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Anonymous
Unregistered
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Hello.
I would like to get some input from you all regarding which one of these scopes you would expect to perform better at lunar and planetary viewing? Does the central obstruction of the Mak bring it down to being equal or less in light gathering power than the refractor?
Any positives/negatives of each scope would be appreciated as well. I currently own both of these scopes, and while I have had a chance to do some limited viewing through each, I would like to compare my own observations with those of some more knowledgeable people such as yourselves! 
Thanks a bunch!
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Bill Grass
Prince Regent
Reged: 10/07/03
Posts: 11652
Loc: Denham Springs, LA
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Right off the bat, I'd say one advantage the refractor has over the Mak is quicker cool-down time.
I don't have a Mak, so I'm not that familiar with them. Somebody with Mak experience can help you better than I can.
Welcome to Cloudy Nights! 
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Anonymous
Unregistered
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Maks will take a while to cool down, maybe an hour or so depending on the Mak's storage temperature versus the outside temperature. The Mak will have a little better light-gathering ability than the refractor, and a little less contrast and maybe slightly lower resolution. But it won't have any false color, and will generally take higher powers better than an achromat. I owned the 127mm Mak from Orion until I got my APO. A nice scope with good optics.
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Blair
Pooh-Bah
Reged: 05/07/03
Posts: 1163
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By the area formula the 102 Mak is equal to an unobstructed 97mm scope. (area of obstruction subtacted from area of mirror converted back to diameter). I have a Celestron 102mm F/5 refractor and with a V-block filter (from Orion) I could not tell a major difference between the two through 133X. Usually seeing keeps me below 100X on Saturn and Jupiter. I'm only assuming now - the 90mm with a V-block would be very close to the Mak in performance. I have owned Orion's 90mm f/10 refractor and it was a very good scope but did not compare it to a 102 Mak nor used it with the V-block filter (as Orion did not have it at the time). I returned the 102 Mak thinking if I was going to go that way I would buy the 127mm Mak. I'm keeping the Celestron 102mm f/5 because of it rich field viewing capability and good performance up to 133X on Saturn and Jupiter with the V-block filter. I rambled a bit but hope this helps.
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Anonymous
Unregistered
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Hello,
I have both the C-102 refractor and the Starmax 127. The 4" refractor outperforms the 5" Mak on the moon and planets. So I`m pretty sure that the 90mm refractor would outperform the 102mm Mak on the moon and planets.
Hope this Helps.
Good Luck and Clear, Steady Skies!
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Anonymous
Unregistered
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Thanks to everyone for the great information. My calculations show that the refractor has a surface area of 6362 sq mm, and the Mak has 7417 sq mm (approximate, with Pi rounded to 5 decimal places...nothing like a refresher course in geometry!). These calcs take into account the 31mm central obstruction of the Mak. This matches with the comparison to a 97mm unobstructed scope. I have heard mention that the reflectivity of the mirrors on the Mak may drop light gathering capability as well, but to what degree is more problematic to compute. Even with this in mind, it appears that the Mak (with a FL of 1300mm), will be superior to the 90mm, 910mm FL refractor in magnification and light gathering power. If it weren't for the chromatic aberration, (i.e. if the 90mm were an APO), then the contrast winner would be the refractor, but as it stands, my views have pointed me to the Mak winning this category as well.
Overall, I've decided to keep the Mak, and sell the refractor due to these reasons, and also the extreme portability of the Mak setup. Cool down time isn't much of an issue for me, as I can set the scope outside an hour or more ahead of time with little difficulty. The lunar and planetary views have been "more pleasing" to me in the Mak, especially regarding sharpness and color. I agree that the 127mm would be an even better option, and I may upgrade in the future if/when an opporunity to do so permits. Overall, I've found the Starmax to be a rugged little scope, that is lightweight, well-built, and views that are more than what you would expect for the price range of the scope.
Thanks again for the great info...clear skies!
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Anonymous
Unregistered
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Any surface in the light path will loose a few percent of the light regardless of if it's reflective or refractive. The Mak has 4 (both sides of corector, primary mirror, secondary mirror), an achromat can have 3 or 4 depending on if the doublet is cemented or spaced.
{rant} As for your calculations a hard science/engineering course would help. You're using more digits of Pi than you need to and are claiming more precision than your inputs allow. Multiplicatively you can only have as many signifigant digits as your least precise input. On the mak you have 3 SD for the primaries size so you're limited to 3 digits on it's area, and using a 5 digit Pi only adds to the wear on your calculators keypad. It's area is 8170mm*mm, with 2 SD the central obsctruction is 750mm*mm.
For addition/subtraction the answer can only have digits that are signifigant in both values (81+25 = 106 (3SD), 81+2.4=83 (2SD), 5523-5521=2(1SD)) but since both areas are accurate to the 10s place it won't change anything giving an effective error of 7420mm*mm.
If you really want to be anal you can keep extra digits in all your intermediate steps and keep track of the SDs to round the final answer, but as with the mak example, it rarely makes any difference, and the last SD is expected to contain some measurement error in any event.
{/rant}
Sorry about that, but it really bugs me and I needed to get it off of my chest. 
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 900
Loc: NYC
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Quote:
{rant} As for your calculations a hard science/engineering course would help. You're using more digits of Pi than you need to and are claiming more precision than your inputs allow. Multiplicatively you can only have as many signifigant digits as your least precise input. On the mak you have 3 SD for the primaries size so you're limited to 3 digits on it's area, and using a 5 digit Pi only adds to the wear on your calculators keypad. It's area is 8170mm*mm, with 2 SD the central obsctruction is 750mm*mm.
For addition/subtraction the answer can only have digits that are signifigant in both values (81+25 = 106 (3SD), 81+2.4=83 (2SD), 5523-5521=2(1SD)) but since both areas are accurate to the 10s place it won't change anything giving an effective error of 7420mm*mm.
If you really want to be anal you can keep extra digits in all your intermediate steps and keep track of the SDs to round the final answer, but as with the mak example, it rarely makes any difference, and the last SD is expected to contain some measurement error in any event.
{/rant}
RANT>
I apologize to the forum, but I found the quoted text/post segment, very condescending and rude. As long as Fritzel is using the similar level of "SD" for all his/her calculations, there really isn't a point to worry/obsess about the level of "SD" applied. In most engineering, SD in/of decimal places is more important than SD of whole numbers. Of course, there's not a single reference as to whether or not Fritzel's calculations are right or wrong.
As far as the comment about wearing out the calculator's keypad, most modern-day, readily available, and quite inexpensive, scientific calculators have a single "Pi" button, which gives Pi, anywhere from seven to ten decimal places, depending on display size, and therefore manually entering 3.14 is actually more keystrokes/button presses, and more "wear" on the keypad. END_RANT<
Once again, I apologize to the forum, but insolence is a peeve of mine.
Best Regards,
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Blair
Pooh-Bah
Reged: 05/07/03
Posts: 1163
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Since this was just a calculation for a comparison I used 22/7 for pie and let Excel determine how many points to carry it out to. For a more precise calculation I would have measured the objective, the mirror and the obstruction diameter directly and have used the accuracy of the measurement device to determine the points of accuracy. But then what is 0.1 or 0.2 of Magnitude limit among friends.
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 900
Loc: NYC
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0.1 mag> .
0.2 mag> .
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Mike28
Carpal Tunnel
Reged: 06/21/03
Posts: 2889
Loc: Morris County,NJ
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If you really want to be anal you can keep extra digits in all your intermediate steps and keep track of the SDs to round the final answer, but as with the mak example, it rarely makes any difference, and the last SD is expected to contain some measurement error in any event.
{/rant}
Hey Hey guys lets not get into dueling calcutators here. We're suppose to be professional amatuers here.  Lets keep it clean.
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Mike
'The only reason for time is so that everything doesn't happen at once.” Albert Einstein
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TAKAHASHI TOA150F
Celestron 80ED
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Anonymous
Unregistered
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Wow! I had no idea I had started such an eventful thread!
With this in mind, it seems like we are short-changing pi. I seem to remember that pi is an infinite number, required to calculate the area of a circle. Whenever we work with pi we are rounding it to some number of digits, so all such calculations are incorrect. The only issue is how much
accuracy we need for a particular application. My thinking was that to extend pi to at least 5 decimal places (my accounting background is showing), my accuracy in computing the area will increase, with rounding occuring at the end of the computation, rather than the beginning, which would introduce more inaccuracy...am I right in my thinking?
Anyway, thanks for the info, and also for the intriguing geometry side-topic.
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 900
Loc: NYC
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Hi Fritzel,
Yes, I believe you are correct, back in my college days, all of my engineering and math professors wanted us to use the value of pi that our scientific calculators used, which was as high as twelve decimal places on the rich kid's HP calcs, when pressing the pi button/key. All rounding-off was done at the end. This gauranteed the highest accuracy, as well as letting our answers match those of the textbooks.
That initial "rant", made it sound like you were completely wrong, and it bugged me, so I had to say something.
I would think that a high level of accuracy, for these types of measurements/calculations, is really overkill, but I always go with the value of pi my calculator spits out when I press the pi key. If anything, it makes the results more right, than more wrong. I guess everyone's MMV.
Best regards,
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Anonymous
Unregistered
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Just a question:
How do you know that the lenses and mirrors are round to five decimal places? Perhaps you should calculate their areas as ovals or ellipses?
(Just kidding) 
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 900
Loc: NYC
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Hey POPC Bill,
You may be on to something, the standard (pi R squared)formula applies to a perfectly flat surface, like a round plateglass window. Since most/all? lenses, correctors and such, are either convex, concave, or even more complex(as SCTs), that formula is actually not entirely accurate. You may actually need to use higher levels of math for areas of spheres, etc. But for just some little'o figgurin', it's close enough, unless someone wants to really be "ANAL".
Best regards,
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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