear glass element will alter the focus, but that is still preferable to having to polish the crazing away or having it develop in the first place.
- Carl.
If you are able to put together a group purchase, I would at least buy a B filter if the vendor turns out to be Aatrodon.
Jim
RE:"Colored glass filters for the Cousins Ic bandpass are usually not bounded on the red edge; the sensor response forms the red limit of the bandpass."
I use Astrodon Ic for my ST-10XME, but was hoping to use the remaining Schuller Ic with another set of Astrodon B&V on my ST-8XE on a 2nd scope......looks like I'll have to get a 3rd Astrodon.
James
I'd be interested in 1.25 inch Johnson/Cousins B, V, and I filters.
Scott
Finally...I'd be interested in 2 inch Johnson/Cousins V
JBD
1 my answer is no
2 I have the whole set ubvri. i got them from a company in connecticut that was very cooperative. I had a problem mounting them in a starlight motorizede filter wheel. i sent them the fiter wheel and they purchased Baader low profile filter rings and remounted them for free. they were very cooperative and the purchase price was about 40 % less than Baader. they work fine and are research grade Johnson/Cousins filters
Regards Bob Brong (phoenix455)
Stella,
A case can be made for encouraging photometrists to use BVI filters rather than buy a complete UBVRI set. Mike Bessell in his 1990 paper discussed these five bands and notes the problems with U and R bands. While V and B-V have traditionally been related to luminosity (Mv) and temperature (Teff) for earlier type stars V and V-I are suitable measures for later types. For redder stars (e.g. SR variables) 'I' is a suitable luminosity surrogate and V-I a good temperature indicator.
I made the V filter I've been using for the past 6 years by cementing a 1.5 mm thick yellow Wratten #12 filter (cost ~ $15) with a 1 mm Schott BG39. (For the yellow filter a Schott GG495 is recommended but both the Hoya Y-50 or a Wratten #12 have almost identical longpass characteristics.) For the I-band filter I use a 2.5 mm Hoya RT=830 filter (it has a very similar response to the recommended Schott RG9). You can currently buy a RT-830 from Edmunds Scientific for S$80. But it is only 25 mm diameter so you need a thin washer to hold it in standard astronomy filter cells.
These filters have cost me about half those sold by Astrodon, Baader or Custom Scientific. Importantly they are dye-in-glass filters with no interference coatings (see Yao &Huang 2002, Chin. J. Astron. Astrophys. 2, 563). . My transformations with these filters using Cousins E-region standards are good (see Moon 2013, eJAAVSO, vol. 41).
Last year I bought a 25mm diameter, 2mm thick, ThorLabs FGB39 (Schott BG39) for AUS$34 and cemented it to a 1 mm thick Wratten #12 as a V filter to use in my second filter wheel. I have 'matched' this with a 3mm thick Schott RG9 filter for the I-band. This ThorLabs version of a RG9 cost me AUS$56 (currently an RG9 is S$70 from Edmunds). I now have 2 VI filter sets - one of 2.5 mm thicknesses and the other 3mm.
I have not, however, been able to construct a cheaper B filter so bought a Baader Johnson B filter.
Tex Moon
T.M. RE1:"Mike Bessell in his 1990 paper discussed these five bands and notes the problems with U and R bands. While V and B-V have traditionally been related to luminosity (Mv) and temperature (Teff) for earlier type stars V and V-I are suitable measures for later types. For redder stars (e.g. SR variables) 'I' is a suitable luminosity surrogate and V-I a good temperature indicator." I totally agree with this and have been told by others at AASVO that BVI will be good for 90-95% of the variables we shoot. U has such poor response with the Kodak chips that you could only use them effectively on a large (>0.5 meter apeture) scope shooting though high quality photometric (mountain top elevation >1500 meter) skies.
T.M. RE2"....so bought a Baader Johnson B filter" I checked the OPT price for Baader filter pack (I assume this is the cheapest US price to buy them at, it was $899.00 for the UBVRI or $179.80 per filter which is only $20 bucks and some change cheaper than the high quality Astrodons (Arne seems to have tested these out with Don Goldman, see: http://www.astrodon.com/uploads/3/4/9/0/34905502/astrodonphotometrcshendentestsummary.pdf
With such a small difference in price, I would pick the Astrodon's over the lesser quality, imported filters.
James
p.s. I hope this happens sooner than latter, need to start my C-11 (2nd scope) with the ST-8XE to do time series soon!
Agree with James that US colleagues are probably best off buying local US product. Please keep in mind when reading my posts that I live way down under (Tasmania) and thus make decisions based on relative exchange rate and availability through local retailer. No company making filters in Australia so no option to buy local!
Those measuring red variables could consider focusing on V and I band measurements. Cheaper filters can then be obtained/made as suggested in my earlier post.
For those wishing to separate a compound filter try methylene chloride which is the active ingredient of many types of paint stripper. Worked okay for me! (I think I found this information by googling 'optical cement'?)
Tex
Hi,
I measured Astrodon Luminance and B, V, Rc, and Ic filters of our small telescope at our radiometry lab. Filters have been used for ~3 year in cold+humid climate, fortunately sealed well from direct contact to elements. Sadly, I don't have reference information from time when filters were brand new.
Still, results in the form of a graph are added. If anyone is interested in more detailed information (i.e. the data itself), I'm glad to provide it.
Best regards,
Tõnis
Hi Stella,
I would be interested in getting a 2" BVI set.
Clear skies
Gianluca
Hi Stella,
Would be interested in purchasing a new BVI set, but NOT custom Scientific; I am on to my 3rd Blue CS in 3 years and wish to get away from the hazing that occurs on their filters. If you organise dielectric (sp?) I will be keen.
Thanks
Simon Lowther
I'm interested in a BVRcIc set plus clear, 50mm/2". Prefer Astrodon.
Jim Seargeant
Friends,
Thank you for the discussion - it is great to see what various people are using for their telescopes, and a thank you to all who shared their experience with filters. At the same time, we have not reached a critical number of interested observers to consider a group purchase from HQ - I was hoping for ~50 people - so at this point, we wouldn't get a significant discount from a ventor. It was worth the try though....
Best wishes - clear skies,
Stella.