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Dual filters for fast imaging systems


Phillyo

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Good evening,

So I had an Optolong l-extreme filter on order to use with my ASI533 and Samyang 135/2 system, however I have just watched a video on youtube where a guy was using that filter with a RASA at f2 and was getting next to nothing in the Ha channel until he dropped it to f6.3. He linked this document: http://karmalimbo.com/aro/reports/Test Report - Optolong L-eXtreme Filter_Aug2020.pdf which shows that at f2, it has as low as 10% transmission at f2 so I have cancelled the order for now (it's on backorder anyway).

I'm wondering if the IDAS NBX filter would be a better option for faster systems? Does anyone have any experience with either filter at fast focal ratios or know of any information I could read about them?

Thanks in advance.

Phil. 

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Having done some research online (should maybe have done this sooner, sorry!) it seems the IDAS NBX was actually made specifically for faster focal ratio systems, so I've ordered one of those instead. They're on back order too at the moment I believe but hopefully once it's in stock I can give it a shot and see how it performs!

Phil

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 I was going to go for the L-extreme until @gorann posted that he had ordered the IDAS NBX dual narrowband filter for his RASA 8 for the reasons you describe, this filter coping better with the band shift and transmission loss that occurs at low f ratios.

His filter was delivered today, so look out for his first light post.

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42 minutes ago, tomato said:

 I was going to go for the L-extreme until @gorann posted that he had ordered the IDAS NBX dual narrowband filter for his RASA 8 for the reasons you describe, this filter coping better with the band shift and transmission loss that occurs at low f ratios.

His filter was delivered today, so look out for his first light post.

Well, it looks like it will now be raining for the next week, so be patient🥴 I also need a holder for the filter on the RASA8 / ASI2600 combo and I have ordered one from Artesky in Italy that is supposed to be in at the end of October:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11535_Artesky-2--Filterhalter-und-T2-Adapter-fuer-Celestron-RASA8-Astrograph.html

(I ordered mine directly from Artesky in Italy since I hope it may be faster)

Meanwhile, if it clears I will just use the ASI1600MM and Baader UFS 2" filter slider with my Baader filters. I have a High speed Baader Ha and the ordinary Baader 8.5 nm Oiii that seems to work fine at f/2, as seen here on the rear behind of the Lion nebula:

https://www.astrobin.com/3g6ee0/C/

Edited by gorann
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4 minutes ago, Erling G-P said:

Quite puzzled here - why would the f-ratio have any influence on how a filter performs?

The filter is an optical element of the imaging train.  Light refracts through it.  And at the relatively steep angles of incidence occurring at small f-ratios perhaps that results in focusing/bloating issues as well as light loss due to the greater amount of material through which the light has to travel?

James

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11 minutes ago, Erling G-P said:

Quite puzzled here - why would the f-ratio have any influence on how a filter performs?

Because with interference filters, width of layer is what determines reflection. When light hits at an angle - it is effectively going thru thicker layer. Fast F/ratio means that some of the light is hitting filter at shallower angle and that light might be reflected instead of passed because of this - effectively creating aperture stop.

j00UtLp3Bpr2RRk9a8CA_rKvt3zABYW7YuoMgCod

This image shows LPS filter and how it shifts towards the blue depending on angle of incidence of light ray.

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