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Why use an ERF on objective, when newer PST's have no erf coating on objective?


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I'm doing a stage one mod, on my coronado pst.

Attaching an 80mm F/10 OTA to the black box via a 2" nose piece adapter.

My question though, is why attach an ERF to the end of the OTA, since my newer coronado pst doesn't even have an ERF on the objective to begin with?

The erf is located below the blocking filter above the penta prism, so the etalon has been at risk this entire time without any mods.

So why bother putting an ERF on my doner OTA, when I have been using the stock pst for months without any erf at the front of the ota?

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The simple answer is the increase in total energy.

An 80mm passes 64/16 = x4 more energy into the scope. This can obviously increase the heat loading not only on the etalon but the other sensitive filters. There is a significant risk of premature failure of the mini erf and final blocking filter. IMHO it's not worth the risk.

A good front mounted ERF (Baader D-ERF recommended) is the way to go.

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but isnt the etalon already at risk? thats what dont understand. I do understand the inverse sqaure of light law, so doubling my aperture will increase my light gathering capability by 4 times. But despite the mod the scopes 40 mm aperture is letting in enough light already to damage.

i really just dont understand the stock design of the pst, and now having to invest in an erf.

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Not an expert but, whilst the components in your PST may be designed to withstand the energy transmitted by the 40mm objective, quadrupling the energy transmission may well push those same components past there design limits and therefore risk damage.

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I unreservedly back the advice from Merlin66, a leading expert in this field. Apart from the increased heating effect, for visual use you would need additional filtering as the image would be too bright. How do I know? I've tried it. :hello2:

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The Lunt internal etalons do infact have their "mini erf" placed before the the etalon proper.. for whatever reason, Coronado, when replacing the external ERF coating, made the decision that the etalon assembly was OK for 40mm and placed their "replacement" mini-erf" infront of the more energy sensitive blocking filter....

I accept Coronado's decision, but would certainly not push my luck to an 80mm aperture without adding an effective external ERF.

On an open forum, I'm not prepared to discuss this further. Drop me an email if you wish to continue this discussion.

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Do you think it would be alright to to do some observing during the upcoming annular eclipse? I will be in the path of annularity, so was thinking maybe a few moments of observation couldn't hurt with an eclipse magnitude of .9439, when it's maximized.

Even with my increased aperture of 80mm, I would figure 94% of the disc being blocked by the moon, would allow safe viewing for a few moments, absent the erf.

I'd just like to get a quick snap shot wit my dslr and webcam.

Thanks if you have any input on this.

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Why not make a cardboard aperture mask of 40mm diameter (securely attached) and make sure - if you want to point the PST before putting the proper ERF on the front? Better safe than sorry. I'm no expert but I do have a lot of experience of life in general and your own health and safety is paramount, not to mention that of your equipment.

Please don't take chances - it's just NOT worth it!

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