Use ERF From External Etalon in a PST Mod?

kornfeld1 · Post by **kornfeld1** » Tue Jul 17, 2012 9:09 pm

I was looking back through Mark Townley's great thread on how he modified his etalons here, and had an idea.

Would it be safe to use the ERF from an external etalon in a PST Modded scope? Based on the transmission curves he posted in the first post, the Coronado ERF's aren't nearly as narrowband as the Baader ERF's, but it would obviously save a pretty hefty chunk of change to not have to buy another filter. I've got a 60mm external etalon, and it would be cool to just be able to pull the ERF out of it and use that.

Post by **marktownley** » Tue Jul 17, 2012 9:28 pm

I would not recommend it. The Coronado ERFs are an absorptive filter; these are fine placed before an objective, but if one was placed in an objectives light cone (as in sub aperture PST ERF mod) the energy levels they would be exposed to would be considerably higher than they were designed for, they would heat up, expand and potentially crack.

kornfeld1 · Post by **kornfeld1** » Tue Jul 17, 2012 9:31 pm

I was worried about that exact issue. Ah well, good to know. Thanks.

rsfoto · Post by **rsfoto** » Tue Jul 17, 2012 9:48 pm

Hi,

¿ What about them placing it in the front of the scope ?

OK I understand that possibly it would only make sense if one does not go over 60mm aperture with a corresponding focal length ... but that would speak against a PST mod ¿ or ?

Post by **marktownley** » Tue Jul 17, 2012 9:57 pm

Placing it infront of the scope is fine...

rsfoto · Post by **rsfoto** » Tue Jul 17, 2012 10:03 pm

Hi Mark,

Thank You :woohoo: My understanding about PST mods is still not ripe inmy head but I guess it is mostly in order to gain aperture and magnification ¿ or ?

Post by **Merlin66** » Wed Jul 18, 2012 1:03 am

It gives both improved resolution and increased light grasp.
A humble PST etalon can provide a cost effective "heart" for a 150mm aperture solar Ha telescope

colinsk · Post by **colinsk** » Wed Jul 18, 2012 1:59 am

It gives both improved resolution and increased light grasp.
A humble PST etalon can provide a cost effective "heart" for a 150mm aperture solar Ha telescope

At what field of view?

Post by **Merlin66** » Wed Jul 18, 2012 3:52 am

Colin,
That comes down to the size of the BF used, the final effective focal length and the eyepiece/ CCD chip size.
(I use the freeware CCDCalc to determine all my FOV etc)
http://www.newastro.com/book_new/camera_app.php/

rsfoto · Post by **rsfoto** » Wed Jul 18, 2012 4:47 pm

Thanks.

Well I have a 60mm ERF taken out from a SM60, I have possibly access to an Etalon from a blind PST and I am getting a BF10 which I will ocnvert to straight look through.

How would that all behave on a TOA 130 ? The TOA 130 is 1000mm focal length and will be reduced to 60mm due to the ERF eg. giving f/16.7

Would such a mod make sense for starting and learn about PST modifications ?

Thanks :thanx:

Post by **marktownley** » Wed Jul 18, 2012 6:08 pm

Thanks.

Well I have a 60mm ERF taken out from a SM60, I have possibly access to an Etalon from a blind PST and I am getting a BF10 which I will ocnvert to straight look through.

How would that all behave on a TOA 130 ? The TOA 130 is 1000mm focal length and will be reduced to 60mm due to the ERF eg. giving f/16.7

Would such a mod make sense for starting and learn about PST modifications ?

Thanks :thanx:

I think, if I was you, I would just buy a cheapy 60mm frac and use the ota as a donor scope rather than the TOA...

kornfeld1 · Post by **kornfeld1** » Wed Jul 18, 2012 7:13 pm

Thanks.

Well I have a 60mm ERF taken out from a SM60, I have possibly access to an Etalon from a blind PST and I am getting a BF10 which I will ocnvert to straight look through.

How would that all behave on a TOA 130 ? The TOA 130 is 1000mm focal length and will be reduced to 60mm due to the ERF eg. giving f/16.7

Would such a mod make sense for starting and learn about PST modifications ?

Thanks :thanx:

As I understand it (and someone correct me if I'm wrong), if you already have an SM60, there's no reason to put a PST etalon on a 60m scope. Both telescopes will have the same aperture, and the PST will have more sweet spotting than the SM60.

The point of the PST Mod is to go up in aperture for relatively little money. Merlin's write up suggests going for at least 80 mm or above.

If you put a small ERF on the front of a big telescope, you've given up the advantage of the bigger scope, and you're right back where you started.

You can, however, get away with using a smaller ERF on a bigger scope if you mount the smaller ERF inside the telescope. That way, you still get the full aperture, but you don't have to pay for a giant ERF. This only works with reflective ERF's because they won't heat up under the partially focused beam they will be exposed to inside the telescope. As Mark mentioned at the beginning of this thread, that won't work the Coronado ERF's because they absorb the energy instead of reflecting it.

rsfoto · Post by **rsfoto** » Wed Jul 18, 2012 7:15 pm

[quote]Thanks.

Well I have a 60mm ERF taken out from a SM60, I have possibly access to an Etalon from a blind PST and I am getting a BF10 which I will ocnvert to straight look through.

How would that all behave on a TOA 130 ? The TOA 130 is 1000mm focal length and will be reduced to 60mm due to the ERF eg. giving f/16.7

Would such a mod make sense for starting and learn about PST modifications ?

Thanks :thanx:

I think, if I was you, I would just buy a cheapy 60mm frac and use the ota as a donor scope rather than the TOA...

Hi Mark,

I understand you but it would not be a final mod. Just for testing

So if I get a 130mm ERF would be better ? I am trying to understand the PST modifications.

Thanks

Post by **marktownley** » Wed Jul 18, 2012 7:20 pm

Hi Mark,

I understand you but it would not be a final mod. Just for testing

So if I get a 130mm ERF would be better ? I am trying to understand the PST modifications.

Thanks

Oh yes, the Baader 135mm DERF would be the dream choice. Change the front relay lens in the to an f6 alternative and you would have a perfect setup

kornfeld1 · Post by **kornfeld1** » Wed Jul 18, 2012 8:52 pm

Is there any functional advantage or disadvantage to using a larger external ERF vs using a smaller internally mounted ERF?

Post by **Merlin66** » Thu Jul 19, 2012 5:39 am

IMHO, an internal ERF should be > 50% the telescope aperture and placed as close behind the aperture as possible. A slight tilt to reduce ghosting and watch for the reflected focused light beam...
No functional difference, just the remote possibility of premature failure of the coatings due to the higher energy loadings....

peter drew · Post by **peter drew** » Thu Jul 19, 2012 10:43 am

I have a range of ERFs from 80mm to 145mm. I have tried them externally and internally and have noticed no difference in performance either way round. My main preference lies in quality, for which the Baader D-ERF seems to be the field leader.

Use ERF From External Etalon in a PST Mod?

Use ERF From External Etalon in a PST Mod?

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Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?

Re: Use ERF From External Etalon in a PST Mod?