Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

ERF energy transmission question

  • Please log in to reply
19 replies to this topic

#1 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 05 July 2020 - 10:11 AM

I am exploring my options for getting into Ha observation and have a few questions.

1) I have a quality 5” ERF from 15 years ago that I originally bought to use with a Lumicon prominence filter. This worked well on my 4200mm FL long focus refractor. My question is, what amount of heat & UV passes through a red glass ERF that could potentially damage other cemented optical elements within the OTA?

2) I’ve read in other posts that people have had success with inexpensive scopes like Chinese 120mm f8 refractors. Is optical quality not as crucial with Ha viewing as with night time observation? 

3) if I understand correctly, a thermally tunable filter like a Daystar Ion 0.5a filter can be tuned to also see prominences?

thanks for your time.



#2 BYoesle

BYoesle

    Fly Me to the Moon

  • *****
  • Posts: 6,767
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 05 July 2020 - 10:49 AM

1) No UV passes through your RG6xx filter. However, all the IR passes through it - so it's not a good solution for large apertures. Your best ERF is and RG filter with a dielectric IR block coating, or the BelOptik or Baader DERF's. The biggest concern is for components near the focus where the IR is most concentrated.

2) An achromatic refractor at f8 will generally be as good and perhaps superior to an f8  apochromatic refractor, as they generally it will have less spherochromatic aberration for H alpha.

3) You mean they can be tuned off-band for doppler shifted prominences - yes - but the time (thermal lag) is longer compared to the tilted and pressure tuned filters, which is instantaneous.


  • MalVeauX likes this

#3 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,547
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 05 July 2020 - 11:11 AM

I am exploring my options for getting into Ha observation and have a few questions.

1) I have a quality 5” ERF from 15 years ago that I originally bought to use with a Lumicon prominence filter. This worked well on my 4200mm FL long focus refractor. My question is, what amount of heat & UV passes through a red glass ERF that could potentially damage other cemented optical elements within the OTA?

2) I’ve read in other posts that people have had success with inexpensive scopes like Chinese 120mm f8 refractors. Is optical quality not as crucial with Ha viewing as with night time observation? 

3) if I understand correctly, a thermally tunable filter like a Daystar Ion 0.5a filter can be tuned to also see prominences?

thanks for your time.

Just to expand on Bob's answer:

 

1) You can transfer UV and IR through the refractor (assuming it's just the lenses and not full of correctors or Petzval or other complex designs) all day, the heat passes through because it's high transmission and not reflection, so the glass doesn't get hot. It gets hot at the point focus as Bob pointed out. Anything at the focus point is where the heat will be experienced. This is why you can employ a front mount or rear mount or internal mount ERF as long as its somewhere in the light cone but not at the point of focus where the heat would be. The ERF reflects the thermal energy before the focus point, so the focus point has less thermal energy.

 

2) As Bob pointed out, inexpensive long focus achromatic doublets can be superior to a more costly ED/APO doublet or triplet. There's compromise to correction of focus of the wavelengths to get CA-free optics. When you're only dealing with a single wavelength of light, not full color, this is not an issue and you do not need color correction optics like ED/APO. This is why so many have excellent results in the inexpensive chinese achromatic doublets and no one around here is going to suggest ED/APO optics for narrowband solar, unless you want a scope that is also ideal for night use in full color. Optical quality still matters. Having a good figure and low SA is important no matter which design or substrate it is. Long focus achromatic doublets generally are well figured in red (656nm is part of visible spectrum, it's not long IR for example) and the longer focal-length will have less SA. A really expensive triplet APO at F3.9 for example would actually be a poor choice for this application compared to a cheap F8~F10 achromatic doublet.

 

3) You should be able to tune the filter to be simply on band, on 656.28nm and you will see both surface and prominences at the same time. You shouldn't need to tune for the surface and tune separately for prominences. It is of course possible that some features show up a bit better as you blue or red shift. For example, spicules show up a little more contrasty when blue shifted, while prominences may appear a little more bright when red shifting a bit. But ultimately if you're on band, 656.28nm, you'll see the the upper chromosphere and its features off the limb, like prominences, at the same time. It's often confused because in a single stack, the disc will be brighter than the limb because the disc has photospheric light leaking through, the parastic continuum, and so it lowers the contrast of the surface features, compared to the limb where there's no photosphere light behind the prominence so its pretty obvious and has high contrast due to no surface brightness competition with the black void of space. A double stack system will suppress that photosphere light leaking through, dimming the disc surface considerably, but then the disc and limb will be a closer relative brightness with higher contrast which makes it much easier to see the surface details and limb structures at the same time without tuning off band.

 

Very best,


Edited by MalVeauX, 05 July 2020 - 11:12 AM.

  • BYoesle likes this

#4 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 05 July 2020 - 11:31 AM

Just to expand on Bob's answer:

 

1) You can transfer UV and IR through the refractor (assuming it's just the lenses and not full of correctors or Petzval or other complex designs) all day, the heat passes through because it's high transmission and not reflection, so the glass doesn't get hot. It gets hot at the point focus as Bob pointed out. Anything at the focus point is where the heat will be experienced. This is why you can employ a front mount or rear mount or internal mount ERF as long as its somewhere in the light cone but not at the point of focus where the heat would be. The ERF reflects the thermal energy before the focus point, so the focus point has less thermal energy.

 

2) As Bob pointed out, inexpensive long focus achromatic doublets can be superior to a more costly ED/APO doublet or triplet. There's compromise to correction of focus of the wavelengths to get CA-free optics. When you're only dealing with a single wavelength of light, not full color, this is not an issue and you do not need color correction optics like ED/APO. This is why so many have excellent results in the inexpensive chinese achromatic doublets and no one around here is going to suggest ED/APO optics for narrowband solar, unless you want a scope that is also ideal for night use in full color. Optical quality still matters. Having a good figure and low SA is important no matter which design or substrate it is. Long focus achromatic doublets generally are well figured in red (656nm is part of visible spectrum, it's not long IR for example) and the longer focal-length will have less SA. A really expensive triplet APO at F3.9 for example would actually be a poor choice for this application compared to a cheap F8~F10 achromatic doublet.

 

3) You should be able to tune the filter to be simply on band, on 656.28nm and you will see both surface and prominences at the same time. You shouldn't need to tune for the surface and tune separately for prominences. It is of course possible that some features show up a bit better as you blue or red shift. For example, spicules show up a little more contrasty when blue shifted, while prominences may appear a little more bright when red shifting a bit. But ultimately if you're on band, 656.28nm, you'll see the the upper chromosphere and its features off the limb, like prominences, at the same time. It's often confused because in a single stack, the disc will be brighter than the limb because the disc has photospheric light leaking through, the parastic continuum, and so it lowers the contrast of the surface features, compared to the limb where there's no photosphere light behind the prominence so its pretty obvious and has high contrast due to no surface brightness competition with the black void of space. A double stack system will suppress that photosphere light leaking through, dimming the disc surface considerably, but then the disc and limb will be a closer relative brightness with higher contrast which makes it much easier to see the surface details and limb structures at the same time without tuning off band.

 

Very best,

Thank you both for taking time for detailed answers. My refractor would be similar to a Petzval with a cemented lens assembly about 26 inches from the focal plane. So even though it’s not at the highest point of heat, it does sound like it may have some risk from IR heat. That rear lens assembly is irreplaceable so I don’t want any risk. So if substantial IR heat is passing through the ERF, would that also make a 5” Mak a poor choice because of risk to the secondary even with a full aperture ERF?



#5 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,547
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 05 July 2020 - 01:18 PM

Thank you both for taking time for detailed answers. My refractor would be similar to a Petzval with a cemented lens assembly about 26 inches from the focal plane. So even though it’s not at the highest point of heat, it does sound like it may have some risk from IR heat. That rear lens assembly is irreplaceable so I don’t want any risk. So if substantial IR heat is passing through the ERF, would that also make a 5” Mak a poor choice because of risk to the secondary even with a full aperture ERF?

With a Mak, SCT or Newtonian, you'll want a full aperture D-ERF in front if you want full aperture of the instrument. It's not uncommon to complete the thermal regulation with a 2nd filter as a secondary ERF before the filters and imaging train too. You may prefer peace of mind of a full aperture D-ERF in front of a Petzval based refractor, with a D-ERF most of the thermal energy is handled so its ok to use that style instrument.

 

That said, even some designs do not handle heat even with a full aperture D-ERF, newtonians for example do not do well here, while Mak/SCT does fine with thermal handling even with the secondary, with a full aperture D-ERF. Newtonian's secondary mirrors in most designs have some kind of issue with thermal or radiative energy even with a full aperture D-ERF over the full aperture that's hard to fix. Refractors are just so much easier to deal with thermal management.

 

Very best,



#6 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 05 July 2020 - 01:29 PM

When you say “D-ERF” , is that something different then the 5-1/2” red glass filter I have mounted in a cell? It has a red glass lens which is 3/8” thick and labeled 25ARED?



#7 FlankerOneTwo

FlankerOneTwo

    Viking 1

  • *****
  • Posts: 853
  • Joined: 30 Aug 2017
  • Loc: Vegas, baby!

Posted 05 July 2020 - 01:46 PM

When you say “D-ERF” , is that something different then the 5-1/2” red glass filter I have mounted in a cell? It has a red glass lens which is 3/8” thick and labeled 25ARED?

D-ERF = dielectric energy rejection filter. The dielectric coatings are typically used to reflect, rather than absorb, unwanted frequencies thus they tend to not heat up as an absorptive filter would do. This reduces the chances of them cracking or otherwise failing from the heat.



#8 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,547
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 05 July 2020 - 03:13 PM

When you say “D-ERF” , is that something different then the 5-1/2” red glass filter I have mounted in a cell? It has a red glass lens which is 3/8” thick and labeled 25ARED?

Right, as Patrick stated. A D-ERF is dielectrically coated so is reflective of unwanted frequencies. A colored filter, even if good substrate optically flat glass (1/4th wave or better) merely absorbs the energy (think white paint is cool to touch out in the sun, while black paint is burning hot to the touch when exposed to sunlight). While they both work at handling thermal energy, having a "hot plate" in your larger aperture imaging train can lead to local seeing in the OTA to be poor as you look through literally hot substrate that dissipates the heat to surrounding air (inside and outside the lens, so inside your OTA too).

 

Daystar's ERF's are just that, yellow glass basically, or red glass. Blocks UV by absorbing. Passes IR. Baader's D-ERF and other D-ERFs are much more costly and better at the job because they're dielectrically coated (just like almost all imaging filters are).

 

Very best,


  • Volvonium likes this

#9 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 05 July 2020 - 03:30 PM

Ok. Sorry for all these questions. So then I assume my red glass filter is ‘not’ suitable for use with my Mak. So is there concern of my red ERF heating up and cracking? So my type of ERF absorbs the energy and heats up, but also allows a significant amount of IR energy through the system possibly damaging the Mak secondary? Will this be detrimental to the thermal tuning of the Daystar filter? Thanks again



#10 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 7,547
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 05 July 2020 - 03:47 PM

Ok. Sorry for all these questions. So then I assume my red glass filter is ‘not’ suitable for use with my Mak. So is there concern of my red ERF heating up and cracking? So my type of ERF absorbs the energy and heats up, but also allows a significant amount of IR energy through the system possibly damaging the Mak secondary? Will this be detrimental to the thermal tuning of the Daystar filter? Thanks again

No worries, it's always good to seek information!

 

Absorption filters can be used in this way, but I wouldn't use it on a mirror based system with significant aperture. It's not likely that your ERF will crack, more that it's allowing significant energy to pass and mirrors are not the same as a high transmission lens that passes the energy so there can be some energy issues. Even with a D-ERF this can be true (as I mentioned before, even a Newtonian will have issues with a full aperture D-ERF, not just an ERF, because of how the secondary usually is situated, but an SCT and Mak can use a D-ERF no problem with the shielded secondary). Depending on the aperture of your Mak, it's possible to use it. It was commercially an option and still is to get sub-aperture ERFs like this for larger aperture mirror systems from Daystar for example. I really cannot say at what point a full aperture ERF will be ok or not ok with various Mak/SCT designs, so hopefully someone else has better answers and sources. You can certainly contact Jen at Daystar for best answer regarding an ERF on a Mak with a Daystar filter. I wouldn't hesitate to use a D-ERF in this way, I use a D-ERF on SCT regularily and it doesn't get hot, but my Newtonian with the same D-ERF gets hot and is not usable this way (exposed secondary). The ERF I used to have was a yellow ERF from Daystar (150mm). I absorbed UV and worked fine, but I wanted more bandpass options so went to a tri-band D-ERF.

 

You're good to go with a refractor and an ERF. You could actually complete the blocking with a long IR filter like KG3 before your focus point.

 

Very best,
 



#11 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 05 July 2020 - 03:58 PM

Thank you for the great info!



#12 markthais

markthais

    Vostok 1

  • -----
  • Posts: 193
  • Joined: 25 Jun 2004

Posted 06 July 2020 - 09:25 PM

The red ERF's  from the old days could also be surplus aero filters(I threw away over 100- 4" ones because how poor optically they where). We would get them from Surplus Shack up to 6 1/2" "cheap".  If the glass has any writing on the edge, it could be one of these filters.  

How dark red is it? RG630 is a deep red, the others are a light red(like a wratten 25). The point of the ERF is to keep as much heat out of the scope. The yellow ones do very little of that. The original Daystar yellow ERF's were the surplus camera filters. Not sure what they use now.

For using a D-ERF(Baader's). I use one on my C-6 with an 3X telecentric and one of my filters, with no problems.

I don't think it would hurt a Mak(not secondary corrected like an Vixen) or a 5" SCT either. But the sure bet is to use it on a refractor.

Mark W.


  • BYoesle, Lost in Space and MalVeauX like this

#13 BYoesle

BYoesle

    Fly Me to the Moon

  • *****
  • Posts: 6,767
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 07 July 2020 - 09:47 AM

For those of you who don't know markthais is an expert on mica-based filter systems, and formerly worked for DayStar Filters until his development of the Solar Spectrum line of filters.


  • MalVeauX and briansalomon1 like this

#14 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,640
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 07 July 2020 - 10:30 AM

The red ERF's  from the old days could also be surplus aero filters(I threw away over 100- 4" ones because how poor optically they where).

  Mark,

     I found just the opposite from the ones I got from Surplus Shed. They tested  out to be  excellent and a real steal at the  prices the were selling for. I took a couple of the smaller 4" ones and ground and polished them into 4" f/30 singlets that are fully corrected fro coma and spherical at 656nm.  Here is picture of one that I made and the back surface that tested out to be very optically flat.  I find these to be the ultimate ERF and solar objective in one   for H-alpha work. Since there is no spherical aberration at 656nm , no coma and less scatter since I only have one piece of glass. Works amazing well with the my older Daystar filter. We are both old enough and been playing this H-alpha game to remember when Jessie Knight sold 60mm  f/15 singlets and singlet barlow 2x  made from red glass to work the Daystar filter.

    By the way to all, crown optical glass like very common BK7 does not transmitt UV so a typical achromat or APO  and even the glass in the eyepiece are all acting like UV filters to fully block it.  When it comes IR you need to look at the transmission curves on the antireflective coating applied to any of the glass surfaces because many times they are blocking IR. So your telescope many already have both UV and IR blocking built. 

 

               All the Best !

                 - Dave 

 

redlens.jpg

redlensbacksurface.jpg


  • BYoesle and Volvonium like this

#15 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 09 July 2020 - 10:05 PM

My ERF lens does have etching on the edge. It’s 5.5” clear aperture

Attached Thumbnails

  • EF28C44D-F82B-44A5-9FB8-C4AF0906E432.jpeg

Edited by Studio11, 09 July 2020 - 10:13 PM.


#16 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 09 July 2020 - 10:07 PM

25-A RED

Attached Thumbnails

  • E19D8DD7-C612-4A7F-987F-3C9FF5D4F50A.jpeg
  • 1266C10C-1262-4311-9FBB-AB7D66E7F2B9.jpeg

Edited by Studio11, 09 July 2020 - 10:18 PM.


#17 markthais

markthais

    Vostok 1

  • -----
  • Posts: 193
  • Joined: 25 Jun 2004

Posted 10 July 2020 - 03:18 AM

Hi Dave,

I don't know if you remember Bob Mortimer? He make an 3" F/30 spar, He used an square tube with fans to move the air through the tube. It work very well.  One time I was at the college and he had it set up. He told me take a look, the image was really good. Then he turned off the fans and you could see the image soften up in no time at all.

I think I still have one or two of those 3"F/30 lens that Bob used . I know I have some 3.25" F/20 that he had made out of BG20"?" He did this because it transmitted K-line and Ha and blocked some of the visible energy. Bob passed some year's ago but I'm sure there are some of his scope still out there. 

 

I have a 3" F/15 with an 2X telecentric using R-25 glass as the objective. If must be 20+ years old. I use this scope to test my filters. You do get more energy through then RG630 or D-ERF but with my system , I just offset the extra heat that would push you off band with the TEC controller.

 

Stan, just because it maybe an Aero filter doesn't mean it's not optical. If you have any concerns, do a star test. Put it on the scope and look at a star , in and out of focus and see how it performs.I think Jim Sweeney sold one's that he would check that way back them. The cell is the cell that the filters can in, just been modified with mounting threads.  

 

 

In the old days I would use that glass sometimes, I would send up a batch to my friend that owned an optical shop. He would re polish them to make sure that they were better then 1/4 wave on transmission. Now days, the cover I sell usually have one of Baader ERF's in them.

Mark W. 



#18 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 10 July 2020 - 04:46 AM

That’s what I was thinking, using a star test to check it. Thanks for the information.



#19 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,640
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 10 July 2020 - 09:57 AM

 That looks to be a WW-II surplus Kodak Aero Ektra  25-A  camera filter. As I said I have a number of them both in that size and the smaller 4" ones.  Today you would pay a couple hundred for just the raw glass if you wanted the make an optically flat ERF or lens from one.  

   A star test will work but  a better test would be to interference test the surface against an optical flat. The surfaces  don't have to 1/4 wave flat but need to be optically smooth. A few wave concave or convex just makes it a very weak lens that slightly changes the focal length the system it is used on. 

  As I said common optical glass cuts off around 390nm so by itself it doesn't transmit UV. As for IR one can use a smaller hence less expensive  IR blocker father done in the optical path. 

              -  Dave 



#20 Studio11

Studio11

    Mariner 2

  • *****
  • topic starter
  • Posts: 221
  • Joined: 20 Oct 2007
  • Loc: Pennsylvania

Posted 12 July 2020 - 08:38 AM

So judging by a star test, the ERF shows about 1/3 wave undercorrection. I assume this is as detrimental as in nighttime observation? I’m not sure how to judge seeing during the day. It seems more difficult then with a point source. Plus not knowing what is normal.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics