Question about QUARK CAH bandwidth?

[alexhuangyan]

Hello everyone：
The QUARK CAH manual mentions a focal ratio>=7 and a bandwidth of 5A; The white paper mentions the relationship between focal ratio and bandwidth;
When the working focal ratio F36-F48 (>F30), what is the approximate bandwidth of QUARK CAH?
Thank you for your reply!

[christian viladrich]

I am sorry to say this curve from DayStar white paper is completely wrong.

It is impossible to have a material (mica, fused silica, or whatever) giving a curve like this. The best possible fit is to have an index of 1.96 for the etalon gap, which is very far from the 1.65 index of mica. See more in the middle of this page :

http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

Back to your question, the correct curves are :
http://astrosurf.com/viladrich/astro/in ... spaced.jpg

http://astrosurf.com/viladrich/astro/in ... spaced.jpg

Regarding the actual FWHM of Quark, I measured five different units. FWHM ranges from better than 0.45 A to 2.1 A. See bottom of this page :

http://astrosurf.com/viladrich/astro/in ... lters.html

Hope this helps.

[Dennis]

Since it is a 5A filter it will remain at 5A at its best i would say.

[christian viladrich]

Since it is a 5A filter it will remain at 5A at its best i would say.

I read the initial question too fast. The curve from DayStar DayStar is for 0.5 A Ha filters, so I thouch the question was about 0.5 A Ha Quark ...

As said by Dennis, at f/30-40, a 5 A Ca K filter will have an effective FWHM of 5 A.

[alexhuangyan]

I am sorry to say this curve from DayStar white paper is completely wrong.

It is impossible to have a material (mica, fused silica, or whatever) giving a curve like this. The best possible fit is to have an index of 1.96 for the etalon gap, which is very far from the 1.65 index of mica. See more in the middle of this page :

http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

Back to your question, the correct curves are :
http://astrosurf.com/viladrich/astro/in ... spaced.jpg

http://astrosurf.com/viladrich/astro/in ... spaced.jpg

Regarding the actual FWHM of Quark, I measured five different units. FWHM ranges from better than 0.45 A to 2.1 A. See bottom of this page :

http://astrosurf.com/viladrich/astro/in ... lters.html

Hope this helps.

Dear christian viladrich,
Thank you for measuring the data.
Thank you very much！

[alexhuangyan]

Since it is a 5A filter it will remain at 5A at its best i would say.

The mica design is 5A, so F>48, the bandwidth is the same, right?

Thank you!

[Dennis]

Since it is a 5A filter it will remain at 5A at its best i would say.

The mica design is 5A, so F>48, the bandwidth is the same, right?

Thank you!

Yes. This is why you can use it with lower f-ratios like F/20 or even below without negative effects on the fwhm.
Different story with H-alpha filters that are around 0.5A. Here you need high f-ratios like F/40 and more to avoid broadening of the design fwhm.

[marktownley]

Since it is a 5A filter it will remain at 5A at its best i would say.

The mica design is 5A, so F>48, the bandwidth is the same, right?

Thank you!

Is it definitely a mica etalon? Do we know this for sure?

[alexhuangyan]

Since it is a 5A filter it will remain at 5A at its best i would say.

The mica design is 5A, so F>48, the bandwidth is the same, right?

Thank you!

Is it definitely a mica etalon? Do we know this for sure?

White paper introduces mica materials;

[marktownley]

Interesting. I would have thought a 5a design would be quite easy to do with filters alone.

[christian viladrich]

Interesting. I would have thought a 5a design would be quite easy to do with filters alone.

Given that the Quark Ca H is thermo-regulated, it is most probable that it is a mica-spaced etalon.

On top of that the DayStar Quantum Ca K (or H) 2 A and 5 A filters are mica-spaced etalons. So the mica used in the Quark is probably of a quality not high enough for the Quantum grade.

Basically, they use the technology there are familiar with.

Indeed, 5A filters can be dielectric (such as Alluxa or Andover). Starting from scratch, hard-coating would be much better for a 0.5 nm Ca K filter (higher transmission, longer life time, no need for thermal regulation, lower back focus). All of this for the same cost.
Maybe newcomers in the field will understand this sooner or later.

[Dennis]

"Indeed, 5A filters can be dielectric (such as Alluxa or Andover). Starting from scratch, hard-coating would be much better for a 0.5 nm Ca K filter (higher transmission, longer life time, no need for thermal regulation, lower back focus). All of this for the same cost.
Maybe newcomers in the field will understand this sooner or later."



Absolutely, i think the market for ca-k is too small for a company like Daystar or Lunt to push any "new" technologies. Antlia on the other hand could be a newcomer in this field and they use modern dielectric filters. But still no 5A ca-k filter on their website to see, just prototypes on their social media.