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Filter for nebula

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#1 ivazquez

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Posted 24 September 2020 - 05:45 AM

Hi all,

 

I'm a fairly newbie in astronomy. I'm just starting to be able to find some deep sky objects using the star hopping technique. But I live in a contamited area (should be suburban but it's very near Madrid) and faint objects are really hard to see. The only nebulas I've been able to observe are the Orion Nebula, a bit of the Lagoon Nebula and the Ring nebula.

 

Looking for a filter, I found one in AliExpress that it's supposed to pass H-Beta and both lines of O-III. It's not a UHC filter since it does not pass the red wavelenghts. It's sold as an H-Beta filter but with a 25 nm passband it also passes O-III. Supposing it meets its specs, do you think it would be a good filter? It's somehow similar to Orion Ultrablock.

 

I've attached the transmission diagram.

 

My scope is a 6 f/8 dobson.

 

Thanks in advance.

Attached Thumbnails

  • Captura.JPG

Edited by ivazquez, 24 September 2020 - 05:46 AM.


#2 SloMoe

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Posted 24 September 2020 - 06:10 AM

You cancompare that to others by going to this site,

https://searchlight....9d-153d7e7c0eb8

 

Now over on the right is a drop down filter list, toggle the selected filter by clicking the box next to the filter you want to compare and you can stack as many as you want on top of each other,

 

that sure looks like a UHC to me

 

If that's the graph of the filter you're getting then it's a bit weak in light pass percentage, if it's a representation from a web site I might thing twice about it, plus Lumicon's Gen III Nebula filter is on sale right now, till the end of the month.


Edited by SloMoe, 24 September 2020 - 06:12 AM.


#3 Redbetter

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Posted 24 September 2020 - 06:34 AM

If it matches that graph it is NOT an H-Beta filter.   If a filter has OIII in high percentage it is NOT an H-Beta.  The red pass band doesn't have much to do with it (that would be H-alpha which visually is only a weak supplement to H-Beta.) Instead it would be a semi-wide "not really UHC" variant.  Estimating the FWHM from the graph, I arrive at a pass band of about 34nm.  There are much worse options, but this one is not what I would classify as "good" either.   

 

It is a no-name filter from China, so you could end up with anything.  My ancient Orion Ultrablock would likely run circles around it...although this Aliexpress version might be a clone or off-spec reject of what the current Ultrablocks are; it is unlikely to perform well for the class.  

 

For urban/suburban skies to get any real benefit you need an aggressive filter with truly narrow pass bands, not a semi-wide.  Either a narrow true UHC or possibly an even narrower OIII line filter is what I would suggest.   (Again, the true pass band is the major factor with either, wide is bad with major light pollution.)  I like true H-Beta filters a lot for the difficult stuff I target in dark sky, but the things they show well are often difficult dark sky targets and quite diffuse for the average observer, not anything you want to try as a novice near a metropolis.  If you were in dark skies and had some particular H-Beta targets in mind then I would point you toward suitable filters (this one is not.)



#4 sg6

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Posted 24 September 2020 - 06:56 AM

Strange they have called it an H-beta filter, basically not as shown narrow enough for H-beta alone.

So it appears to pass H-b and OIII, reasonable I suppose as long as you appreciate that.

 

Would suggest that the curve depicted is the theoretical one. They are often just too neat to be real, and that applies to many others as well.

 

The peak of 92-95% seems normal.

 

First aspect is that wheatever you look at needs to be emitting in H-b and OIII, so looking at an Ha target is useless. Yes people do get a "blue" pass filter and then wonder why a "red" nebula looks poor.

 

Everything will be blue, think Cyan is the correct term.

 

Being honest, if the price is good and you feel like taking a bit of a chance, then go for it. Filters are I feel a bit of chance anyway these days. Shouldn't be but are.

 

Also some UHC filters pass Hb, OIII and Ha, some pass just Hb and OIII. So against some UHC it is a match.

 

Useful if a link and cost were given.

 

Just had a play with the Searchlight site. The one you have a graph for looks similar to the Fairpoint UHC filter. Basically Hb+OIII and nothing else.


Edited by sg6, 24 September 2020 - 07:39 AM.


#5 Redbetter

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Posted 24 September 2020 - 07:31 AM

So it has a supposed spectrum demonstrating it is NOT a narrowband UHC (and most assuredly not an H-Beta as it claims.)  It is a no-name AliExpress of unknown providence that is likely to have random characteristics...the internet equivalent of buying designer handbags from a guy's trunk on the highway.  

 

Being honest, avoid, regardless of the cost!  No need to take chances on stuff that is set up to disappoint even based on what the vendor admits to...   Novices won't know what is good or bad because they have no experience to compare to.  Why is this simple concept so hard to understand for someone like sg6 who has been on CN for over a decade?  (Hint: the OP is a newbie with Sept. 20 signup date.)

 

I don't need a link or cost to say, "Run, Forrest, Run!"



#6 ivazquez

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Posted 24 September 2020 - 07:56 AM

First of all, thanks everyone for your responses. 

 

My doubt is between purchasing an O-III filter or the one I put on the post. The appealing part for me is that this filter can pass both O-III and H-Beta.

 

If it matches that graph it is NOT an H-Beta filter.   If a filter has OIII in high percentage it is NOT an H-Beta.  The red pass band doesn't have much to do with it (that would be H-alpha which visually is only a weak supplement to H-Beta.) Instead it would be a semi-wide "not really UHC" variant. 

I know it's not an H-Beta filter since it also passes O-III. For the moment, I don't want to purchase a real H-Beta. The price is around 47€ ($55) including shipping. I think the price is good and maybe I will give it a try. Pus it has 15 days return policy.

 

The cities around me are mostly using now LED streetlights so I want a filter that blocks most of the light and the UHC I've seen for the moment pass most of the red light. I understand that I cannot look at H-Alpha objects with this filter.

 

 

 

It is a no-name filter from China, so you could end up with anything.  My ancient Orion Ultrablock would likely run circles around it...although this Aliexpress version might be a clone or off-spec reject of what the current Ultrablocks are; it is unlikely to perform well for the class.  

 

My first option was, indeed, the Orion Ultrablock but I found this and the Orion is out of budget at this moment (**** covid).

 

 

You cancompare that to others by going to this site,

https://searchlight....9d-153d7e7c0eb8

 

Now over on the right is a drop down filter list, toggle the selected filter by clicking the box next to the filter you want to compare and you can stack as many as you want on top of each other,

 

that sure looks like a UHC to me

 

If that's the graph of the filter you're getting then it's a bit weak in light pass percentage, if it's a representation from a web site I might thing twice about it, plus Lumicon's Gen III Nebula filter is on sale right now, till the end of the month.

Great source! Thanks. Purchasing the Lumicon is impossible for me at this moment.

 

Again, I really appreciate your help.



#7 SloMoe

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Posted 24 September 2020 - 08:08 AM

Well, for one, LED light can't be filtered out, no matter what filter you buy.



#8 Redbetter

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Posted 24 September 2020 - 08:19 AM

 

The cities around me are mostly using now LED streetlights so I want a filter that blocks most of the light and the UHC I've seen for the moment pass most of the red light. I understand that I cannot look at H-Alpha objects with this filter.

 

LED's are bluer, not redder.  You want to block the extra blue that the wide "UHC" passes since the extra band pass is all light pollution from the LED's.  In an LED environment, the UHC's that pass red would actually perform better by comparison if the nebulae had red components.  Our eyes are orders of magnitude more sensitive to blue at night than to red.  At worst the red band pass does little or nothing with LED's because they transmit more heavily in the blue.  Signal-to-noise wise, the red isn't really hurting, but the wider main blue/green pass band is causing damage.

 

Maybe the overall effect of the advertised 20+% wider blue/green band pass is not large, but it is in the wrong direction for what you describe.  More importantly, you have no idea what actual product you will get or its actual performance--other than they can't even describe it accurately.  That is the difference between some known reliable makers, and anonymous Chinese stuff.  

 

Caveat emptor.



#9 JoshUrban

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Posted 24 September 2020 - 08:23 AM

Hey there!

 

  Something else to keep in mind is - deep sky objects can be tricky to see, period!  It wouldn't seem like practice would apply here, but I've found that observing is definitely a skill that builds.  Street lights absolutely wash things out, but with some careful preparation, draping a towel over your head to block stray light, getting more practiced with averted vision (that's a lifetime journey there), using the right eyepiece/magnification for the target (and trying different ones) will not only let you see more, but when you get out to a dark site, that'll let you go WAY further.  A galaxy that's obvious to my suburban-trained eyes when observing under dark skies might be invisible to someone else.  

 

  Also, bet you could snag a used name-brand filter here on the classifieds for the same price.  Additionally, $50 worth of gas and camping fees at a dark site is another thing to keep in mind.  Cheap binoculars under dark skies can outperform big scopes in the city.  

 

  Lastly, although light pollution is a bummer, limitations can lead to creativity.  A study of variable stars, splitting doubles, perfecting planetary observations, and lunar explorations are all great topics of study for the city dwellers.  



#10 Napp

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Posted 24 September 2020 - 08:57 AM

Rather than buying some no-name filter with dubious performance you really would be better served taking your scope to a darker site occaisionally.  When buying filters you really do get what your pay for.  The good ones are worth saving up for rather than making do.  LED lighting is broadband.  To effectively filter it out you would really not have much if anything left.  And truth be told, UHC, OIII and Hydrogen beta filters all work better in a dark sky.  All of us in light polluted areas are dealing with this.  I drive 65 miles each way to get to a Bortle 3 sky when I can.



#11 ivazquez

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Posted 24 September 2020 - 11:42 AM

Again, thanks everyone for your feedback. I really appreciate.

 

Rather than buying some no-name filter with dubious performance you really would be better served taking your scope to a darker site occaisionally.  When buying filters you really do get what your pay for.  The good ones are worth saving up for rather than making do.  LED lighting is broadband.  To effectively filter it out you would really not have much if anything left.  And truth be told, UHC, OIII and Hydrogen beta filters all work better in a dark sky.  All of us in light polluted areas are dealing with this.  I drive 65 miles each way to get to a Bortle 3 sky when I can.

 

I'm not sure if I hava a Bortle 3 sky near me. I live near Madrid, where the city and its metropolitan area spread their light as far as 100 miles. Madrid is one of the most light-polluted cities in Europe even more than Berlin or Paris.

 

I think I may wait a while to purchase the filter and look for a good second hand unit although there is not a very active second hand market for astronomy in Spain.



#12 SloMoe

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Posted 24 September 2020 - 12:05 PM

Hi, I'd suggest waiting to purchase until 1) you have enough for a quality name brand filter, & 2) you find a reputable vendor to purchase it from.



#13 Napp

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Posted 24 September 2020 - 12:16 PM

I'm not sure if I hava a Bortle 3 sky near me. I live near Madrid, where the city and its metropolitan area spread their light as far as 100 miles. Madrid is one of the most light-polluted cities in Europe even more than Berlin or Paris.

Check the light pollution maps.  It doesn’t have to be a Bortle 3 sky though it would be better if it were.  Anything better than the sky you have is good.  I drive 45 miles each way for my other club’s Bortle 4 site.  I see others posting about driving some distance to get to Bortle 5 sites.  As long as it’s better you will likely enjoy it.  Are you a member of an astronomy club?  A local club will know of or actually have a dark sky site.  



#14 Dave Mitsky

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Posted 24 September 2020 - 12:36 PM

I drive 65 miles each way to get to a Bortle 3 sky when I can.

I'd be happy to have to drive only 65 miles one way to reach Bortle 3 skies.  It's a 113 mile trip for me that takes over two hours.  I consider myself lucky to have lesser but reasonably dark sites within an hour to an hour and a half drive. 


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#15 Napp

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Posted 24 September 2020 - 01:22 PM

I'd be happy to have to drive only 65 miles one way to reach Bortle 3 skies.  It's a 113 mile trip for me that takes over two hours.  I consider myself lucky to have lesser but reasonably dark sites within an hour to an hour and a half drive. 

I understand, Dave.  We are lucky to have access to this site.  



#16 ivazquez

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Posted 25 September 2020 - 01:52 AM

Check the light pollution maps.  It doesn’t have to be a Bortle 3 sky though it would be better if it were.  Anything better than the sky you have is good.  I drive 45 miles each way for my other club’s Bortle 4 site.  I see others posting about driving some distance to get to Bortle 5 sites.  As long as it’s better you will likely enjoy it.  Are you a member of an astronomy club?  A local club will know of or actually have a dark sky site.  

I'm not a member of an astronomy club. I'm looking for one near me to join. My sky may be a Bortle 5 or so. To the southwest the sky is quite bright, but to the north and the east it's reasonably dark. In good days I can see a little of the Milky Way to the cenit and north. 

 

I may have Bortle 3 skys around 60 km from home (around 40 miles). The problem of this site is that it's in a valley so objects near the horizon cannot be seen. But this weekend moon is quite high during the night so I think I will stay at my backyard.



#17 LDW47

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Posted 25 September 2020 - 08:34 AM

If it is reasonably cheap try it and let us know your findings thats the only way to know if it is another hidden gem in the big pile of filters and it can be fun and challenging, it might make you a leader, an expert in that filter. I would do it but I have so many da*n filters as it is and they all perform great in their own way. When Orion raises its head a little earlier I will give a couple or 3 new Svbony filters another run. Good Luck !



#18 LDW47

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Posted 25 September 2020 - 08:37 AM

Did you give us a link to this filter ? Did I miss it ?



#19 ivazquez

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Posted 26 September 2020 - 03:16 AM

Did you give us a link to this filter ? Did I miss it ?

Sorry, I didn't give it:

 

https://es.aliexpres...1076445507.html

 

The supposed pass-band is almost the same as the orion Ultrablock. There are not feedbacks. It's not a lot of money so may be I could give it a try. 

 

Yesterday, I received a (supposed to be) 7x50 double illuminated crosshair finderscope for 23€. Of course, as I already knew, it was a single crosshair non illuminated finder. It also came with some minor scratches on the tube. I opened a dispute and Aliexpress has refunded me in full and I don't have to return the finder. It works great for the cost of 0€. I could easily find Uranus in my (may be) Bortle 5 sky. Andromeda can be found at first sight. I couldn't test more because of the weater.

 

If there were a cheap way to check the characteristics of the filter it would be great since you can get a full refund if the filter does not meet the advertised specs. 

 

In this case, also, if you are not happy, you can return the filter in 15 days at no cost, at least for Spain.


Edited by ivazquez, 26 September 2020 - 03:19 AM.


#20 ivazquez

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Posted 27 September 2020 - 06:53 AM

Finally I'm not going to buy it. I asked the seller because I saw that they had an UHC filter with exactly the same (supposed to be) transmission pattern. So I asked and he told me that the diagram was incorrect. He sent me other two diagrams without any reference. 

 

Looking at a second hand platform in Spain I found an Astronomil UHC filter for just 27€ including shipping, so I'm going to try to buy it. 

 

The transmission in the H-Beta and O-III is quite narrow and in the H-Alpha part seems to be also quite well adjusted. Also, is a known brand and the price, I think, is nice.



#21 Starman1

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Posted 28 September 2020 - 03:57 PM

Astronomik is one of the best 4 brands, so you did well!




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