What kind of scope for Lunt Cak filter.

[Nigella Bryant]

Hi all, I'm considering adding Cak imaging to my HA telescope. Would a 90/100mm achromatic refractor suffice or would it be better to get an ED?

[Geoff Lister]

As none of the experts has posted a reply; here is my take on it. The apo/achro/ED/doublet/triplet lens systems are designed to get a common focal point with the widest range of optical wavelengths. When you use a narrow-pass-band Ha or CaK filter, there is only one wavelength to consider. The CaK and Ha focus points will be different, but it does not matter, as you only use 1 filter at a time.

A quick look at the SGL "Imaging -Solar" section, seems to advise a minimum f10 (even better, f15) refractor. A 90mm f10, stopped down to 60mm may work well.

Geoff

[Nigella Bryant]

13 minutes ago, Geoff Lister said:

As none of the experts has posted a reply; here is my take on it. The apo/achro/ED/doublet/triplet lens systems are designed to get a common focal point with the widest range of optical wavelengths. When you use a narrow-pass-band Ha or CaK filter, there is only one wavelength to consider. The CaK and Ha focus points will be different, but it does not matter, as you only use 1 filter at a time.

A quick look at the SGL "Imaging -Solar" section, seems to advise a minimum f10 (even better, f15) refractor. A 90mm f10, stopped down to 60mm may work well.

Geoff

Thanks Geoff, appreciate your insight. 

[Merlin66]

Nigella,

A couple of things.

Yes, using an extreme narrowband filter suppresses any residual chromatic aberration...

But not all scope are well designed to work in the UV - spherical aberration can be a real issue. Luckily stopping down the aperture helps.

I use an ED80 f7.5 stopped down to 60mm to give me the "minimum" recommended f ratio. Better still would be a larger aperture, longer focal length stopped to f15 (or greater)

Resolution in the CaK is 1.66 better than that in H alpha - based on the shorter wavelength - this effectively means a 60mm in CaK works like a 100mm in H alpha.

The CaK images (significantly reduced for upload) show some results with my DIY Cak filter (1A bandwidth).

Hope that helps.

[Nigella Bryant]

1 hour ago, Merlin66 said:

Nigella,

A couple of things.

Yes, using an extreme narrowband filter suppresses any residual chromatic aberration...

But not all scope are well designed to work in the UV - spherical aberration can be a real issue. Luckily stopping down the aperture helps.

I use an ED80 f7.5 stopped down to 60mm to give me the "minimum" recommended f ratio. Better still would be a larger aperture, longer focal length stopped to f15 (or greater)

Resolution in the CaK is 1.66 better than that in H alpha - based on the shorter wavelength - this effectively means a 60mm in CaK works like a 100mm in H alpha.

The CaK images (significantly reduced for upload) show some results with my DIY Cak filter (1A bandwidth).

Hope that helps.

 

Thanks, I've got a couple of fracs a 70mm actromatic, not sure of focal length probably an f6, an 80mm ed f6.7.

 

 

1 hour ago, Merlin66 said:

 

 

 

[Nigella Bryant]

Just an update. I've only brought the baader K line filter 395nm at the moment and got an 80ed f6.8.. saving for the biggie, the Lunt cak. Preliminary results seems a good match. 

Edited February 2, 2021 by Nigella Bryant

[Carl Au]

Lovely 

[Nigella Bryant]

14 hours ago, Carl Au said:

Lovely 

Thanks, wasn't sure what the baader k line filter would be like because it's quite a bit winder than the Lunt cak. 10angstrum instead of the Lunt 2.4 but pleased. It's not a cheap filter at £250 but far less than Lunt at £1000 to £1500 plus. Also the baader k line can eventually be used with the Lunt Cak to increase contrast so nothing lost.

Sorry, quoted 10A for the baader, that's wrong, it's 80A, thanks to merlin66 for putting me right. ed. 

Edited February 2, 2021 by Nigella Bryant
Added comment

[Carl Au]

I have always found Baader filters be excellent, I am a bit of a Baader fan as it goes.

[Merlin66]

Hhmmm

The quoted bandwidth (FWHM) for the Baader K-Line filter is 80A ( https://www.baader-planetarium.com/en/baader-k-line-filter-1¼"-(double-stacked).html)

whereas the Lunt CaK is said to be <2.4A bandwidth.

 

 

[Nigella Bryant]

1 hour ago, Merlin66 said:

Hhmmm

The quoted bandwidth (FWHM) for the Baader K-Line filter is 80A ( https://www.baader-planetarium.com/en/baader-k-line-filter-1¼"-(double-stacked).html)

whereas the Lunt CaK is said to be <2.4A bandwidth.

 

 

My bad, quoted wrong, I'm a numpty, lol 80A as apposed to a 2.4A. never intended to mislead anyone. I blame my age, thought I'd read something about baader K line  filter being 10  something. Again, my bad. Thank goodness for more knowledgeable people than me. I don't know my angstroms from my nanometres. 

Edited February 2, 2021 by Nigella Bryant

[Nigella Bryant]

Also had to get some step rings so I can get a decent full disk with the 0.5reducer with the baader K line filter. 

Edited February 2, 2021 by Nigella Bryant

[Ibbo!]

I'm using my Lunt with an ED 80 and flat fielding with a plastic bag (more successful than with Ha)

I will be trying out the 127 1200 Bresser when the sun gets a bit more elevation from here.

Getting better results with the larger pixel sized Pt Grey Blackfly Cam, though again that could be down to elevation.