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#1 PNWkarma

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Posted 06 October 2017 - 09:05 PM

As far as I'm aware, there seem to be a few options for ERFs:

 

Older red glass filters, front mounted

More modern "yellow" glass filters, front mounted

Narrow pass filters, mounted internally before the first optic that receives the concentrated light beam.

 

I understand that you can calculate the amount of energy passing through the ERF of just over one watt per square meter per nm of bandpass.  I've been told that some etalons can dissipate the energy to safe levels with telescopes below 80mm.  For Apos and OTAs over 80mm with three or more lenses it seems that the only option that's suitable is the front mounted ERF.

 

My question is for simple achromatic telescopes in the 100-150mm range (in my case 127mm), is a 7nm HA filter just before the etalon adequate to reject enough of the heat energy?  Based on the crude energy value mentioned above and comparing it to the power of the peltier device in my etalon, I think it should work. I just thought I should ask the experts here before trying it.

 

Thanks in advance,

 

Jonathan

 


Edited by PNWkarma, 06 October 2017 - 09:07 PM.


#2 bob71741

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Posted 07 October 2017 - 09:12 AM

Jonathan- Can you explain the following?

 

"Based on the crude energy value mentioned above "  ????

 

"... the peltier device in my etalon" ?????



#3 FlankerOneTwo

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Posted 07 October 2017 - 10:16 AM

If you're talking about a Daystar Quark for the etalon and if you mean the Baader 7nm HA filter and if all the elements in your scope are air spaced (no Petzval or oil spaced elements, I don't think they're commonly used in achros), you are close. The only problem, as pointed out to me by icecreamcat, is that the Baader 7nm HA filter does not adequately reject UV/IR, whereas their 35nm HA filter does. Baader's UV/IR filter will work as well.

 

From other threads,

 

1. "I spoke to John at Televue today and the answer is "No [front ERF required]". Apparently they have used the Daystar Quark with 2" IR/UV block filter with no issues on the NP127. The reason being that the rear elements are air spaced and 5 inches away from the focal point so all that heat reflecting through is of no issue to the telescope itself. He said that they had it out imaging for at least 4 hours with no ill effects." -cloud cover

 

2. "Hi Marktownley,  I e-mailed DayStar about  reducing the ES 5" Apo to 4" and then putting a 1.25"

Baader UV/IR filter on the end of the diagonal with this was acceptable.  The response by DayStars
representative Tiffany is below.

De Lorme,

   Thanks for your note.   We have been discussing these ES triplet scopes lately.
We were talking with Greg Bragg and Scott Roberts at the Winter Star Party about them specifically.
The ES 127 triplet does not use oil in its APO lens spacing.
With just air spacing, there is no risk to any of your telescope's components to gain heat or degrade under the UV light of the sun.

Greg and Scott at Explore Scientific use their Quark regularly on these telescopes with the rear mount UV/ IR cut filter alone without any heating issues.
So I would actually suggest there is no reason NOT to use the UV/IR cut filter on your ES 5" triplet.

We would worry about you using any RG610 colored glass after the objective.
Colored glass absorbs energy, where the UV IR cut filter is reflective.
So a concentrated beam will absorb all that heat and expand, breaking.

Just a UV/IR cut filter before your diagonal should be fine.
If you want to stop down, you can, but it is not necessary.

Tiffany" -De Lorme

 

3. "I have been use a Quark and the IR/UV cut filter with my Williams Optics 132mm with no problem.

I setup with a 2" extension , 2" diagonal , UV / IR filter, Quark and Denk Binos. I have to have the extension to reach focus.
I mentioned this somewhere else here. I bought my Quark at Heart of America Star Party last summer . Daystar was there demonstrating the Quark and I was impressed. I spoke with Jen Winters , Daystar owner, in length about he scope I planed to use the Quark with and she insisted that the IR / UV filter would be ok on the 5" scope , no full appature EGR filter needed.
My scope has an Air Spaced Triplet Apochromat lens. I have never noticed any heat build up anywhere through the system. 
I don't use a tracking mount, I have a Disc Mount 6 , don't think there would be a problem with a tracking mount either." -bigdob24

 

To my understanding, there is not a safety issue, but there may be an image quality issue for imaging, due to the potential for thermal air currents in the OTA.



#4 PNWkarma

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Posted 07 October 2017 - 10:39 AM

Hi Bob,

 

The peltier device is the thermoelectric heating/cooling device that maintains the correct temperature in my Daystar Quantum, or indeed most modern thermally regulated etalons to my knowledge (vs. tilt or pressure tuning).  Some older devices used resistive heaters.

 

The value I was referring to was the approximate ~1w/m2/nm.  Based on this value and calculating for my particular telescope, the amount of transmitted thermal energy is very small and far less than the power rating of the peltier device.  Depending on the direction and amount of current passed through the peltier device it will heat or cool (actually it just transfers the energy to the opposing end of the device).  My reasoning is that the peltier device can therefore compensate (cool if needed) for the transmitted thermal energy and maintain the temperature setpoint.

 

Best,

 

Jonathan



#5 PNWkarma

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Posted 07 October 2017 - 10:49 AM

Patrick,

 

That's very helpful information.  In particular the energy rejection rates of the different filters.  I'll do some more research on this. I had assumed that the narrower bandpass would reject more energy, but I now realize that this doesn't take into account the transmission efficiency across the spectrum.  

 

Yes, you're correct that I'm paring an air spaced achromat with a Daystar Quantum etalon.  

 

Nice to know that others are using these simple filters safely (no damage to hardware) on larger objectives.

 

Many thanks,

 

Jonathan



#6 PNWkarma

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Posted 07 October 2017 - 10:54 AM

Oh, and good point on the thermal currents inside the OTA.  I have plenty of surface area and volume in my scope (127x1200) but I'll have to see how those currents 'travel' internally and how well it can dissipate. A good reason not to flock in this instance.



#7 FlankerOneTwo

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Posted 07 October 2017 - 01:41 PM

Here are the energy rejection graphs from Baader. The 35nm HA seems to reject more of the deeper IR above 1100 nm than either the UV/IR cut or the 7nm HA. I didn't have the UV/IR filter graph before, it actually appears that the 7 nm is actually better than the UV/IR in this zone of the spectrum. Since many people use the UV/IR without adverse effects, I would think that you could probably do the same with the 7nm, however I think I'd prefer the broader IR coverage (7nm passes 50%). For solar imaging, I would guess that both of these are probably far too wide bandpass to have a significant effect on image quality in any case.

Attached Thumbnails

  • baader-h-alpha-7nm-ccd-narrowband-filter-1-1-4--022.jpg
  • baader-h-alpha-35nm-ccd-filter-1-1-4--13c.jpg
  • baader-uv-ir-cut-l-filter-1-1-4--740.jpg


#8 PNWkarma

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Posted 07 October 2017 - 09:22 PM

Thanks Patrick.  In summary the three filter options from Baader are:

 

Baader 7nm HA

Pros - very effective energy blocking although some slight transmission at longer wavelengths.

Cons - only transmits ~88% of the needed light at 656nm.

 

Baader 35nm HA

Pros - about 95% energy transmission at 656nm.  Very good energy block for the remaining spectrum.

Cons - none?

 

Baader UV/IR cut

Pros - almost 98% transmitted light at 656nm.

Cons - pretty wide bandpass and so a lot more unwanted energy transmission. Also transmits quite a bit of energy above 1100nm.

 

I'm choosing the Baader 35nm because I can live with a 3% loss in transmission (vs. the UV/IR cut) and I like the idea that pretty much nothing else but the 35nm FWHM is getting through and therefore protecting my etalon.

 

If someone has a strong argument for a full aperture ERF, then I'm willing to try that also.  I assume there must be one, because it'd be hard to justify the price difference without a good reason.


Edited by PNWkarma, 07 October 2017 - 09:23 PM.


#9 PNWkarma

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Posted 07 October 2017 - 09:43 PM

Just two weeks ago Daystar updated their website with a Technical Note on this very subject.  Here is the link:

 

http://www.daystarfi...-or-Front-mount

 

Reassuring to have this from the manufacturer.  They recommend the UV/IR cut, but I'm still going with the 35nm HA.



#10 BYoesle

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Posted 08 October 2017 - 09:28 AM

 

My question is for simple achromatic telescopes in the 100-150mm range (in my case 127mm), is a 7nm HA filter just before the etalon adequate to reject enough of the heat energy?

The larger the ERF and the less energy concentrated at or near the etalon the better. My first choice would be a Baader DERF of the appropriate size placed ahead of the objective:

 

http://www.baader-pl...5---180mm).html

 

In the US these are available through a number of distributors.

 

Next for your particular telescope, I would go with the BelOptik 125 mm ERF on RG630:

 

http://beloptik.de/e...jection-filter/

 

After this I would consider an internally mounted sub-aperture filter, again as large as possible.

 

Also consider the BelOptik UV/IR KG3 filer over a standard IR/UV filter for smaller apertures:

 

http://beloptik.de/e...locking-filter/


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#11 PNWkarma

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Posted 10 October 2017 - 11:58 PM

Lots of useful information as usual, thanks Bob!

 

I think I understand that the main advantage of the front mounted ERF is to keep the heat out of the OTA and away from the filters.  This would obviously help reduce the internal air currents that could otherwise lead to poor seeing.  I expect this would be exaggerated if viewing for long periods.

 

I like the Beloptic site.  Looks like he's doing some interesting stuff.  I will go with your recommendations on the Baader ERF unless I can't find one in stock in which case I order from Beloptic.  They seem very similar in design but with Baader having a slightly higher efficiency.  Fascinating to read how they have such a manufacturing challenge to equalize the film stress on each side of the lens to avoid deformation.

 

I recently purchased a 0.45A Daystar also so I'm really looking forward to finishing this OTA and do some side by side comparisons of the 0.6A and 0.45A.  Not sure if these will double stack well because if I do, I'll be viewing through two sets of blockers and trimmers so I expect it'll be pretty dark. It might work well for camera work though.  Always fun experimenting!



#12 BYoesle

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Posted 11 October 2017 - 01:05 PM

Hi Jonathan,

 

The difference between the 0.45 and 0.6 filters likely will not be too great:

 

https://www.cloudyni...-created-equal/

 

You would have to make great efforts to be sure you will reach such theoretical band-passes, particularly optimizing the f ratio to about f 50 using a true telecentric lens system. Christian Viladrich's articles are mandatory reading regarding these filter systems:

 

http://www.astrosurf...nt/solar/FP.htm

 

Here again, the Baader or BelOptik telecentrics are recommended.

 

For double stacking, Christian has tried some approaches:

 

http://www.astrosurf...-contrast-2.htm

 

What is needed (other than a front etalon) to effectively DS the DayStar and Solar Spectrum filters is a high quality 1.0 to 1.5 angstrom drop-in filter. Sadly, no such animal is available other than a custom basis and the price is very great.


Edited by BYoesle, 12 October 2017 - 07:56 AM.


#13 PNWkarma

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Posted 13 October 2017 - 11:13 PM

Thanks once again Bob!

 

It's taken me a couple of days to read and then fully absorb all the information. It's nice to have the science behind the reasoning. I've contacted Oliver at Beloptik and am waiting for a response.

 

I'm shooting for a f40 setup using my f10 and a 4x telecentric.  That gets me pretty far down the graph that plots the f ratio vs bandwidth.  I would like to try f50 but I'm not sure I'll be able to notice the difference. 

 

The information on Christian's site is encouraging. I will definitely try double stacking these two Daystars and will report back.  It looks like the preferred method is placing the widest bandwidth etalon upstream of the tighter etalon. 

 

I spoke to Televue who said that the optimum distance for their Powermates from the etalon is essentially as close as possible by using the t-adapter (at least this is true for the 4x).  I'm not sure if I understand telecentrics fully yet because it seems that a completely telecentric lens wouldn't have an optimal distance as the beams should be perfectly parallel.

 

I've sent an email to Baader also about an ERF and a telecentric lens for a 1200mm focal length.




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