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Refractor vs. Cassegrain for Planet Viewing

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#1 Brian Johnston

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Posted 22 March 2010 - 04:10 PM

It will be a while before I can afford anything, but I'm window shopping....

I love my big dob for light gathering, I love my little dob as a grab and go, but for planet viewing I'm frustrated with both more than I should be.

I've gotten collimation best I can tell by star test and a high-end laser (barlow method has actually made views worse afterwards, so I know its not the laser), but in the smaller dob, even though it's rated to 225x, I find the clarity starts dropping rapidly at about 100x depending on seeing conditions. The larger dob I can get to about 150x before the clarity starts dropping depending on seeing conditions. But often I've wondered whether it was a function of alignment that was the cause of the degraded views as sometimes the seeing didn't seem like it should be as much of an issue as it was.

Now I've no doubt that the mirrors could probably be aligned better by a pro, but it's frustrating always trying to tweak the last millionth of an inch out in these scopes with a flow F value. Whoever invents the true auto-aligner for a mirrored scope is going to make a fortune.

So my question is this - for planet viewing on my next scope should I save up and get the biggest refractor I can afford? Or do I go with a longer FL where there's more room for error in collimation at get a cass?

I know with a smaller glass scope I might end up losing clarity due to the smaller aperture, but there's no alignment required which is becoming very attractive to me - enough to justify the higher cost of the glass scope.

What attracts me to the cass are the longer focal length's meaning my alignments allow more room for error plus the added benefit of being able to get higher magnifications with lower power EP's - a huge frustration with the smaller dob at it's 450 FL. Since this would be primarily for planetary viewing, the lower FOV isn't a large concern to me.

This would be for visual only, not for pictures. Thanks all!

#2 dsohunter


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Posted 22 March 2010 - 04:25 PM

I don't have a refractor, but I can add this:

I have a 14" dob, an 8" dob and an 8" SCT. I've observed the best planetary details out of the 14". That's pretty obvious since I have more light to crank up the magnification.

Perhaps of more interest to you is the comparison between the 8 inchers. The longer FL of the SCT helps me grab more magnification, but assuming both scopes are collimated, I don't notice a large difference in detail. If anything, some would argue that the larger secondary obstruction makes contrast lower.

Seeing is the hardest obstacle to overcome where I'm at and that makes star tests with the SCT difficult for collimation. My SCT is also a beast to cool down compared to my dob.

Bottom line, is given the same or less aperture, I don't think an SCT is going to improve you situation drastically for planetary. You're probably struggling with collimation and seeing problems. With (rare) good seeing, I've had some 3D like razor sharp views in my reflectors. SCTs do eliminate diffraction spikes, which is nice.

I can't speak for a Mak or refractor, so I'll let those owners chime in.

#3 sailor70623



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Posted 22 March 2010 - 04:29 PM

I get great viewes with my Z10. Right up there with my F8 6" refractor, if not better.
First, check that your primary isn't pinched. This can cause all kinds of problems. Second, make sure the center spot is, well in the center. A misplaced center spot can blow all your attemps at getting good collimation. I do use an auto collimator on mine, but you should be able to get your collimation good enough with a barlowed laser.
Do you use your cooling fan? Even if you do, make sure you don't have boundry layers of warm air on your primary. Lastly, the weather has been horrid lately, maybe seeing just won't support over 150X.
What eyepieces are you using? The 9mm that came with the scope is not a high quality EP. I usually use Baaders in my Z10 and sometimes Naglers. Ashame to buy a new scope because of a cheap bad EP. New scope or not, you will want some good EPs, why not try some of the Meade 5K SWAs while they are on sale.
Now since you have Newts, do think about getting an auto collimator. They sure make getting the last bit of performance out of your scope easy.
My 8" SCTs can't get near the views my Z10 gives. And they take longer to cool. But the tracking on my SCTs and refractor are very nice for planetary viewing. Then if I want to view planets, I can throw my Dob on my Eq platform and enjoy the same. As for your idea that the longer focal length will give you more room for error, not really, and while SCTs hold their collimation very well, most find them much harder to get a good colliamtion on them. A few short FL EPs will be much cheaper than any good SCT and can be bought with long ER if that's what you are looking for.

#4 Bob Griffiths

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Posted 22 March 2010 - 05:00 PM

I own a 100 mm f/6 and a 127 mm f/9.3 refractors (both achros and NOT expensive ones) neither can match my 8 inch Sct which also can't match my 11 inch Sct for planetary detail...

Both Refractors fall apart at about 150x...

I honestly can say why you are having problems with the 10 inch Dob... (well besides just plain rotten seeing conditions) it should d just fine...

Bob G

#5 astroalex


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Posted 22 March 2010 - 07:55 PM

Brian, I sense your frustration, but your 10in dob will out shoot an 8in SCT any day of the week, especially on planets. You might consider improving your dob with full flocking that will improve contrast. Check your laser on a "V" block aiming it against a wall and spinning it to see if it is putting out a straight beam. Collimation does take a LOT of patient and you must stick with it. Try using an Orion Collimating Eyepiece unit BEFORE using a laser. It will help you get the secondary square under the focuser. And after you get done, there is always some lingering doubt in your mind about how good a job you did. It's the nature of the beast. You just have to do the best you can and keep refining your technique.

An 8in SCT with "tracking" will give you a more relaxed view of planets because they will stay steady in the eyepiece. This ONE thing makes an SCT a joy to use, or any scope for that matter. I was able to look at AND study M42 trapezium much easier with my AT102ED on a CG5GT and enjoy the view better than with the XT10I. With tracking, the human eye has more time to adjust to the view and you naturally see more detail. No so with a dob w/o tracking. This is why I am seriously considering the 8in EdgeHD SCT OTA to go on the CG5-GT mount. I would be using this combo for the study of binaries. However, it would no doubt, be a planet killer package as well. The downside of this setup is that it would be a lot more work to assemble all the equipment and make it work with mount, weight, scope, battery supply, etc. I move my XT10 outside in 2 min and start viewing in 5 min. So, there are trade-offs, as we all know.

Take care and Clear Skies! :)

#6 brianb11213



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Posted 22 March 2010 - 08:11 PM

I move my XT10 outside in 2 min and start viewing in 5 min.

How long does it take to cool ... 1 hour? 2 hours? 'Cos, until it is pretty close to ambient temperature, the view at high magnification will be poor ... that applies to ANY scope!

A tracking mount certainly helps when using a medium to high power, but there's nothing special about a SCT from that point of view. In fact there's very little difference in the view you get between scopes of different design but similar optical quality, provided they're properly collimated & allowed to cool to ambient. The thing that makes a SCT "special" is its light weight in proportion to its aperture; and more aperture is, well, more.

The biggest factor of all is the steadiness of the atmosphere ("seeing"), this varies widely & is often very poor. There is no point in trying to use a high magnification in poor conditions, and any attempt to collimate when the seeing is less than good is likely to do more harm than good.

#7 Tom Andrews

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Posted 22 March 2010 - 09:21 PM

I have to throw my experience in here because it's somewhat different than the others but the conclusion is the same. I've had 2 dobs (8", 12.5"), 1 refractor (80mm) and 3 SCT's (5", 8", 11"). For me the best on planets and everything else has been the 12.5" dob. The next best on everything except planets is the 11" SCT. The next best on planets is the 8" dob.

BUT the biggest difference is in the seeing conditions and this is where my experience is a little different. My wife and I are full-time RV'ers and move around the country. After 5-1/2 years of traveling (but only being in about 30 states), northern California has offered the best views of all. And last year I had the opportunity of going from northern CA with my 12.5" dob seeing incredible views of Saturn to central CA, southern CA, Arizona and New Mexico in just a few days and the difference was unbelievable! The views got a little worse as I went and by the time we got to Albuquerque the views were so bad that I didn't even care to look at Saturn anymore. And this was all through the 12.5" dob. So my conclusion was/is, seeing conditions (meaning everything: light pollution, atmospheric disturbance, smoke, dust, etc.) make all the difference.

You MIGHT have the opportunity to try 10 different scopes from the same place and decide that one or two really outshine the others. But if you took those same 10 scopes to a different location, you might decide differently. And the scope that you thought wasn't giving you good views might show outstanding views you didn't think possible.

Now where does that leave you? All you can do is try different scopes side-by-side to see what works the best for your seeing conditions. But, as my experience shows (and as others have experienced), the overall best views for me have come from the largest aperture.

#8 Time on my hands

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Posted 22 March 2010 - 11:32 PM

Brian, find a way to look through the scopes side by side or as close as you can. Star parties are great for this. I found that I prefer the refractor views to SCTs. I found this out after I dropped $2000 on an SCT. Comparing a 14" SCT to a 4" refractor convinced me. I will admit that the views through a 12" portaball with a curved spider has equalled my TMB 130 SS so there is potential for the Netonian design. Your preferences are what is important. Others that made the same comparison I did chose the SCT and one picked a standard Newtonian. Look for yourself, then decide.

#9 David Pavlich

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Posted 22 March 2010 - 11:45 PM

Resolving power equals aperture, all things being equal. The best view I've had of Jupiter was through a 16" Nightsky. Simply gorgeous! This is a well collimated and cooled scope that belongs to a club member. It's just brute power. And you have to keep in mind that the big mirror allow for some pretty high magnification.

But above all, if the seeing isn't good, the views will just not be good as power increases. I have a C14 and 300X is fairly easy unless the seeing is really bad. However, when you have that rare night that everything is just right, getting a big scope up to eye opening magnification is a thing of wonder!!


#10 Vondragonnoggin


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Posted 22 March 2010 - 11:54 PM

There is a really informative thread on premium mirrors in mass produced dobs. I thought about going with a premium, but after reading it, found out that I can send my mirror off for testing and refiguring if neccessary at 1/3 the cost of a new premium mirror, and have it come back with 1/10th wave or better optics. replace the secondary if needed too and still come out better than buying a new scope with the same aperture. Tracking can be had for any type of scope if you want to spend the money. A really good mount with tracking will cost some bucks too. Some of the people that sent theirs in for work found out that the center mark was off also. A 10" dob should be able to get some good magnification on planets at least some nights you are out. I find mine views best after a few hours outside and particularly if I start observing at around 9 or 10 at night and wait till 11 or 12 to view the planets. I imagine there is poorer seeing when the earth is still cooling from the days warmth. Usually have no problems viewing at anywhere from 150x to 240x on average nights out and 300x + on great nights, but I'm sure that depends on the area too.


#11 HiggsBoson


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Posted 23 March 2010 - 01:13 AM

Hello Brian

I am a relative newcomer to this activity but I will add my prospective on your concerns. I work with a 6” Newtonian made with a mirror that I made myself with no previous experience. I routinely work at 175x and on good evenings I push up to 275x on Jupiter, Saturn and Mars. With your larger aperture you should be able to do as well. I suspect two problems:

First, you clearly do not have confidence in your ability to collimate your scope. You are favoring a particular design to eliminate this task. Many here are able to collimate their scopes using a wide range of tools from simple things that come with a scope to the more expensive sets that include autocollimators. The first time I collimated my scope I used an old Kodak film canister with a hole in the bottom. Everything was done by eye using this kluge. It worked and I was able to perform a star test. Based upon this test I sent my mirror off to be coated with aluminum, therefore one can collimate a Newtonian using modest equipment. In my case it was an F/9 scope which is quite forgiving. On the other hand your 10” is F/4.9 will require quite good collimation for proper performance.

Taking advice offered here, I purchased the CATSEYE tool set. While it is a little pricy it allow one to collimate ones scope as well as it can be done and be sure that it is good. I can do this in daylight without loss of observing time. I spend 10 minutes performing a collimation during the day and transport my scope. When I reassembly the scope at the site I spend only a few seconds verifying that collimation has not changed. I can then view with confidence. If the view is fuzzy, it is not collimation. There may be other sure ways to get confident collimation but I can only tell you that this absolute beginner has never lacked confidence in collimation since I acquired these tools. I allow others to star test my scope with confidence at star parties and club events without a concern that my errors will be exposed.

My second concern is a combination of cooling and seeing. I view in my front yard which is completely paved with interlocking stones. I find that these stones give up heat for about 3 hours after sundown. Nothing I do will clear up the view. I must wait at least this long if I want to use 175x or have a shot at using 275x. Next, there is a hill to the east of me and there are homes on the hill. I find it unproductive to view in this direction. I must wait for objects to be to the south and west where there is no development. I spent a frustrated year before I realized that heat matters in high powered viewing. Now I set the scope out at least one hour prior to the end of the 3 hour wait after sundown. I use planetarium software to plan what objects will be in my preferred field of regard to the south and west during the time I will view.

These steps have saved me no end of frustration. Now I do not lose time viewing with a poorly collimated scope. I do not spend time viewing over heat sources or standing on a heat island ( my front yard). I use the star test to ensure that the mirror is cool and if I do not get a sharp view it is a night of bad seeing. This does not frustrate me because I know that there is nothing I can do about bad seeing. Slowly I am getting better at understanding the weather that is highly correlated with poor seeing. The better I can predict the viewing conditions the less frustration I experience.

With a fast Newtonian I strongly suggest the CATSEYE tool set. While I am sure that lasers can be used successfully I find a predominance of complaints here in these forums about these tools. I have never heard of anyone who lost time due to a miss-collimated autocollimator. The tools I recommend are passive, always collimated, require no batteries and work at any temperature. In 3 years of use I have not had one second’s trouble with them or ever had a lack of confidence in my collimation. When it is right you know it is right. When it is not right you know by how much.

With a 10” Newtonian you should be getting views that are superior to any 5” refractor. I said ‘any’ 5” refractor.

#12 brianb11213



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Posted 23 March 2010 - 04:20 AM

Brian, find a way to look through the scopes side by side or as close as you can.

I own a CPC1100 and a WO FLT 110. Now the FLT 110 is a nice scope, producing tidy images with a classic disc & faint ring diffraction pattern on in-focus star images (at high magnification, in good seeing).

The images in the 4.3" refractor certainly look tidier than the images in the 11" SCT, but, on 90% of nights, the SCT shows much more detail ... it feels more like 3x than 2.5x better resolution at the eyepiece, and when imaging moon or planets, the difference is even more marked (the brighter image gives shorter exposures which cut through bad seeing). On the one night in 10 when there isn't a big advantage to the larger scope, the seeing is so wobbly and/or blurred (jet stream) that the issue is moot.

This does of course depend on the scopes being properly set up (accurately collimated, free from "pinching" from overtight cells etc) and allowed ro cool to ambient temperature (which can take a long time, especially with large scopes with sealed tubes (SCTs), though a "cat cooler" helps).

I enjoy using a 'frac, and the FLT 110 is an excellent "rich field" instrument - but, for resolution of planetary details, aperture rules, optical quality is an important second factor and design comes a distant third.

#13 Tony Flanders

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Posted 23 March 2010 - 05:43 AM

I suspect that your problems are mainly due to poor atmospheric conditions, and can't be fixed by buying a different scope. As for the scopes you currently own, the Starblast is a lovely little scope, but planets are not its strong point. Collimation aside, the fast f/4 focal ratio places inordinate demands on the rather crude focuser. When observing the planets in my Starblast, I find that the view is limited by my ability to achieve accurate focus. In any case, I don't really enjoy using this scope much higher than 100X. Its strong point is deep-sky viewing at low to medium powers.

With your 10-inch scope, 150X sounds like an altogether reasonable limit on nights of typical seeing. When the air is really steady, you can probably push it to 250X or higher, but don't expect that to happen on a regular basis.

With a little practice, you should be able to collimate the 10-inch scope well enough for viewing at 150X in less than one minute. My own favorite collimation device is a Cheshire eyepiece, but I don't doubt that a laser can be equally effective.

To get a comparable view at 150X through a refractor, you would need at least 4 inches of aperture, and preferably considerably more.

#14 SteveG


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Posted 23 March 2010 - 06:02 PM

I can't speak to the SCT as I've never owned one, but have not cared for them as planetary scopes when given the chance to look through one. You have a 10" reflector, which should give detailed views up to 300x if seeing and collimation are good.

In my opinion, a great complementary scope is a 4" ED refractor. It is lightweight and easy to transport and use. It does not require a long cool-down time (15 min). And best of all, it provides sharp contrasty views, with more pin-point stars on nights of fair seeing or better. I purchased my ED100 for $400, and mounted it on a Polaris mount purchased used for $200. I added a $30 drive motor that runs on a 9v battery for a fully self contained setup weighing a total of 30 lbs.

#15 Brian Johnston

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Posted 25 March 2010 - 03:59 PM

Thanks all for the great responses! My laser is a Hotech and I have no doubt it's accuracy.

I have Cheshire also, but apparently I'm not a good user for those because collimation always seems worse when I do it and it appears (to me) that it's a lot easier for me to put in errors I've noticed, especially in the StarBlast that depending on the angle I look in the peep hole or the slop in the focuser, the reflected 'circles' appear in different places. And I've read the instructions it came with and watched things on the internet several times. I know it works for others, but not for me.

I won't say I don't get similar issues from the laser - I can get different points of light depending on how I clamp it down; but it's always significantly better in the end than the Cheshire. Not to mention the Cheshire is pretty much impossible to use on a scope that is about 5 feet tall - can't look through that tiny hole and turn knobs on the back of the mirror at the same time and going back and forth gets ridiculous after about 5 'tweaks' of a thumb screw and still not having it right.

As far as an auto-collimator - I'm not very familiar with those...but isn't that basically just another name for a Cheshire?

I think the comment about heat off the drive way might explain some of it, in the summer, but definitely not an issue in the cold and I let the Z10 cool down for at least an hour.

And yes, I'd say a good part of my looking for a different style of scope is because I'm tired of the fickleness of these mirrors and having to collimate every single time I take it outside and plan to view at high power (lower power nights, I don't worry about). To vent a little - how come they can make a telescope with 'light switch' technology but they can't make a scope than can align it's own mirrors? I'm sure the technology has to be there to do it.

#16 sailor70623



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Posted 25 March 2010 - 10:06 PM

An auto collimator is not just another cheshire. They are at leat twice as accurate. They allow very accurate setting of your secondary's rotation. Very percise collimation of the primary, and are, to me, easier to use than a cheshire. A good laser (well collimated) allows setting of the secondary fast and easy, but you must barlow the laser to get top knotch primary alignment, and even then, the auto collimators can give even more acuracy. For planetary viewing, especially at high magnifications this is important.
If you use yor lap top for a planetarium program, thus have it out anyways, try a cheap web cam. Set it up to look through your cheshire, and place the lap top where you can see it while adjusting the primary. I just have the other half turn the screws, while I look through the auto collimator, or cheshire.

#17 FLYcrash  Happy Birthday!



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Posted 25 March 2010 - 11:13 PM

To vent a little - how come they can make a telescope with 'light switch' technology but they can't make a scope than can align it's own mirrors? I'm sure the technology has to be there to do it.

There is, and it's expensive. Some of the largest professional telescopes have thin segmented mirrors, each actuated by a servo to minimize the continuously monitored wavefront error. At the seeing level, there are systems that phase-modulate the light cone to correct for atmospheric distortions. Look up "active optics" and "adaptive optics" for some interesting reading.

In my own field of work - experimental physical science - the difficulty of aligning is well known. Some of the crowning achievements in modern optical physics - laser cooling and trapping, cavity quantum electrodynamics - are famously expensive and labor-intensive partly because of the need for electronically managed alignment. If you study physical chemistry and your Ph.D. project involves a lot of laser methods, you need to expect that it will take a year or two longer than normal. That's literally a year or two of a person's life spent collimating laser cavities and optical setups, 50 or 60 hours a week.

Now, I wouldn't do well ruling out what the free market can accomplish - I'm still trying to comprehend that one can buy commercial image-stabilized binoculars or a computer that fits in your pocket - but do understand that acquiring an image, interpreting an error from that image, and making appropriate precision mechanical adjustments is a difficult task requiring some serious hardware. Just ask the astrophotographers who have gotten into autoguiders.

#18 Greg Keys

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Posted 26 March 2010 - 10:32 AM

I work in industrial automation, designing computer control systems for automating equipment. Raman is correct. I could design a system to align the mirrors in a dob, but it would triple the cost (or more) of the scope. In automation, cost is directly coupled to two things. First is percision, and the second is size. Little tiny precise servo motors for each alignment point would rapidly drive the cost out of reach. Look at what it costs to produce a good goto mount. That is only two servos and sensors. Primary mirrors have 3 to 6 alignment point, secondarys typically have 3 plus 2 or more for centering the secondary.... add in electronics for controlling those, and a video system to interpet and feed back info to the controller.... well, you get the idea.

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