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Grumpy Martian

Where do achromatic refractors figure in today's world?

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15 hours ago, Vondragonnoggin said:

I was out last night with my Startravel 150 and a Williams Optics binoviewer. Highest I took it was 150x with a semi-apo in the diagonal and this was the first time I had tried that scope above 50x.

It was actually some great views with a pair of 9mm BST Planetary EP’s. I used the 1.8x GPC with the Binoviewers to achieve focus. The previous owner told me he hand picked the scope after testing three of them and said it was surprisingly a good figure for that model. He was right.

 

I usually use my mak or 6” F/5.9 achro, but the F/5 was actually putting up some stunning views in my opinion.

6 bands on Jupiter. Not large enough image scale to see the blue eddies in the bands, but what a steady view with no shimmer at all. Saturn was the same with several bands visible, Cassini visible but not Encke, 5 moons around Saturn although two of them I had to use averted vision to get them to blink in. Unbelieveable steady views though.

 

Maybe I’m just too impressed by anything I’m able to see where views are very steady and some amount of detail present. I even pointed at a few nebula with no filter and was able to make out the Lagoon, Triffid, Omega, and just barely a haze for the Eagle, a few globs, and Wild Duck cluster was stunning.

 

It surprised me. I normally steer people away from high power views with the short tubes, but maybe I’ve just been echoing advice from other members that don’t use short tubes and frequently debate best apos. I guess I’ll change the advice to “try it if you feel like it. The worst case is you’ll find you don’t like it at high powers”

It's so easy to echo the low opinions, based on the often inexperienced or prejudiced views of others. But it takes a measure of courage to go against the flow and point out the reality. Youve got to be commended!

I've always found the ST refractors impressive, and only seen one poor one out of twelve. The poor one, a 6" F5, was replaced by the vendor and all was well. In 2004, a friend who'd bough a ST102 brought it to me to see what I thought. I'd used several prior to that date, so knew what to expect. I aimed the little Star Travel at a rapidly shrinking Mars, fit a 3X Televue barlow, and saw a perfect view of Syrtis Major on a 5.5 arc second disc. The ST150's are superb RFT's, but I was impressed at the magnification they could take, considering their presumed limitations. I found that 200X was reasonable, and I never used a fringe killer on any of them,  as to me they performed better without. :icon_cyclops_ani:

 

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Here's just one example of where the achromat would fit into today's world, and there are many:

311272373_2019-05-3116_13_51.thumb.jpg.49237af2918bbfc855fb42912c4d61f2.jpg1155825737_2019-05-3116_14_47.thumb.jpg.0b603300ef15ae921a3b1a00ccafbc42.jpg

Starlight Nights is one of the finest, most inspirational books I've ever read. It's a great fireside read and a real motivational force, written by and telling the story of one of the world's greatest armature astronomers.

Buy it! Read it! Lust it! Live it!!

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1 hour ago, mikeDnight said:

Starlight Nights is one of the finest, most inspirational books I've ever read. It's a great fireside read and a real motivational force, written by and telling the story of one of the world's greatest armature astronomers.

Buy it! Read it! Lust it! Live it!!

Spot on Mike. Best book ever!

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18 minutes ago, John said:

This one will probably show quite a bit of false colour - I'd still like to have a look through it though :grin:

https://www.astrobuysell.com/uk/propview.php?view=150342

 

I'd buy that in a heartbeat if I could afford it John, and I'd invite you to the inaugural first light on Vega. If everyone clubs together to buy it, I'd be happy to build an observatory for it in my garden, with free viewing to all who contribute to the initial purchase.

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This thread has got me thinking whether I really should upgrade my ST80 to an ED72. Maybe I should double down on the achro goodness and get an ST102 instead?

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Posted (edited)

The oft-quoted table! <G> But I do think there is significant wisdom re. Chromatic Aberration! 😎

CA-index11.jpg.f247ad825520f6051294c877fb599850.jpg

I do think there is (review) evidence that "cheaper" Achro scopes have other aberrations as well.
For my Solar (monochromatic H-Alpha) Frankenscope I chose an ED80 over other Achromats?
But ED80s are *relatively* cheap. With increased aperture... an APO 'Frac: the "Sky's the Limit" 😸

Achromats are good FUN -- Particularly if you are on a limited budget! 🙃

Edited by Macavity

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Gotta be paper for me. I'll run the Archive.org text through a publishing platform and get it perfect bound.

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9 minutes ago, johninderby said:

Id rather have it on Kindle though. 🙂

Fair enough.

I don't have a kindle, but have a bought a lot of "1p" books off amazon (+£2.79 p/p). I first went looking for this there, but the cost of a physical copy was a bit ott for me for this sort of work. A pdf suits me though.

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2 minutes ago, Ags said:

Gotta be paper for me. I'll run the Archive.org text through a publishing platform and get it perfect bound.

Best of both worlds.

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It will be a bit of editing work to clean up the OCR text and restore formatting, but worth it I think. I will look at the copyright status to see if I can share the results publicly...

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24 minutes ago, Demonperformer said:

Fair enough.

I don't have a kindle, but have a bought a lot of "1p" books off amazon (+£2.79 p/p). I first went looking for this there, but the cost of a physical copy was a bit ott for me for this sort of work. A pdf suits me though.

On my iPad actually. Find being able to adjust the text size and style to make it easier to read makes reading a lot more enjoyable and easier.  Just works for me. 👍🏻

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23 minutes ago, Peter Drew said:

I already have a 220mm F12.5 and the CA is quite noticeable.   😀

My 180/12 R35 iStar shows very little false colour to my eyes.
The claimed R35% reduction in ca offers an f/16 equivalent correction in an f/12 tube length.
That's a 28" reduction in moment arm. iStar offer a range of faster, R% objectives.
My 7" happily holds 200x, or more, on the moon and sun.
While my 6" f/8 Celestron would never allow more than 120x even with a Fringe Killer.
I spent years blaming the seeing conditions.
The 6" now works better as an H-alpha telescope at 120mm/10 equivalent rather than white light.

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Posted (edited)
3 hours ago, Rusted said:

My 180/12 R35 iStar shows very little false colour to my eyes.
The claimed R35% reduction in ca offers an f/16 equivalent correction in an f/12 tube length.
That's a 28" reduction in moment arm. iStar offer a range of faster, R% objectives.
My 7" happily holds 200x, or more, on the moon and sun.
While my 6" f/8 Celestron would never allow more than 120x even with a Fringe Killer.
I spent years blaming the seeing conditions.
The 6" now works better as an H-alpha telescope at 120mm/10 equivalent rather than white light.

I almost bought an Istar Phoenix with R35 lens. Actually ordered it and then cancelled the order after 5 months wait and no delivery in sight. Ales was having issues lens deliveries at the time and was very nice about it, but I didn’t feel like waiting any longer and bought my Astro-Telescopes AT152 from a classified ad. 

 

Moonraker Telescopes has an amazing “Wide Boy” 6” F/5 that you can get with either a Jaegers lens or Istar R35 lens. A bit spendy, but then his scopes are works of art in fit and finish. Someday....

Edited by Vondragonnoggin

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Posted (edited)

My very good friend Nick (cotterless45) got me into Achromatics I had a 8" reflector which I sold I now have a Tal 100rs f10 and a Meade 127mm F9. 3 I love them both. The Tal is great for solar and a quick grab and go.

The Meade is brilliant on double and multiple star's but also great on clusters.

I may in the future get myself another 8" but as I don't have dark skies  it's not worth it having  stuck gathering dust.

Plus I would have to get a better mount a 8" is too big for the EQ5 IMHO.

 

Edited by wookie1965
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Thinking through the achomatic refractors that I've owned over the years, I came up with this list:

Tasco 60mm F/13.3 ** (1st scope and it got me hooked so it gets two stars)

Vixen SP102M F/9.8 **

TAL 100RT F/10 (3 of these) ** (to be fair the 3rd was not as good as the other two)

Skywatcher Evostar 120mm (F/8.3)

Meade AR5 127mm F/9.3 (2 of these)

Bresser Messier 127L F/9.3 **

Skywatcher (and other brandings) ST80 F/5 (3 of these)

Celestron ST102 F/5

Skywatcher (and other brandings) 150mm F/8 (4 of these) * (** with Chromacor fitted)

Meade AR6 152mm F/7.9

Istar 150mm F/12 (monster !) * (superb figure to the objective bit it showed more CA that I was expecting and it was a so and so to get mounted properly)

Of these the ones that I found most enjoyable are marked with either one or two stars, depending on the level of enjoyment and performance delivered. The ones without any stars were probably OK but did not leave any lasting impression with me.

If I had another chance to own one of these, bearing in mind the ED doublet scopes that I now own, I think it would be the TAL 100RT and an early one at that, for old times sake :icon_biggrin:

tal100rt.jpg.7e04278ffd65286fae50e3355c03df7b.jpg

 

 

 

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On ‎30‎/‎05‎/‎2019 at 14:47, mikeDnight said:

I've always found the ST refractors impressive, and only seen one poor one out of twelve. The poor one, a 6" F5, was replaced by the vendor and all was well. In 2004, a friend who'd bough a ST102 brought it to me to see what I thought. I'd used several prior to that date, so knew what to expect. I aimed the little Star Travel at a rapidly shrinking Mars, fit a 3X Televue barlow, and saw a perfect view of Syrtis Major on a 5.5 arc second disc. The ST150's are superb RFT's, but I was impressed at the magnification they could take, considering their presumed limitations. I found that 200X was reasonable, and I never used a fringe killer on any of them,  as to me they performed better without. :icon_cyclops_ani:

 

Interesting that somebody else found the Startravel 102 worked well with a Barlow. Mine seemed to work well with a kit Skywatcher X2 Barlow which appeared to counter the chromatic aberration and give good planetary views.

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4 minutes ago, Cosmic Geoff said:

Interesting that somebody else found the Startravel 102 worked well with a Barlow. Mine seemed to work well with a kit Skywatcher X2 Barlow which appeared to counter the chromatic aberration and give good planetary views.

The CA is still in the system though - using a barlow lens does not actuallty reduce it, despite the longer virtual focal length.

 

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16 minutes ago, John said:

The CA is still in the system though - using a barlow lens does not actuallty reduce it, despite the longer virtual focal length.

 

In theory barlow can reduce CA - but not because of shape of the light cone, or "longer focal length". It can do that with fast achromats if inserted deeper inward because it can "vignette" light cone acting like sort of aperture stop - blocking outer parts of lens.

This will depend on barlow used, its clear aperture and speed of scope. For F/5 scope, at 100mm from focal position beam is 20mm wide. In theory barlow with small clear aperture and that needs to be more than 100mm from focal position can introduce this sort of vignetting and act as aperture stop - lowering CA a bit.

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41 minutes ago, vlaiv said:

In theory barlow can reduce CA - but not because of shape of the light cone, or "longer focal length". It can do that with fast achromats if inserted deeper inward because it can "vignette" light cone acting like sort of aperture stop - blocking outer parts of lens.

This will depend on barlow used, its clear aperture and speed of scope. For F/5 scope, at 100mm from focal position beam is 20mm wide. In theory barlow with small clear aperture and that needs to be more than 100mm from focal position can introduce this sort of vignetting and act as aperture stop - lowering CA a bit.

This may explain why using a PST etalon and filter can improve the performance of my 150/8 when used for H-a.
The focal ratio is altered to f/10 and the aperture reduced to an equivalent 120mm.  I find my PST Etalon group is weakly negative like a Barlow.
The very narrow filtration passband will obviously reduce CA but will not alter the spherical figure of the lens.

Ironically, I never found any improvement from simply stopping down the 150/8 using a ring baffle in front of the objective.
Always preferring the view with the full aperture in white light. The same holds true for the 180/12 R35 iStar.
Adding an aperture stop never seems to provide the supposedly "magical" benefits of a "slower" lens when it really ought to.

D&G Optical claimed there was little difference to choose between their 6" f/12 and their f/15 and it was more a matter of taste.
No doubt the elderly observer, with natural yellowing of their eye lenses with age, would agree. Saving 18" on tube length has greater advantages.
A "Fringe Killer" filter provided obvious benefits with my 150/8 [and was always fitted for white light] but not with the 180/12 R35.
I have never tried the green "Solar Continuum" on the 150/8 but use one routinely on the 180/12 R35 for both solar and lunar and for both visual and imaging.

Unfortunately iStar lenses are now becoming very expensive in comparison with complete Chinese ED scopes.
I would not have had much change from £900 for a 150/10 achromat in its cell via the UK dealer for my H-a plans. Perhaps I should have tried the 180/12 for H-alpha?
A Chromacorr, with my name on it, has never crossed my path.

 

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56 minutes ago, Rusted said:

...A Chromacorr, with my name on it, has never crossed my path.

 

I'm fortunate to have owned 3 of them over the years. They are remarkable devices but are/were very expensive and now that the cost of ED doublets has dropped, the market for Chromacorr's has dried up in the main.

At one time Istar were developing a similar optical corrector but the project did not reach fruition. Astro Physics produced a very early SA / CA corrector but that never reached more than single figure production volumes.

 

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