Rho Ophiuchi widefield Proline 16803 165mm Pentax

[multiweb (Marc)]

Originally Posted by gregbradley

Thanks Marc. Where did you get it from?

Did it need an adapter to fit the lens?

Greg.

I got it from Peter Tan. Pentax Lenses come in 52/49mm. The baaders are all M48x.75 so you'll need a step up/down ring. There are stacks on eBay for $10-$20. You can also use them as aperture masks. Stops some lenses nicely without the annoying diffraction spikes.

[gregbradley]

Originally Posted by multiweb

I got it from Peter Tan. Pentax Lenses come in 52/49mm. The baaders are all M48x.75 so you'll need a step up/down ring. There are stacks on eBay for $10-$20. You can also use them as aperture masks. Stops some lenses nicely without the annoying diffraction spikes.

Thanks Marc.

I'll contact him and check out Ebay. Thanks for the tips.

Greg.

[midnight (Darrin)]

Very striking Greg! This is an excellant combination for such widefields. Marc also put me on the idea of the fringe killer as without one, my images are being killed at the moment.

Well done and looking forward to seeing more!!

Darrin...

[gregbradley]

Thanks Darrin!

Greg.

[Ross G]

Hi Greg,

As you are imaging through the Pentax lenses in RGB with a monochrome camera, shouldn't being able to focus the 3 colours independently allow you to get rid of the chromatic aberrations...or am I barking up the wrong tree?

Thanks.

Ross.

[gregbradley]

Originally Posted by Ross G

Hi Greg,

As you are imaging through the Pentax lenses in RGB with a monochrome camera, shouldn't being able to focus the 3 colours independently allow you to get rid of the chromatic aberrations...or am I barking up the wrong tree?

Thanks.

Ross.

Good question. It does not seem to work out that way Ross. I suppose if you refocused on every colour filter change which I do not then it may limit it. But there's more to it than that. I think the performance of the lens in the blue channel is less than in other channels.

There is a German guy Rohr who posts detailed optical testing of various telescopes and one of these tests shows the Strehl ratio for various colours. It is interesting to note not many have a high strehl ratio in all colour channels and often good in a few and poor in another.

So that to mean means that the lens or telescope will not be as sharp in certain colour frequencies and also within the blue band you would have various shades of blue as per the spectrum with deviating performance within that band.

That is how I see it unless someone can chime in here with more data.

So in the case of these lenses the bright stars seem to be magenta.

I had a fluorite doublet FS152 and it would have blue haloed bright stars often in LRGB images. In narrowband it was perfect. That's because doublets even high quality doublets do not focus all colours at the same spot like a high end triplet will.

These lenses are also not exotic glass and they don't perform as well across the spectrum as a result.

Greg.

[Ross G]

Thanks Greg,

I felt it couldn't be that easy.

How about shooting in narrowband?

Would photo through a Ha filter through an achromat be the same as one through an equivalent apo?


Ross.

[gregbradley]

Originally Posted by Ross G

Thanks Greg,

I felt it couldn't be that easy.

How about shooting in narrowband?

Would photo through a Ha filter through an achromat be the same as one through an equivalent apo?


Ross.

Its my understanding that narrowband does reduce the chromatic aberration so it should turn out fine. Lenses also. Would it be as sharp as a sharp APO? I suppose there are other aberrations besides chromatic that tend to be in achromats only because APOs also correct for these other aberrations as well.

It'd be worth finding out.

Certainly my FS152 was extremely sharp in narrowband images.

Greg.

Greg.