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Is there a F/5 150mm+ ED doublet?

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#26 Jeff B

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Posted 25 January 2019 - 12:07 PM

An achromat can bring two colors to a common focus.  The conventional approach is to bring the red and blue to that common focus.  You see that in spot diagrams that has green as the focal point.  The red and blue out of focus color blurs are typically much larger than the airy disk but are of equal size.  That out of focus red and blue is the purple "CA" border around stuff you typically see in achromats.

 

My take on the Istar R series lenses is that the "reduction" in CA is more of a "redistribution" than an actual reduction.  Just take a look at how the spot diagrams are presented.  A good, clear example is the 6" F8 R35.

 

The designs appears to bring the focus of the blue portion of the spectrum much closer to green/yellow than that of red.  As presented, the on-axis spots for the green and blue fill the airy disk but the red is puffed much further out.  You also see this in the contrast plot for the different colors, where the green and blue are grouped together and the red falls well below them. 

 

Now take a look at the same spot diagram for the 6" F5 achromat.  You see that the red and blue spots are much closer to each other in size, which is very much like a conventional Fraunhofer achromat design.  

 

So to me, it seems that the two colors that Istar has chosen to bring to a more common focus in the R series lenses, are the blue and green/yellow, letting the red fall off.

 

From a visual point of view, pulling blue much closer to the green/yellow focus and letting the red focus bloom, may make sense.  In dimmer light, the eye/brain is much more sensitive to blue than red.  Puffing the red focus out even more will make it much dimmer and much more difficult for the eye to detect since it's spread out much more.  With the blue also now being much closer to green, it seems to give a more white image on some objects as some have reported. 

 

Jeff


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#27 starcanoe

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Posted 25 January 2019 - 12:15 PM

Good point Jeff...

 

As a long time dabbler in the theory of these things the thought has often been in the back of my mind that bog standard "correction" of achromats might not be totally optimized once ALL the factors are considered.

 

I've even contemplated designing a lens where it is specifically corrected for Mars, Jupiter, or Saturn...with a specific (and probably mild filter) in mind. I suspect you you would not actually HAVE to design 3 different lenses but that you could design a design (heh) that was a modest improvement over the standard achromat with each planet having a filter that made sense for that planet and the achro in question....



#28 Jeff B

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Posted 25 January 2019 - 12:31 PM

The old lens makers from the 19th century did exactly that, but in that case they let the blue portion of the spectrum hang out there, bringing the red closer in for scopes set up for studying the planets like Saturn, Mars, and Jupiter as they have considerable yellow/red content.

 

Jeff


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#29 MSWcdavis

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Posted 25 January 2019 - 02:45 PM

What about a 130 f6 doublet with fpl 51 Ed glass?

#30 Marcsabb

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Posted 25 January 2019 - 04:24 PM

What about a 130 f6 doublet with fpl 51 Ed glass?

With the right eyepieces it can also be a good 'richfield' telescope, no doubts about that. A 150mm, especially at F/5, could be a bit more effective with difficult, faint DSO when the opportunity arises and it's also the closest thing to a fast newtonian, with the added bonus of somehow better convenience and 'ruggedness' required by somebody who often spend weekends outdoors, camping.


Edited by Marcsabb, 25 January 2019 - 04:31 PM.


#31 beanerds

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Posted 25 January 2019 - 06:31 PM

You piqued my interest. I've read the description of the R50 "anastigmatic" lens on the Istar page but there is little info on how this lens achieve better correction. Spot diagrams also seems to display astigmatism confused1.gif  I've read somewhere that Istar lenses are made more corrected towards the blue at the expense of red; for visual use at medium magnifications, especially with a prism, it is usually a good enough compromise to be considered a semi-apo. Is that the case?

I could not answer that but I too have an iStar lens in my home made scope , a 127mm f8 and it has very good CA and SA  correction , so good in fact that after I finished the build I tested it side by side many times with my 127mm f7.5 triplet and in the end sold the triplet .

 

I still have and love my iStar after 4 years , yes you should look into one of their 150mm f5 R50 lens sets , if anything like mine it will be awesome . Their coatings are very well done as well , here is mine .

 

iStar do a good quality product at reasonable prices  .

 

Good luck with your search and please keep us updated .

 

Beanerds.

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Edited by beanerds, 25 January 2019 - 06:36 PM.

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#32 ValeryD

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Posted 25 January 2019 - 07:12 PM

An achromat can bring two colors to a common focus.  The conventional approach is to bring the red and blue to that common focus.  You see that in spot diagrams that has green as the focal point.  The red and blue out of focus color blurs are typically much larger than the airy disk but are of equal size.  That out of focus red and blue is the purple "CA" border around stuff you typically see in achromats.

 

My take on the Istar R series lenses is that the "reduction" in CA is more of a "redistribution" than an actual reduction.  Just take a look at how the spot diagrams are presented.  A good, clear example is the 6" F8 R35.

 

The designs appears to bring the focus of the blue portion of the spectrum much closer to green/yellow than that of red.  As presented, the on-axis spots for the green and blue fill the airy disk but the red is puffed much further out.  You also see this in the contrast plot for the different colors, where the green and blue are grouped together and the red falls well below them. 

 

Now take a look at the same spot diagram for the 6" F5 achromat.  You see that the red and blue spots are much closer to each other in size, which is very much like a conventional Fraunhofer achromat design.  

 

So to me, it seems that the two colors that Istar has chosen to bring to a more common focus in the R series lenses, are the blue and green/yellow, letting the red fall off.

 

From a visual point of view, pulling blue much closer to the green/yellow focus and letting the red focus bloom, may make sense.  In dimmer light, the eye/brain is much more sensitive to blue than red.  Puffing the red focus out even more will make it much dimmer and much more difficult for the eye to detect since it's spread out much more.  With the blue also now being much closer to green, it seems to give a more white image on some objects as some have reported. 

 

Jeff

+100



#33 NC Startrekker

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Posted 25 January 2019 - 08:12 PM

What about a 130 f6 doublet with fpl 51 Ed glass?

How about a double doublet so to speak. The TeleVue NP-127 has a Petzal lens design. Essentially this design employs a doublet objective up front followed by another doublet at the focuser end which corrects for field curvature. The NP-127 has 127mm of aperture at f/5.2 with no CA and a flat field. It is light at less than 15lbs and fairly compact. But all of this does not come cheap. 


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#34 iKMN

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Posted 25 January 2019 - 09:56 PM

I “settled” on a Mak/Newt ES152mm to get close to the goals as OP. Flat, color free field fast F4.8 - I really like it. It’s not for everybody but it compliments my other scopes very well. Otherwise the TV101 is the way to go and the scope I want but I don’t have that kind of money.

K

#35 Tjn58

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Posted 25 January 2019 - 11:12 PM

Did you build this yourself (using the Istar lens cell)?

Post a pic if possible.

Also, I'm intrigued by the lack of CA as my understanding is a 150mm F5 R50 would be equal to a 150mm F7.5 Achromat which should exhibit significant CA at 200X's.

I actually bought the original prototype scope from Ales at Istar.  I don't know how to post pictures, but if you go to this website and scroll about half way down, you will see my scope (it was used for all the original advertisement photos).  Like I mentioned earlier, I have old eyes and I suppose my tolerance to CA is somewhat higher but it was a great addition and continues to be a joy to use.

 

 https://www.peak2val...ge_2238911.html

 

Tom


Edited by Tjn58, 25 January 2019 - 11:28 PM.


#36 junomike

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Posted 27 January 2019 - 12:13 PM

I actually bought the original prototype scope from Ales at Istar.  I don't know how to post pictures, but if you go to this website and scroll about half way down, you will see my scope (it was used for all the original advertisement photos).  Like I mentioned earlier, I have old eyes and I suppose my tolerance to CA is somewhat higher but it was a great addition and continues to be a joy to use.

 

 https://www.peak2val...ge_2238911.html

 

Tom

Nice Looking RFT!



#37 Marcsabb

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Posted 27 January 2019 - 02:39 PM

I actually bought the original prototype scope from Ales at Istar.  I don't know how to post pictures, but if you go to this website and scroll about half way down, you will see my scope (it was used for all the original advertisement photos).  Like I mentioned earlier, I have old eyes and I suppose my tolerance to CA is somewhat higher but it was a great addition and continues to be a joy to use.

 

 https://www.peak2val...ge_2238911.html

 

Tom

I wonder how difficult would be to adapt the Istar lens cell to an old Celstron Omni 150r or like my friend's Travelstar. Used, they come very cheap and they are already dimensioned to hold a 6" F/5 doublet. 



#38 junomike

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Posted 27 January 2019 - 06:12 PM

I wonder how difficult would be to adapt the Istar lens cell to an old Celstron Omni 150r or like my friend's Travelstar. Used, they come very cheap and they are already dimensioned to hold a 6" F/5 doublet. 

I thought about this also as I have a SW 150 F5 however the OTA would most likely need to be cut as the F/L of the Istar 150 is 690mm (actually F4.6) which is over 2" shorter than the F5/750mm versions.  Otherwise it would Ideal.



#39 Marcsabb

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Posted 28 January 2019 - 01:41 AM

I thought about this also as I have a SW 150 F5 however the OTA would most likely need to be cut as the F/L of the Istar 150 is 690mm (actually F4.6) which is over 2" shorter than the F5/750mm versions.  Otherwise it would Ideal.

I don't have a Omni/Startravel at hand but, from what I remember, the cell supports structure, a die cast monolithic piece, extends for 60-70mm. Look at this pic I've found here on CN's ads. I suppose it's being screwed onto the main tube, so designing a shorter ring with a flange should do the trick. Can you please check on your tube if my guess is correct?

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#40 25585

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Posted 29 January 2019 - 06:45 PM

I always wonder/fantasise what a Tele Vue NP150 would have been like, been made from etc.


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#41 jay.i

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Posted 29 January 2019 - 09:28 PM

I always wonder/fantasise what a Tele Vue NP150 would have been like, been made from etc.

The rare TV-140 is pretty close... I believe it is rumored to use fluorite but Al won't confirm it.



#42 Marcsabb

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Posted 30 January 2019 - 04:55 AM

The rare TV-140 is pretty close... I believe it is rumored to use fluorite but Al won't confirm it.

Odd that he wouldn't confirm... It would be a premium feature to advertise!

 

At any rate, the more I think about that 150mm F/4.6 Istar doublet the more I want to try it and see how it fares against my friend's Travelstar 150 and against a high end fast refractor like my Takahashi Sky-II with its flattener/reducer at 4.5.

 

Btw, I found a rare review of the Travelstar 150 http://www.astrotest...er-150mm-achro/ in which the author also tries to use it for astrophotography. Obviously, he recommends narrowband filters but surprisingly he finds it useful also for planets and dobules despite the obvious limitations. Of course a fast refractor, especially an achro one, wouldn't be the first choice for many tasks, beside low power 'star sweeping', but it's dfficult to argue against the ruggedness and convenience of a refractor.



#43 Riccardo_italy

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Posted 30 January 2019 - 06:52 AM

This will tell you something. I've picked some of the fastest triplets I know.

 

80mm: f5 (SW ESPRIT)

100mm: f5.5 (SW ESPRIT)

115mm: f5.5 (ES FPL53)

130mm: f6 (APM LZOS)

140mm: f6.5 (ES FPL53)

150mm: f7 (SW ESPRIT)

 

As you see, the greater the diameter, the slower the f-ratio. That's the reason why in refractor the telescope's size growth exponentially with the diameter: not only a 100mm f5 is longer than an 80mm f5, but you also need to increase the f-ratio (from f5 to f5.5)


Edited by Riccardo_italy, 30 January 2019 - 06:55 AM.


#44 salico

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Posted 30 January 2019 - 06:57 AM

CFF 160mm: f6.56



#45 Marcsabb

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Posted 02 February 2019 - 04:51 PM

Some nice pictures of a homemade refractor based on a istar 150mm R50 doublet:

 

https://maximusphoto...ractor-is-born/




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