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Equipment Discussions >> Refractors

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tomharri
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Reged: 09/19/08

Loc: USA
Re: Istar 5" f/8R30, first light new [Re: Gord]
      #5673852 - 02/11/13 11:14 AM

How do you tell if coated/uncoated? When light is reflected off lens front there are no colors, just natural reflections. The SS was from last batch with the all aluminum collimatable cell. Tried a prism 90 in SS with no changes I could see. When Istar says their lenses have collimatable cell, was expecting both sides like the SS came with. But no, you have to source your own tube to cell connection.

Empty magnification? Put a coin 6' away from your eyes. How much detail can you see? Now hold that coin as close to your eye as possible. See the difference? Quality accurate optics can make as much power as the sky will allow. You need to separate the details to clearly see them. Mars is the best example of the need for alot of powers.

The Istar 5" has the perfect range of powers/quality that can be used around here, 200-300x. Though it does seem kinda pricy at $700 vs. $200 for the Surplus Shed one, of which there may be no more cause the Lens Description with the out of stock label is gone from his site. Oh well, things can't last forever.


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Gord
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Reged: 01/06/04

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Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5674841 - 02/11/13 09:35 PM Attachment (24 downloads)

Hi Tom,

Here's a picture of a couple of lenses and differences in coatings. One has the common broad-band multi-coatings (the green), and the other seems to have none (both are IStar lenses). I believe your IStar is the same as the achro in this image. The other possibility is MgFl which have a kind of purple/violet color to them.

Sounds like your SS may be uncoated.

On the optimum magnification, how much more detail are you seeing at 200x (or 250, 300) vs. at 150x on the surface of Jupiter? Things like the sizes of the disks of the moons will benefit from higher powers if the seeing and optics will support it, but details in the banding are usually lost if overpowered. Eddgie could explain the why better in terms of the MTF relationship, but I know that brightness is a component and when you go to those higher powers in a small scope, the exit pupil gets very small and contrast suffers.

Clear skies,


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tomharri
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Reged: 09/19/08

Loc: USA
Re: Istar 5" f/8R30, first light new [Re: Gord]
      #5675654 - 02/12/13 11:35 AM

Even Jupiter is so small at 150x. It is much easier to see the details at 250x which the Istar can support but is near the limit of the SS achromat. Yes, 300+ power is a bit much for a 5" but in the Istar it is able to do this with an atmosphere that cooperates.

Takahashi states you can use 100x per inch in their scopes, This 5" looks able to do 60x per inch without the image fuzzing up.


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5675716 - 02/12/13 12:12 PM

But for many planetary aficianados, anything beyond 30X per inch is empty because no more details can be seen, the image is only physically larger. The larger the image gets beyond that, the more "grainier" it becomes. I have a magnificent 6" APO and anything beyond 200X for planetary is just too dim for my taste. Yes the scope will handle more, but that does not change the exit pupil.

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Kevin Barker
super member


Reged: 04/22/09

Loc: Auckland, NZ
Re: Istar 5" f/8R30, first light new [Re: johnnyha]
      #5675835 - 02/12/13 01:14 PM

Johnny
I agree with your and Gord's magnification comments.

I usually view Jupiter somewhere from 125 to 170 X in My 130 mm apo. At 200 X subtle detail is not normally improved but I have had a couple of rare occasions when it did show a bit more detail.
For my little AS80/840 Zeiss achromat 100-140 is the range that seems to work on Jupiter. The 0.6-0.8 mm exit pupil range seems to work best from my experience.

I also had an APQ100/1000 for ten years, this worked well in the 125-166X range on Jupiter. I never had a night when 200 X worked better.

A couple of exceptions are Mars and Saturn. I have used using higher mag to see the smaller detail in Mars. Possibly the contrast is different and also the damn planet is usually so small. I have used smaller exit pupils around 0.5 mm (250-260 X) on these targets. Sometimes even higher but not to gain anything.

The bright moon can usually take a higher magnification than Jupiter does. I guess the contrast here is also very different. Seeing smaller craterlets easier for my eyes sometimes requires 200 +.

Tighter and difficult doubles however can benefit from extreme mag 40-50X aperture in inches. Seeing dependent.

I would expect a good 5 inch achromat would operate best in the 125-170 X on Jupiter.

Both of Tom's scopes seem to be working well, the differences seem minor. The length of the tubes and mass of the lenses/ota seem to be the main differences.

Kevin


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t.r.
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Reged: 02/14/08

Loc: Upstate NY
Re: Istar 5" f/8R30, first light new [Re: Kevin Barker]
      #5675878 - 02/12/13 01:37 PM

Yep on the Jupiter mags for detailed views...C80SS ~ 100x, TMB 92L & Tak Sky 90 ~ 155x, AP 130 ~ 220x, TEC 140 ~ 245x...on GOOD nights of seeing! For an average night I run the AP 130 @ 165x on Jove.

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Gord
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Reged: 01/06/04

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Re: Istar 5" f/8R30, first light new [Re: t.r.]
      #5676607 - 02/12/13 09:17 PM

I'll add that my experience is in line with Kevin's on those other targets as well. They take magnification much better than Jupiter. It seems Jupiter is more the odd-ball than any other as it takes really good seeing and big aperture to be able to push things.

Clear skies,


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mikey cee
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Reged: 01/18/07

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Re: Istar 5" f/8R30, first light new [Re: Gord]
      #5676820 - 02/12/13 11:42 PM

Tom I believe in what you're saying about the Istar quality. Scaling your 5" up to my 10" F/11 R30/35 I hardly ever go below 515x with my 65° plossls. Jupiter is actually still quite sharp at 685x with my 15mm plossls. At 815x with my VT 12.5mm orthos it really does begin to lose definition. Things that are linearly larger like the blue festoons become less distinct. At lower powers like 410x in my 25mm plossls the planet is so bright and contrasty sharp but to me the festoons are too small to see easily. This probably has more due to my eyesight becoming more used to the higher powers at 515x-685x. As far as I'm concerned the R30/35 lenses really excel. I'm as happy as a clam. Mike

Edited by mikey cee (02/12/13 11:44 PM)


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AlienRatDog
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Re: Istar 5" f/8R30, first light new [Re: mikey cee]
      #5677114 - 02/13/13 07:19 AM

Not to be a negative nancy but I noticed the Istar 127 R30 OTA is about $2k. What would be the advantage of this scope in comparison to, let's say, an Explore Scientific 127apo?

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tomharri
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Reged: 09/19/08

Loc: USA
Re: Istar 5" f/8R30, first light new [Re: AlienRatDog]
      #5677147 - 02/13/13 07:41 AM

You buy the lens for $700 and build your own tube assembly, my total cost is around $1000 and it is lighter than the way Istar builds them.

I build, therefore I am.


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Kunama
professor emeritus


Reged: 10/22/12

Re: Istar 5" f/8R30, first light new [Re: AlienRatDog]
      #5677149 - 02/13/13 07:44 AM

Quote:

Not to be a negative nancy but I noticed the Istar 127 R30 OTA is about $2k. What would be the advantage of this scope in comparison to, let's say, an Explore Scientific 127apo?




Build quality, the Istar is built to last a lifetime. But you do need to consider the size of the mount as the Istar is no lightweight.


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tomharri
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Reged: 09/19/08

Loc: USA
Re: Istar 5" f/8R30, first light new [Re: Kunama]
      #5677342 - 02/13/13 10:08 AM

Mikey Cee: You must have excellent atmoshereics in Nebraska, way better than the eastern guys, or even me. I'm on the western side of the mountain from Portal AZ. Only had my 10" Zambuto newt up to 570x with a Pentax 2.5 XO. Something to be said about a longer focus scope. You don't need to peek thru a tiny lense to get high powers.

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t.r.
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Reged: 02/14/08

Loc: Upstate NY
Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5677387 - 02/13/13 10:40 AM

Roland and I had an enlightening exchange over on the 'Mart awhile ago. I wonder if this R30 lens is indeed the C-e achro Roland discusses. The above reports sounds like it may be...

http://www.astromart.com/forums/viewpost.asp?forum_post_id=606701&poll_id...


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Gord
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: Istar 5" f/8R30, first light new [Re: t.r.]
      #5677475 - 02/13/13 11:17 AM

Hi t.r.,

Thanks for the link to the discussion. However, these R30's are not C-e, they are F-e corrected (green/blue focus as opposed to the green/red C-e focus Roland is talking about). They are the opposite of what Roland discusses, at least as I understand what C-e means.

And that would seem to hold for his comment about Mars. Green and red are the primary colors for Mars, so an objective that lets red go out of focus is going to perform poorly. In this regard, I believe the classic archo's from IStar are closer to C-e than are the R30's.

Clear skies,


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t.r.
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Reged: 02/14/08

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Re: Istar 5" f/8R30, first light new [Re: Gord]
      #5677533 - 02/13/13 11:47 AM

That clears it up! So, the opposite...now the question...WHY? If planetary detail is mainly in the red/green channel as Roland confirms, why, why, why let the red go out?!?! The only reason to do so is to preserve star color information (blue)and grab up the visual primary of green I suppose...so technically, the standard c-f achromat is still going to perform better than this R30 lens for planetary. Let me ask it this way, what is the R30 supposed to accomplish, what is its virtue? Maybe that will make it crystal clear... Is it simply to reduce/eliminate the defocused blue CA one sees and trade it for defocused red which the eye is less sensitive to? To me, that tradeoff is not worth the price of giving up the red channel information if I use the lens for planetary. What am I missing?

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Gord
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Re: Istar 5" f/8R30, first light new [Re: t.r.]
      #5677619 - 02/13/13 12:39 PM

Quote:

That clears it up! So, the opposite...now the question...WHY? If planetary detail is mainly in the red/green channel as Roland confirms, why, why, why let the red go out?!?! The only reason to do so is to preserve star color information (blue)and grab up the visual primary of green I suppose...so technically, the standard c-f achromat is still going to perform better than this R30 lens for planetary. Let me ask it this way, what is the R30 supposed to accomplish, what is its virtue? Maybe that will make it crystal clear... Is it simply to reduce/eliminate the defocused blue CA one sees and trade it for defocused red which the eye is less sensitive to? To me, that tradeoff is not worth the price of giving up the red channel information if I use the lens for planetary. What am I missing?




Ding! Ding! Ding! Ding! I think we have a winner!!

Seriously though, I believe it is just that. The secondary color is less obvious, so it must be better, right? To Roland's comments (and other ones made in the other color correction thread), it very much looks to be a marketing exercise. People seem to be getting conditioned to "CA is BAD!" and must be avoided. We see it all the time in the kinds of discussions and questions asked. So you can see how it could be appealing to have someone present an option for less CA.

But as seems to be coming up with more discussions around this, things just aren't that simple. Tom's own experience here with the SS lens that shows more traditional CA, but showing better detail is a good example. You can't just look at it in a simple "I see less CA, so it must be better".

Anyway, I look forward to more user reports, especially on side-by-side tests to get a really good picture. And on the alternate side, where are these alternate correction designs excelling. Double stars have been mentioned, and thinking about it the moon would seem like a good target as well.

Clear skies,


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t.r.
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Reged: 02/14/08

Loc: Upstate NY
Re: Istar 5" f/8R30, first light new [Re: Gord]
      #5677649 - 02/13/13 01:00 PM

...Yep , yep. I too look forward to more reports, always good to read what others are experiencing and add it to the CN collective knowledge base. I still believe, if a manufacturer tried a C-e design and admittedly let the blue go out of focus, even though the CA would be more obvious, the planetary detail (red/green) would be enhanced thus producing an improvement over either the R30 or standard Fraunhofer for planets. For stellar it would be nasty! But that would truly be a change and an improvement in achro design strictly for the planetary observer...a dedicated achromat "Planet Killer"!!! Just look at the pic of Jupiter in the C-e that Roland posted on that A'mart thread. There is more blue CA halo yes, but look at the enhanced belt definition! It makes the low contrast detail pop out! If you really study the images, the C-F achro image is blurry in comparison. But you would have to contend with that blue halo in the C-e. My

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Kevin Barker
super member


Reged: 04/22/09

Loc: Auckland, NZ
Re: Istar 5" f/8R30, first light new [Re: t.r.]
      #5678651 - 02/14/13 12:29 AM

Tim, Gord et al
The old Zeiss AS lenses were supposedly CDe corrected. They also used a special short flint glass as well which reduced secondary spectrum by 40 % or so for KZF6 and BK7(from memory)

I think planetary observers would love a scope with this type of correction if it did have significant benefits.

I would love to know definitely the glass types Istar are using, it is relatively straight forward to calculate the amount of secondary spectrum. Do they sit amongst normal glass types. Recently there are suggestions on another thread that Chinese do not have access to short flint and /or it is too expensive.

Also if they are not using special glass of some sort for the R30/35 etc lenses why are they using so much glass. They are truly very massive. The 250 mm version is close to 14 kg. The 127 mm R30 has more mass than a fast 6 inch standard Crown Flint.

Surely they could achieve different correction than C F if that is just what they are doing without all of the trouble of a massive lens.

I am sure there is an expert or two out there who can enlighten us all. Valery etc

I would not say conclusively from reading this interesting comparison of two 5 inch scopes whether R30's are better or worse with respect to planetary observing. The earlier comments about lower power could be due to cooling issues of the more massive lens. And even there what was better, same etc

More reports with different/same lenses will however start to help ascertain some certainty one way or the other.

I reckon an interesting comparison would be two Istar f-8 lenses of the same aperture, one an achromat and one R30. Of course I am assuming the normal achromat is corrected to C F??

I still suspect the R30/R35's's use slightly different glass types.

Are we comparing different levels of CA or different wavelength points of correction ??

Kevin

Kevin


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tomharri
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Reged: 09/19/08

Loc: USA
Re: Istar 5" f/8R30, first light new [Re: Kevin Barker]
      #5678813 - 02/14/13 06:12 AM

The only reason I started this thread was because there was no information out there about the Istar R30. Figured to start with a small scope just to see, then move up the ladder to the most I could handle/afford.

First objective has been accomplished. These lenses are a step above a regular achromat. I feel sorry for you armchair observers. Winter must really be brutal back east. But it sounds as if your atmosphere never really steadies up the rest of the year.

MOVE! The country is pretty much the same from sea to shining sea. During my military career seen pretty much all of it and decided the west was the best. Then you got Florida. If it hadn't been for that shark attack while kayaking, would probably be there.

Put down your musty/dusty books on theory and get out there and do something real. This is beginning to seem like Galileo vs. the Pope, or a good old fashioned witch hunt. Anything new is VERBOTEN!


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t.r.
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Reged: 02/14/08

Loc: Upstate NY
Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5678906 - 02/14/13 07:46 AM

Tom, you seem to be taking things personal and becoming defensive for NO reason. No one here is attacking you, your observations, Istar, the R30 lens or the like. When manufacturers remain tight lipped as to what their latest creation involves it creates speculation. They could end it easily by providing details, but most choose not to and allow the customer to sift through truths, half-truths and even lies to draw a conclusion about the product. This thread isn't even a spirited debate yet, so relax a little. Nothing new is VERBOTEN...but it does bring about intelligent discussion on whether it has anything "new" to offer. If it does, it will stand up to a little scrutiny.

The reason the lens doesn't show CA in your observation is two-fold IMHO. First, it is correcting the blue to focus, taking what you can see easily and replacing it with defocused red, which you can't see. That red channel does however, contain planetary information that is sacrificed! This explains why the SS standard achromat lens showed a better image. Second, you are magnifying to a point where the image is stretched and the color error left over drops below the point of perception. The same thing happens in observing reports of the Short Tube 80 on the moon in particular, which I have seen for myself (I do leave my armchair from time to time to actually observe ) when folks magnify over 100x or so with it, the CA "goes away" because the image is enlarged(stretched) and dimmed to a point where that defocused light drops below visual perception. But, it is also in the realm of "empty" magnification and most objectives will begin to break down in contrast. Hence, NO FREE LUNCH. There is nothing magical to the attenuation of the CA in this case, it doesn't happen by lens design, glass used or a "new" breakthrough. It simply comes down to trade-offs, but one must choose them wisely! For myself, thanks to this thread and a better understanding, I can cross the R30 off my list and go back to the standard C-F achromat design from Istar or D & G for consideration and simply filter it selectively. Thank you for starting it!


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