Rho Ophiuchi widefield Proline 16803 165mm Pentax

[gregbradley]

I have been using these Pentax 67 lenses for a little while now.
The 55mm F4 is OK but needs to be stopped down a bit and run at 2x2 binning to get rid of aberrations but is nicely very widefield. The 165mm F2.8 though runs fully open with little to no aberrations except a bit of chromatic aberration.

It lacks the resolution of a telescope but still fun. Bright stars come out a bit magenta and that needs to be processed out but otherwise it shows up a nice area reasonably well.

I took this one a while ago. I don't think I've nailed the processing yet but it didn't turn out too bad.

3 hours 15 minutes. 5 minute subexposures, F2.8, 2x2 binned, LRGB:

http://www.pbase.com/gregbradley/image/138661800/large

Greg.

[Lester]

Exceptional view, thanks Greg.

[jjjnettie]

I love this region, there's so much happening. Thanks for the view. :)

[SkyViking]

Great view and excellent colours. It's a spectacular area, and well captured!

[batema]

Beautiful colours and a lovely image. It looks fantastic.

Mark

[h0ughy]

its great to see images like this - reminds me of what the sky looks like - havent seen it for quite sometime

[RickS]

Lovely colours and dust, Greg!

[Ross G]

Hi Greg,

Your photo looks great.

Love the colours and the wide FOV.

The quality of your Pentax lens is obvious.

Thanks.

Ross.

[gregbradley]

Thanks Lester. I like these widefields. This represents about 12 panels of a mosaic with an FSQ and reducer and the 16803.



Thanks JJ. It is the probably the most colourful part of the Milky Way.



Thanks. I am happy with this little Pentax 67 165mm lens. Its a keeper.
The 55mm one shows more coma but is still usable. The 300mm is like a telescope and also shows no coma.



Thanks Mark.



Yes it has been cloudy lately hasn't it? I like this lens its a good match with the Proline.



Thanks Rick.



Thank Ross. These Pentax lenses are a bargain. I forget exactly how much I paid for it but it was something like US$250 or perhaps less on Ebay where there are lots of them.

Greg.

[Stevec35]

Nice colours Greg. A well composed shot.

Cheers

Steve

[strongmanmike]

Wow what a field Greg :eyepop:...you are going to have some fun with that setup I can tell. Looks like you have worked on the stars, I assume to remove the magenta halos? If you can improve this aspect I think you will produce some corkers with this setup...man, how many amazing optical trains do you have now :eyepop: :thumbsup:

Mike

[gregbradley]

Thank Steve.



Thanks Mike. Yes I have a few!

Yes some work on the stars and not perfect either. I was thinking of reprocessing from the ground up but other projects to work on.

I have a mosaic to stitch together.

Greg.

[atalas]

Nice work Greg,a unique looking composition.

[gregbradley]

Thanks Louie. I wanted to get the Blue Horsehead in it as well so I positioned the image that way.

Greg.

[tornado33]

In the old days of film it would take a dedicated schmidt camera to take images like that. Well done
Scott

[gregbradley]

Thanks Scott.

I have a 4 panel (intended 6 panel if I can get it) mosaic to put together as well from the same lens.

I have to get my processing routine down pat for this lens as it has slightly different problems to my normal telescope images.

Greg.

[multiweb]

Huge FOV and very vivid colors. Nice one. :thumbsup:

[gregbradley]

Thanks Marc.

It turned out to be a good lens. The 300mm F4 seems to be good as well.
What was that filter you got for your lens? Did that reduce the chromatic aberration (the odd magenta bright star)?

Greg.

[multiweb]

2" Baader Fringe Killer. They retail at approx. $50.00. Worked a treat for me. A must for an OSC. Not so much for a mono IMHO.

[gregbradley]

Thanks Marc. Where did you get it from?

Did it need an adapter to fit the lens?

Greg.

[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.

[gregbradley]

Thanks Marc.

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

Greg.

[midnight]

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]

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]

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.