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Who’s Afraid of a Phantom: Istar Phantom 140mm F/6.5, that is?
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Who’s Afraid of a Phantom: Istar Phantom 140mm F/6.5, that is?
October 26, 2021
By ABQJeff
So just in time for Halloween a Phantom arrived at my door in mid-September. No not a ghost, although it wears white, I am speaking of the new Istar Phantom 140mm F/6.5 Triplet APO made with Hoya FCD-100 and Lanthanum glass (www.istar-optical.com or from www.starizona.com). I ordered this ‘first-run’ 10.1 kg (22 lb) telescope to compliment and balance my Celestron 9.25 EdgeHD in dual Alt-Az mode on an Atlas Pro mount. It will provide the wide fields (~3 degrees) that my SCT will not do, and be a decent backup for planetary observing when seeing hampers full use of the C9.25’s aperture. I am currently visual only, and due to space limitations, this combination now completes my initial telescope collection (which you can see in the picture below and is listed in my signature). I have been doing astronomy just over one year, taking it up during COVID like many others. I pride myself on not picking just what is popular, but what gives best value for my needs. In this case, I chose a scope that all reports say are in the same class as a TEC-140, but for thousands of dollars less expensive and with no delay in receiving.
Istar guarantees a minimum Strehl of 0.90 and the telescope currently lists for only $4495 and includes a 4” dual speed R&P focuser, tube rings, Losmandy (ADM D-series) sized dovetail, handle, retractable dew shield and sturdy Aluminum carrying case. For a triplet APO with high-end glass of this size, with a Strehl guarantee, that is a GREAT bargain. Teleskop-Service Optics, Sharpstar and Explore Scientific all have triplet APO 140mm f/6.5 products, but none offer a Strehl guarantee. Additionally, while the Sharpstar and TS Optics version are in this price range, the Explore Scientific scope is $6000. Furthermore, while TEC-140 is a bell weather in this class, it runs over $8000 for the equivalent set-up.
But can this small boutique brand F/6.5 140mm really hold up to real world conditions? Will there be some CA? How is its light gathering ability? Does it arrive collimated? What is its build quality? Let’s find out!
First the unboxing. The Phantom comes well packaged in two boxes. The exterior box is heavily taped. The inside of the interior box is lined with foam around the hard aluminum case.
The aluminum case arrived unscathed and opening the telescope was in perfect condition as well.
The telescope comes with the sales invoice, Warranty certificate for your serial numbered telescope and Ales Krivanek (the owner) will send you a link to the optical test report that you can download.
My scope arrived with a 0.955 Strehl rating. And actually I had access to ALL the test reports from this first batch of Phantoms. All but one of 11 had Strehl above 0.95, and that one had 0.944 (Ales, you should guarantee a 0.94 Strehl!) So their source of glass and lens fabrication is obviously excellent.
The telescope has a BIG 4” R&P focuser that is very sturdy for the heaviest camera or binoviewer gear. It includes a finder scope shoe and has 80mm draw length with measurement indicators. My impressions of the focuser movement and build quality is discussed below (bottom line: not a Starlight Instruments focuser.)
The lens cap is a very nice aluminum metal threaded to twist on and has a threaded stop down aperture as well.
The dew shield is very nice and retracts with the loosening of two large black thumb screws.
However, you will notice, it is such a new scope, they have the wrong F/# on the dew shield. They provide stickers to correct it (if and when I get brave enough to change it).
Removing the focuser and peering inside, there are multiple baffles (I counted 4, but there may be 5) and the entire interior is covered in black felt velvet (very nice, this is the only one of my telescopes to have that!)
So how about mechanics performance. Well, this is where there are issues.
One: when it arrived, two screws from the focuser had come loose in shipment and fell off in the case. These screws helped hold the draw tube in place and a rattling draw tube alerted to me their absence. Easy enough to place back in (but one shouldn’t have to for a $4500 telescope, my $500 Orion refractors didn’t come this way).
Two: A larger issue is the 2” “twist tight” mechanism that holds the eyepieces (twist tight attaches via worm screws and M63x1mm thread to backplate of focuser). The twist tight doesn’t do a good job of holding EPs or diagonals, and in fact the provided 2” to 1.25” adapter fell right out and landed on my garage floor (denting the edge). I prefer, and ordered, a Baader 2” to 1.25” clicklock adapter replacement for the damaged adapter. I replaced the Istar twist tight adapter design with a Teleskop Service Optics short adapter for 2” to M63 (https://www.teleskop-express.de/shop/product_info.php/info/p11236). The issue with the twist tight is a known issue that Istar says on their website they are going to address.
“Replaced the Istar 2” twist tight (in my hand in image) with TS Optics short adapter for 2” to M63” (attached to focuser)
Three: Backfocus. I not only replaced the loose twist tight because it doesn’t hold EPs and diagonals well, but also to shorten the optical path length of the focuser. The Phantom has a 138 mm backfocus from the back of the 2” adapter with focuser fully retracted. However, I use a 2” filter wheel and 2” APM Amici diagonal. I could not get focus in the base telescope configuration with my Explore Scientific 2” eyepieces (82 degree-18, 24, and 32mm and 68 degree-40mm). I was ~5mm off of achieving focus. The new TS Optics short adapter reduces the optical path length by 9.5mm, so now I can use the telescope with my 2” Explore Scientific EPs (yay!).
The second production run of Phantoms now has a removable 80mm section, this is great for binoviewers so you wouldn’t need a glass path corrector. However, this would not have helped me as then I wouldn’t have been able to use my 1.25” EPs which were able to achieve focus in the base configuration and cutting 80mm from optical path length would put it out of the focuser range.
(Note: just for info, the focuser backplate attaches to the draw tube with a M102x1mm thread)
Four: Focuser movement. The focuser is robust, as mentioned. However, it is not buttery smooth. You can feel the gearing as you move the focuser. I ordered a 3” Feather Touch focuser (this scope uses same attachment as a Takahashi FS-128, M145x1, to attach the focuser to the main tube). The 3” focuser is even a shorter optical path length, which will allow me to use the rest of my 2” EPs. When that focuser arrives (Starlight Instruments currently has a pretty big backlog) that will effectively add ~$1000 to the cost of the telescope. But still a bargain compared to other Super APOs on the market.
Five: Dovetail bar and rings. The dovetail bar and rings are custom machined for this telescope. They are of very nice quality and have good fit and finish. But, the holes for the rings and dovetail I measured to be ~50mm spacing, which is not standard. Thus, it is not possible to change out rings or dovetails. If someone wants to do this they have to buy a whole new ring and dovetail set which runs $500+ (for this size telescope). Also the dovetail itself is very slightly narrower (like 1-2 mm) than an ADM D-series dovetail. I investigated this because my Optec Libra telescope position fine adjuster, which I use on the left secondary scope side of my Atlas Pro, would not securely hold the Phantom’s dovetail (dovetail would slip in mount when pointed to Zenith), no matter how hard I tightened the Optec Libra’s mounting screws. The Optec Libra holds my C9.25 (with ADM D series dovetail or the base Celestron orange CGE dovetail) just fine. Since the 140mm Triplet APO is very end heavy (ie puts a lot of torque on the left side Atlas Pro secondary scope mount), it is best if it is on the right side anyway. So that is how you see my set-up configured.
Onto optical performance. It turns out all the ‘meh’ performance is for the mechanics of the scope, so the ‘critical things’ I had to say about the Istar Phantom are done. The optics are incredible; I was truly amazed as you will see below.
For the optics battle, I used the October new Moon week and my family fall vacation to put my current inventory of scopes up against the Phantom. The viewing location was Canyon Madness Ranch, near Roy, NM. It is a Bortle 1 location, and luxury adventure ranch. So I had nighttime activities to do, and my family could go horseback riding, ATV riding, do archery, skeet shooting, feed the peacock, etc. during the day.
Seeing for the evaluation was typical New Mexico 1-1.5”. First night was totally clear, second night some clouds.
For my comparison, I put the best representing telescope of each category versus the Phantom. So who was afraid of the Phantom?
Color Correction/Chromatic Aberration:
Altair Wave 102 F/7 (715mm FL) vs Istar Phantom 140mm F/6.5 (910mm FL)
When spending the money to get a triplet APO, the first thing you want to be sure of is zero false color. However, 140mm and f/6.5 is a new-ish pairing in astronomy triplet APOs. Is 6.5 just too fast for a 140mm objective (eg TEC 140 is F/7)? So for this comparison I put my Altair Wave 102mm F/7 0.98 Strehl FPL-53 triplet APO versus the Istar Phantom 140mm f/6.5.
Targets: Vega, Capella and Sirius
Used same diagonal: Orion dielectric diagonal (to be sure no CA from my prism diagonals)
EPs: 1.25” Televue Delite 3mm in Altair Wave (238x) and Televue Delos 3.5mm in the Phantom (260x)
Result: First note, no collimation needed. The Phantom arrived perfectly collimated. ZERO color in either scope on each of these targets. As air currents changed seeing (and put the scope slightly out of focus), a small portion of both scopes outer airy rings would very briefly show a very, very slight tinge of coloring as an air current passed, but if anything the Altair Wave was worse. But this again was from seeing putting the scope out of its focus point. In focus under good seeing, total bright white on all three of these stars.
Verdict: NO CA in either scope. Altair Wave 102 F/7 is NOT afraid of the Phantom but neither is the Phantom afraid of the Wave
Planet and Lunar Performance:
Celestron 9.25” Edge HD vs Istar Phantom 140mm f/6.5
The standard rule of thumb for telescopes with central obstructions, like an SCT, is that you subtract the diameter of the obstruction from the overall objective to get its relative contrast performance, ie its planetary performance. The C9.25EDgeHD has an 85mm CO, thus it should behave like a 150mm unobstructed telescope under this guideline. How did the C9.25 Edge HD and Phantom stack-up versus each other?
Targets: Jupiter, Lunar Gassendi crater (10 days later when there was a moon)
Diagonal: Orion Dielectric diagonal
EPs: 1.25” 8mm Stellarvue Ultrawide on SCT (~300x), 1.25” Televue 3mm Delite on Phantom
Result: The contrast was about equal between these two scopes, however the resolution and color on Jupiter, provided by the 9.25 inches, provided to much for the 140mm APO to beat. I was able to see double the number of distinct color bands in Jupiter with the 9.25 SCT, just because I could better detect the color. And on the Moon, Gassendi crater, there are three riles in the lower right hand corner. These showed like scratches in the Phantom. However, in the C9.25, they were deep much more easily noticed grooves.
Verdict: C9.25 Edge is NOT afraid of the Phantom for planetary and lunar.
Double Star Performance:
Celestron 9.25” Edge HD vs Istar Phantom 140mm f/6.5
With the extra resolution, one would expect the C9.25 to beat out the Phantom on double stars. However, the C9.25 has a significant central obstruction which pushes energy out of the center portion Airy disk.
Targets: Double-Double in Lyra, Almach (Gamma Andromeda), Eta Cassiopeiae, Iota Cassiopeiae
Diagonal: Orion Dielectric diagonal
EPs: 1.25” 8mm Stellarvue Ultrawide on SCT (~300x), 1.25” Televue 3mm Delite on Phantom
Result: True to expectations, the C9.25 better resolved all targets with more clear separation between stars. HOWEVER, the individual star Airy disks appeared better in the Phantom as the central part was brighter and more distinct (as the SCT pushed energy out the second and third rings). So resolving doubles the larger aperture SCT won, for admiring doubles the Phantom wins.
Verdict: Tie. Both scopes performed well on doubles and each had their own ‘pros’ to their use on doubles (glad I own them both :->). C9.25 Edge is NOT afraid of the Phantom for doubles but neither is the Phantom afraid of the C9.25 Edge on doubles.
Small DSO (aka ‘fuzzies’) Performance:
Orion 150mm Maksutov Cassegrain Telescope (MCT) vs Istar Phantom 140mm f/6.5
Similar to planetary contrast, there is a rule of thumb for telescopes with central obstructions, like an MCT. The rule is you subtract the area of the obstruction from the overall objective area to get its relative brightness performance, ie its deep space object (DSO) performance. And since Cats, like SCTs and MCTs, have a narrow field we are looking at small DSO, aka “fuzzies” performance. The Orion 150 MCT has a 47 mm CO, thus it should behave like a 142mm unobstructed telescope under this guideline. Using a variety of eyepieces to from low to high magnification, how did the Orion MCT and Phantom stack-up versus each other on smallish Deep Space Objects?
Targets: Ring Nebula (M57), Swan Nebula (M17), Lagoon Nebula (M8), Sculptor Galaxy (C65/NGC253), Pinwheel Galaxy (M33), Andromeda Galaxy (M31) (center and edge), M32
Diagonals: APM Amici 2” Diagonal (with 2” filterwheel attached) in each telescope
EP combinations: ES 40mm-68 in MCT150/ES 20mm-68 in Phantom; ES 30mm-82 in MCT150/Stellarvue 15mm-82 in Phantom; 20mm Orion 80 degree in MCT150/10mm Celestron Luminos in Phantom; 17.5mm Baader Morpheus in MCT150/9mm Baader Morpheus in Phantom; 12.5mm Baader Morpheus in MCT150/6mm Orion 80 degree in Phantom; 9mm Baader Morpheus in MCT150; 4.5mm Baader Morpheus in Phantom; 8mm Stellarvue 82 degree in MCT150/4mm Meade PWA 82 degree in Phantom.
Result: For whatever reason, the extra mirror bounces of the Mak, the felt lining the interior of the APO, the refractor’s baffling, magic pixie dust, the Phantom was about 10% brighter on all targets at all magnifications (and verifying sizes of objects were same in each scope). The MCT150 was more performing like a 130mm unobstructed telescope. And on Andromeda, the extra contrast of the APO was kicking in as well so I could better see arms and features against the space background (wasn’t just a light grey blob).
Verdict: The Orion 150mm Maksutov Cassegrain IS afraid of the Phantom on Deep Sky Objects.
Very Large DSO Nebula Performance:
Orion 120ST 120mm F/5 Achromat doublet vs Istar Phantom f/6.5
While the Phantom APO beat the MCT150 on smaller DSOs, how will it do against a fast achromat on the big DSOs? Of course the 600mm of the 120ST will give it a wider potential field of view. So seeing the entire Veil Nebula or North American AND Pelican at same time with good framing, that is an automatic win for the ST120. But for the 3 degrees the Phantom can see with an ES68-40mm…which is better?
Targets: North American Nebula, Eastern Veil Nebula, Andromeda Galaxy (in entirety)
Diagonals: APM Amici 2” Diagonal (with 2” filterwheel attached) in each telescope, used filter wheel set to none, Orion Skyglow and Orion Ultrablock filters for Nebula comparison.
EPs: ES82-30mm in 120ST (6mm exit pupil, 4.1 degree FOV), ES68-40mm in Phantom (6.15mm exit pupil, 2.99 degree FOV)
Result: Very close. Of course, as mentioned, the 120ST wins if wanting to see the whole of very big objects with good framing, but on just the 3 degrees they both can see, the Phantom wins. It is about 10% brighter, more than the extra .15mm of exit pupil would indicate. Better baffling? Felt lining? And again the extra contrast (and resolution of 140mm vs 120mm) meant the wisps of the Eastern Veil just showed a bit more in the Phantom. See more (120ST) or see better (Phantom)…
Verdict: Tie. Like with double stars, each scope brings its “pros”. 120ST is not afraid of the Phantom on very large DSO nebulas, but neither is the Phantom afraid of the 120ST.
Summary:
The Istar Phantom 140mm f/6.5 is a new entry in the large (5”+) triplet APO world. For the price to performance it is an excellent option. The vendor is already implementing upgrades (eg 80mm removable section from OTA tube) and recognizes need to make further improvements in its mechanics (eg 2” twist tight clamp). The optics are superb and are even better than the advertised 0.9 Strehl minimum guarantee. Another independent reviewer who provided a review to Istar even said the optics beat out a TEC 140. I have never viewed thru a TEC 140, but based on my comparison tests, I don’t doubt it. It arrived in perfect collimation and had zero chromatic aberration. It completely blew away a 6” Mak on deep space performance, held its own against a C9.25 on doubles and an ST120 on large nebula, and its planetary performance vs the C9.25 was only beat because of the extra light gathering and color that the larger aperture brings to bear.
If you are interested in a super high performing 5+” triplet APO, that is much less than comparative peers, don’t be scared, I highly recommend you take a look at a Phantom!
Clear Skies (and Happy Halloween)!!
ABQJeff
Binos: Oberwerk 12x60 LW & Deluxe III 20x80
Scopes: Orion:ST120 w/GSO 2xspeed, ED80 w/GSO 2xspeed, 150MCT w/FeatherTouch/Baader 2" clicklock; Altair:102 f/7 EDT; C9.25 Edge HD w/FeatherTouch/Baader 2" clicklock; Istar:Phantom 140 f/6.5
Mounts: Atlas Pro/Berlebach Planet, 2xStarSeeker IV/Berlebach 212 tripod, VersaGo II/SSIV tripod, Paragon Plus/Versa Go II tripod, Az-Gti
Diagonals: 2xOmegon 1.25" RACI/2x APM 2" RACI/Baader T2 Zeiss/Orion Dielectric
Filter Wheels: 2x1.25"(planet, nebula), 2x2"(nebula)
Top EP: Daystar Quark Gemini; Orion LHD 6/20, 20mm Reticle; ES 82(30/24/18), 68(40/24/20/16); Celestron Luminos 10/23; Meade PWA 4-82; Morpheus 4.5/6.5/9/12.5/14/17.5;Pentax XW 5; Stellarvue 8/15 ultrawide, 26mm Reticle; TV 3mm Delite,3.5mm Delos
- Bob Campbell, Psion, TheBigEye and 19 others like this
39 Comments
Thanks for the nice, thorough review.
APOs like that are best admired for inch per inch pristine images, build, ease of use and WF photos. But aperture will-out, and the result with the 9.25" SCT doesn't surprise. Nice that you have good enough seeing to exploit a large scope!
It sounds like you have a great scope. I ended up paying almost as much as you did with my ES127 carbon fiber if I include the aftermarket focuser I put on it. I would say you got a great deal!
I have experienced some of the same things you have, comparing my 5 inch apo to my much larger SCT’s. My refractor has sharp optics, is awesome for imaging, and does very well for its size. The “for its size” thing is the main qualifier.
Thanks, it has been fun using it!
Truth be told I was originally planning on just pairing my 120ST with my C9.25, one scope for purely wide fields, one for everything else. No overlap. BUT then I got the 4” triplet APO, and its double star and planet performance vs my 150mm Mak (I pair those two together) “spoke to me”. I realized a good quality APO is a very nice thing to have paired with a CAT for when seeing is ‘iffy’ and for general viewing together on the same objects it provides a nice alternate scale view to see different framing at same exit pupil.
Of course, I always have my 120ST RFT available for seeing those massive objects and sky sweeping.
Hmm. Surprised that the st120 vs the 140 mm apo comes out as a tie. That is certainly not the case between my very nice synta F8.3 achromat and my Pre ED glass AP 150 mm Triplet. I am also surprised that the difference between the refractor and your Cat was that pronounced on solar system objects. I agree that the 9.25 inch cat SHOULD out perform a 5.5 inch Refractor, but your description seems to indicate it was by a lot. Curious, what do you estimate your seeing conditions were during the observing session? Where I live, seeing is often poor, which may skew my experience with my larger mirrored scopes. Anyway, nice report. Thanks for sharing!
JMD
Thank you for your comments and questions.
The ST120 ‘tied’ for big DSO because of its shorter focal length allowing 4+ degree viewing. The 140mm APO can’t do 4+ degrees and is slower. Those are both negatives for seeing big extended DSOs.
Optical performance-wise for the same target that could fit in both scopes FOV, the 140mm APO won for brightness, contrast and resolution.
As I summarized, if you want to “see more” (as in area covered with large exit pupil) the ST120 wins as it is my fastest scope, but for “seeing better” (contrast, brightness, resolution) the 140mm APO wins. Kind of like what’s best for dinner: all you can eat buffet or a small gourmet meal. It is a tie between them as they each have their place/role.
Re: C9.25 and seeing, as mentioned my typical seeing is routinely/most of the time sub-1.5 arcseconds, often even close to 1 arcsecond (I base this on the doubles I am able to cleanly split).
For planets/lunar I can run 350x no problem (unless bad weather which does happen on occasion). Note: I dont want to give the impression I have no air currents/air cells, after all I live in the mountains (lol), but between the sub-second-1 second blurs of air currents I get a couple seconds of clear seeing (that is what I use for my observations).
While both scopes have similar contrast, the resolving ability of 9.25” just overwhelmed the 140mm on lunar riles, and the extra light capture of the C9.25 brought out colors more vividly on Jupiter (thus able to see multiple additional bands). Those two factors were just too much for the APO to overcome in good seeing conditions.
If It is true that the level of contrast on an SCT compared to an apo is the aperture minus the secondary obstruction then even a C8 will be very close to a 5 to 5.5 inch refractor. The major thing smaller scopes don’t have is the same level of resolution. If smaller details aren’t there, it probably doesn’t matter how good the contrast is. Using the same formula as above, a 9.25 SCT would have the same contrast as a 6 inch refractor and superior resolution.
When I have my SCT’s out doing visual in bad seeing I often stick with my least powerful eyepiece which gives me a magnification on par with a lot of refractors. I seem to get about the same stability with the images that way, but the images are also brighter and more colorful than with smaller scopes. I am sure this goes against the rule of thumb so many people like to quote with aperture and seeing, but it what I have seen.
I really do love my apo refractors, but they are still used best with their limits in mind and the same goes with SCT’s.
Great comments. Using lower mag in worse seeing is a great way to deal with unsteady air. I find that is not needed as much in my APOs as their resolution ability is not as affected by poor seeing (since they are resolution limited already).
Re: rules of thumb, yeah I am not too sure about the accuracy of either of the rules of thumb regarding obstructed aperture (contrast based in diameter and brightness based on area), some say CATS perform better than this because of resolution, brightness, thinner glass, erc. others say worse because of the extra mirror bounces and poor quality control in mass produced CATs. There is definitely an impact from having a CO, but I don’t have precision measuring equipment to compare to theoretical plots from Rutten and Van Venrooij. I just have my eyes to do a qualitative assessment. That assessment says a C9.25 Edge beats a 140mm APO on planets/lunar and a 140mm APO beats a 150mm Mak for DSO brightness.
I like your very meticulous review of the various scopes, but I tend to derive a different conclusion, namely, that if the 9.25" Celestron -- always the "sweetie" in the Celestron line up -- already beats the Istar 140mm on brightness and resolution, while losing only very little on contrast, while costing ~$3000 less for the OTA new, wouldn't it make more sense to spend another ~$400 or ~$4900 to buy a new C14 OTA? Certainly that is a scope that would blow away the performance of the ISTAR 140 on every variable! Of course, it would also require a larger mount and two people to set up comfortably remotely. But as a permanently mounted home observatory scope it would be a better way to go, -- at least in my humble opinion. It's amazing to me how a few batches of lousy, quickly churned out SCTs from both Celestron and Meade to feed the Halley's Comet frenzy of 1984-86 have turned people off to SCTs to this day, and that even though since the early 1990s the quality standards have been much higher again. It's true there is nothing quite like the view through a top-notch refractor. But the idea that SCT images are somehow inherently "softer" than APO images is simply a myth. Especially at the same price point, the SCTs usually are the better performers. That is, after all, why they were so wildly popular when first introduced at the end of the 1960s! And the Questars have always been first rate since their introduction into the market [in 1959?] No 90mm Maksutov can perform quite as well optically as an unobstructed 90mm APO of course, but Questars have many other advantages over virtually all APOs as well!
Interesting result on your C9.25 as compared to the ISTAR 140. I did a similar planetary comparison of my standard C9.25 (with Starbright coatings), my APM140 doublet APO, and a Takahashi FC 100. This was in Virginia under moderate seeing conditions observing Jupiter. The clear winner with the most detail and color was the Takahashi FC100. A close second was the APM 140 and in third place was the C9.25. There is an excellent report on the APM 140 and it's optical capability on the Cloudy Nights web site.
Thank you for the comments. I am a fan of both Cats and Fracs, each have their purpose. And I certainly appreciate the ‘bad press’ SCTs get (see my comments in the “SCTs are underated” thread in Cats and Casses forum, which is basically a mini-version of this review). If it can fit in the FOV, I primarily use the C9.25. So it is my main scope.
Why did I spend $4500 for an APO triplet vs a C14? Pleiades, Sagittarius Star cloud, Andromeda, North American Nebula, Veil Nebula, etc. (and yes pretty high mag double star views). C14 Edge is a great scope, and I may get one someday, but the handling/logistics (I observe solo and dont have a permanent spot) combined with narrow FOV put that on hold (for now). Truthfully my next BIG scopes will likely be a C11 Edge (can use Atlas Pro still) and/or (egads) a Dob, like an 18” Obsession.
CS!
Thank you for your comments. You are not alone in rating your APOs above your SCT on planets (many, many a thread on CN in this debate). I am ‘secretly’ modeling my telescope quiver after Jim Barnet’s setup and in the past he stated he prefers his TEC140 over his C9.25 Edge (which he is fond of) on planets.
So, I too was a bit surprised how the aperture of the C9.25 was able to have it pull so clearly away from the 140mm on planetary and lunar. And I had several family and friends at the eyepieces who had the same impression. But those are our eyes in our conditions on these scopes. To us the C9.25 Edge was superior on planets/lunar, but like taste, senses are subjective.
CS!
Yep! With that quality of seeing, double(roughly) the aperture should soundly trounce the refractor! I would be happy to live with a 4 inch refractor under those skies!! I am very jealous:-). My skies are usually 2-3 arcseconds or worse, lol. Thanks for the clarification! Sounds like you have a good scope! Hope it brings you lots of fun.
cheers!
JMD
In my experience, my refractors will hold their own with CATS that have an inch or two more aperture. Once you go beyond that, the Cat starts to pull away. Differences become more noticeable with more stable skies, which are few and far between from my back yard:-).
JMD
The Effective area of aperature on a 9.25" is 58.44si after the secondary area is deducted.
The Effective area of aperature on a 6" with no CO is 28.27si
Thats a huge friggen difference.
The 9.25" is effectively a 8.62" D aperature.
I was not resorting to hearsay nor relying on the other tests results for my results. I specifically mentioned I have never looked through a TEC140 and that I have not done the comparison, I was relaying facts/information from others that have tested and reviewed this telescope as a means of providing more information for the reader. Also based on my personal experience, I provided my take on that other person's review.
Fact 1) another person did a comparison with a TEC140, that independent review is posted on Istar's site publicly available for others to read.
Fact 2) I don't doubt the other person's test results, that is my emotion/opinion. I don't have doubt. You may. And that is fine.
Now that being said, you can bet if there is a TEC140 at a star party I am at, I would love to do the comparison.
The following statement is incorrect: "Planet and Lunar Performance:
Celestron 9.25” Edge HD vs Istar Phantom 140mm f/6.5
The standard rule of thumb for telescopes with central obstructions, like an SCT, is that you subtract the diameter of the obstruction from the overall objective to get its relative contrast performance, ie its planetary performance. The C9.25EDgeHD has an 85mm CO, thus it should behave like a 150mm unobstructed telescope under this guideline. How did the C9.25 Edge HD and Phantom stack-up versus each other?"
It is the area of the secondary that is subtracted from the area of the primary, not the diameter of the secondary from the primary.
Dan Kahraman
The comparisons should not end there:.All things being equal a refractor should have a darker field of view in addition to distortions caused by obstructions. The metallic reflective deposits cannot be equated to a transmission through a glass medium as homogenous as possible. Reflective films cause contrast destroying effects which make the background less dark than that of a refractor.
Paradoxically, fainter objects are easier to see in a refractor than a reflector of twice the diameter even though the objects will be much brighter in the larger reflecting telescope.
Selective use of a few equations don't tell the whole story. You would need a thesis for that..
Dan Kahraman
Very nice, informative review. Thank you so much. We are taking all of your criticism regarding mechanical shortcomings very seriously and we will fix them based on level of importance.
The most serious one was the insufficient grip on heavy eyepieces and camera system and that has been taken care of already. We are now looking into easier way of attaching the lens cap. Even though some people do like the threaded versions, some do not (like yourself). I need to gather info from more buyers and then make a decision about how to improve our existing lens cap. Again, great job on this review, much appreciated from all of us at iStar!
regards,
Ales
Thank you for your comments, Dan. What you are saying is 100% true. Regardless, the review is nicely put together and informative even though some of the facts about different types of optics could have been explored in more depth but on the other hand that may be a subject for yet another article and debate. Thanks to all of you who looked and participated!
Thanks Ales!
I am really enjoying my IStar Phantom 140. I just had it out for a New Moon all-nighter Saturday evening under Bortle 1-2 skies alongside my C9.25 Edge. As mentioned these make a great pairing for two different image scales and to give me a great range of observing with excellent optics.
My friend joining me was amazed at the quality of the instruments.
Clear Skies to you and Happy Holidays to the entire IStar team.
Jeff
Thank you Dan for the comments and input. Regarding the "rule of thumb", that is a popular rule here on Cloudy Nights, that I agree is incorrect in detail, but the overall intent is correct: contrast performance on a CAT is more effected by central obstruction and its mass-produced mirror performance (because of impact on Mean Transfer Function) than overall brightness performance.
Another similar "rule" (which funny enough is probably more accurate despite even less math involved) is for CATs you subtract 1 inch for DSO (ie brightness) performance and 2 inches for planet/lunar (ie contrast) performance (again same intent). With that rule, my C9.25 would behave more like a 7" refractor and thus indeed is unfair comparison to a 140mm refractor.
For readers, Jared Willson did all the optical calculations in 2009 on equivalent aperture:
https://www.cloudyni...mparision-r1901
Looking at that, for performance comparsion between a CAT and a 140mm refractor, it would have been better if I had used an Intes Mak MN65. If anyone wants to donate me one, I will be happy to do that comparison :->.
On your second comment, that is certainly paradoxical. I am not really sure how to do that comparison to test that a 140mm triplet APO is equal to a CAT or reflector twice its aperture in seeing faint DSO (even should someone donate me a C11 with 280mm aperture - again happy to be the receipient of said donation :->). I effectively do this comparison all the time with the C9.25 and 140mm Phantom. I use both at same exit pupil and get the different image scales (Phantom magnified (140/235)x the C9.25 magnification). The effect is the object is much smaller in the Phantom (35% the area) than the C9.25. So at equal brightness, I actually find it is easier to see the object in the C9.25 because it is 2.8 times bigger. Now when I increase the magnification so the DSO is at equal size (ie equal magnification), then my exit pupil (brightness) using the Phantom is now 35% the area of my exit pupil with the C9.25 and thus the DSO is again harder to see (to me). So again the C9.25 'wins'.
Perhaps there is an optimum exit pupil or magnification ratio to do this comparison to test your statement, because setting exit pupil equal or magnification equal, I personally don't see that result on faint DSOs between a 235mm SCT and the 140m refractor. But again, that is me.
If only there was some discussion/disagreement on CN comparing performance between Cats, Dobs and Fracs...just kidding!! These are some of the most entertaining exchanges on CN; us astronomers certainly have our favorites :-> (I am currently a Cat and Frac guy, obviously). So I will leave it to those forums to discuss further.
CS and Happy Holidays!
Jeff
I bought my Istar Phantom 140mm f/6.5 today. I will do a comparison with my TS-Optics 150 mm f/8 FPL53 Doublet Apo . I don't have a comparable sized Takahashi or Astrophyisics refractor. Because of the inclement weather I will limit my report initially to visual comparisons. I do have an Orion Maksutov D=180mm, F=2700mm and GSO's 8 inch Classical Cassegrain. While I would not consider the 140mm f/6.5 triplet APO a planetary scope 140mm is 5.5 inches in diameter which is not a small size especially if it happens to be free of chromatic aberration. I have pushed the 150mm TS APO Doublet well over 100x/inch without the image breaking down or colour showing. The Istar Phantom D=140mm should be able to handle 200x/inch.
It will be interesting to see where the comparison takes me.
Look for a report comparing the Istar Phantom 140mm to the Orion D=180mm Maksutov and the GSO D=190mm f/12 Classical Cassegrain....Imaging will have to wait for a clear calm night but I don't trust the rapidity with which storm systems move in the winter (Alberta Clippers).
Dan Kahraman
Ooh, I will definitely look for that report
!
I think the 7 in (180mm) Orion Mak vs the Phantom 140 should make a REALLY interesting comparison. It is kind of funny Ales amd you mentioned planetary performance and here I got it mainly because of its shorter focal length (vice TEC 140) and faster F# allowing me to get 3 degrees FOV and a pretty big exit pupil for visual viewing of large DSO
.
And of course, congratulations on the new Phantom!!
Did you get the new model that the OTA comes apart for binoviewer use?
CS!
Yes I got the new model of the Phantom. The issue with the extension will be fixed within the week. The Mak. is an older version with only 1.25" adapter, not the 2". You would have to use an ocular approx. 1/3 the focal length used on the Maksutov to get comparable magnifications in the Phantom.I don't have delivery of the scope yet.
Dan Kahraman