39 replies to this topic

[LMcKeen]

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

Edited by LMcKeen, 07 December 2021 - 06:15 AM.

[LMcKeen]

Jeff:

 

Thanks for simplifying my proposed comparison. I will confine the test to the 180mm Mak. Since the test will be conducted in the daytime initially and because the Orion Mak. has a dark tube colour it may be at a disadvantage with seeing. But we shall see. Maybe I can wrap it up in aluminum foil. Seriously.

 

Dan Kahraman

[LMcKeen]

Getting back to the area of obstruction versus a diameter by diameter comparisons:

 

The Orion 170mm Mak has an obstruction 41mm in diameter

 

The ratio of areas is 5.82%....whereas the ratios of diameters is 24.1%. This is a significant difference 

 

What does this do to the Airy disc energy distribution patterns? How does the aluminum deposited coatings further destroy image contrast?

Lastly you have the number of surfaces with say 88% reflectivity versus the multicoated lens surfaces of *98% transmission each....What is the final result? 88.6% for the triplet, 68.6% for the Maksutov.

 

88.6% of 140mm is 124.04mm

68.6% of 170mm is 116.62mm

 

Should I further substract 5.82% from the Maksutov but none from the Phantom? In that case 109.83mm for the Maksutov.

 

Light Gathering power will be 18.70x human eye for the Mak, 23.84x eye for the Phantom triplet. Final figure of 7.11" diameter for the 9.25" Celestron without the diagonal 50.64x LGP of human eye.

 

 

*I don't know the actual values

 

 

Dan Kahraman

 

*Calculation for Celestron 9.25"

 

Celestron 9.25 obstruction by area 13.12%

Transmission corrector plate 0.98*0.98=.9604
Primary mirror and secondary mirror 0.96*0.96=.9216

0.8851% of 9.25=8.19"

subtract 13.12%...7.11" without diagonal

Edited by LMcKeen, 07 December 2021 - 09:41 AM.

[ABQJeff]

Jeff:

 

Thanks for simplifying my proposed comparison. I will confine the test to the 180mm Mak. Since the test will be conducted in the daytime initially and because the Orion Mak. has a dark tube colour it may be at a disadvantage with seeing. But we shall see. Maybe I can wrap it up in aluminum foil. Seriously.

 

Dan Kahraman

I wrap both my Cats in Reflectix insulation (basically aluminized bubble wrap) found at hardware stores next to the duct work.  I find it eliminates need for cooling on my 6” Mak and C9.25.  You can see the wrapped scopes in their ‘shining’ glory in my telescope “family photo” in the review.

[ABQJeff]

Getting back to the area of obstruction versus a diameter by diameter comparisons:

 

The Orion 170mm Mak has an obstruction 41mm in diameter

 

The ratio of areas is 5.82%....whereas the ratios of diameters is 24.1%. This is a significant difference 

 

What does this do to the Airy disc energy distribution patterns? How does the aluminum deposited coatings further destroy image contrast?

Lastly you have the number of surfaces with say 88% reflectivity versus the multicoated lens surfaces of *98% transmission each....What is the final result? 88.6% for the triplet, 68.6% for the Maksutov.

 

88.6% of 140mm is 124.04mm

68.6% of 170mm is 116.62mm

 

Should I further substract 5.82% from the Maksutov but none from the Phantom? In that case 109.83mm for the Maksutov.

 

Light Gathering power will be 18.70x human eye for the Mak, 23.84x eye for the Phantom triplet. Final figure of 7.11" diameter for the 9.25" Celestron without the diagonal 50.64x LGP of human eye.

 

 

*I don't know the actual values

 

 

Dan Kahraman

 

*Calculation for Celestron 9.25"

 

Celestron 9.25 obstruction by area 13.12%

Transmission corrector plate 0.98*0.98=.9604
Primary mirror and secondary mirror 0.96*0.96=.9216

0.8851% of 9.25=8.19"

subtract 13.12%...7.11" without diagonal

Just confirming, is it 170mm or 180mm? Orion had a 180mm f/15 Mak (7”).

[LMcKeen]

Jeff where would I find your photo? Thanks for the aluminized bubble wrap. Excellent idea esp. for closed OTAs even more so for darkly coloured tubes.

 

Dan Kahraman

[ABQJeff]

Jeff where would I find your photo? Thanks for the aluminized bubble wrap. Excellent idea esp. for closed OTAs even more so for darkly coloured tubes.

 

Dan Kahraman

It is embedded in the review article.  If I cant attach it in this conversation I will PM you.

 

On insulation: According to Jon Isaacs a decade or so ago the Dutch figured out that it was internal difference in temperature that caused thermal currents for closed tubes (ie Cats) not difference with outside temperature (as effects open tube reflectors).  So goal is to have mirror and tube walls as close as possible in temperature, thus increase thermal mass of thin walls using insulation and insulate the thick mirror region less (if at all) so they both cool at close to the same rate.  Also best to insulate around dew shield as well so tube doesnt see as cold of an ‘exit’.  It has taken Americans a decade and many debates on CN for most Cat users to go with insulation, although there are still strong proponents of active cooling and waiting hours for scopes to acclimate (I am just impatient I guess).  Just go to Cats and Casses forum and search insulation, lots of fun .  Personally I use two reflectix layers on my 6” Mak walls and one on the mirror (which doesnt have vents).  For my C9.25 I use one layer on the walls and none on the large mirror area (which has vents).  So far so good.  I dropped from 55 degrees to 16 degrees F Saturday night and did not have any noticeable thermals.  My friend’s uninsulated C8 was a mess.

[LMcKeen]

Thanks Jeff I will check that thread.

 

You can PM me anytime you want.

 

Dan

[ABQJeff]

Copy, PM sent; moving insulation discussion to that since not relevant to Istar Phantom 140mm review.

[ISTAR Optical]

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

Jeff,

That is the best kind of news a manufacturer can receive. Keep enjoying your new Phantom 140 and show others at star parties. As you most likely know, iStar is a tiny, family owned business with only two employees and our survival depends on reviews from satisfied customers like yourself. 

 

Clear Skies!

 

Ales

[ISTAR Optical]

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. 

Jeff,

just a short comment on your post. You did mention "magnification on par with a lot of refractors". For example 140mm Phantom (fluoride-lanthanum-dense crown optical design) can handle 400x magnification, for example on double stars, without the image breaking up. 

Our 250mm f/11 R30-T doublet can be pushed to 750x to 1000x (depending on quality of seeing). So I believe that the level of magnification between F/10 SCT and a typical refractor should be viewed similarly. 

[lwbehney]

I directly compared the contrast of my C8 against my FS-128 side by side. The target were the threads of a screw located inside of the taillight of a neighbor’s car. The resolution of the C8 was superior in it’s ability to increase the separation of the threads, however, the refractor view was superior in actually seeing the individual dark lines of each one of the screw threads buried inside of the transparent red plastic of the taillight. 
Basically, it was as if the refractor lifted away a slight fog from the view.  
Atmospheric seeing cannot be blamed as a reason; the comparison was made through < 200 meters of air. 
 

-Larry 

[ABQJeff]

To all, In my review article I mention the focuser attaches via an M145x1 thread (like a Tak FS128). Unfortunately, that was the dimension provided to me by Istar. I ordered a 3” Feather Touch with the M145x1 adapter, and it didn’t fit. Turns out the focuser attaches via an M133x1 thread (maybe M134). Either way, my Feather Touch upgrade wont work and I will have to eat the 15% return cost ($150). Consider yourself warned.

[Bearcub]

Hey not fair... where is dobsonian vs this one?

I might be late to comment and i dont even have a telescope yet but i just wanted to tell you that some sort of test with dob would have been amusing and perhaps fun to read.

[ABQJeff]

Hey not fair... where is dobsonian vs this one?

I might be late to comment and i dont even have a telescope yet but i just wanted to tell you that some sort of test with dob would have been amusing and perhaps fun to read.

I don’t have a Dob, but I will be at Okie-Tex with my friend who has an 8” F/6 Dob.  Call it battle of the the F/6-ish scopes.  It should be interesting as he will have a CO and mirror losses on mass produced mirrors (NOT Zambuto, etc.)

 

Jeff