Author Topic: Considering 2 (used) reflectors for 1st scope  (Read 867 times)

Brandon Belknap

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #15 on: January 18, 2018, 07:34:41 AM »
"To focus at high power in an f/5 scope, you really need a high-quality Crayford focuser, preferably with dual speeds."

Two-speed focussers are for imaging purposes primarily, as I've no problem focussing with my f/5; nice try, though.

"...so there's no incentive to rotate it."

Synta et al are not going to equip their mass-production Dobsonians with rotating tube sections or tube rings; too expensive.

Bresser(Jinghua?), however, has, and no doubt in response to demand...

http://www.telescope...ian-8-inch.html

Now, that's what I'm talking about.

"...but it comes at the price of purchasing two mounts."

Not "purchasing", as though you have to run out for a gallon of milk and a dozen eggs; rather, acquiring, over time, over the years, and as I have.

trualolalun

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #16 on: January 20, 2018, 01:27:37 PM »
For these prices you might consider buying both and selling the one you don't want to keep. GN

Charlie Carpenter

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #17 on: January 20, 2018, 02:46:45 PM »
I figure if I'm going to spend $400 I might as well go ahead and get an XT8, Omni XLT 150, or Meade LX70, you can only look through one at a time right?

The whole reason I was considering the dob was due to my concern about the tripod on the skyview, but if the sky-watcher has a sub-par collimation assembly that doesn't do me much good either. So now I'm leaning back towards the skyview...

Paul Nyuon

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #18 on: January 20, 2018, 05:36:32 PM »
As I had stated earlier, the focusser is very important, and a most essential part of the optical train. A successful collimation is dependent upon the focusser to quite an extent. Unfortunately, Synta, the manufacturer of these and other telescopes, does not take 1.25" focussers too seriously.

Behold, Synta's plastic 1.25" focusser, and in all its awe-full and horrid "magnificence"...
As you can see, I tried my best to make it nice, but ultimately, to no avail. I am close to replacing it with an all-metal unit, and eventually, you would want to as well.

Precision escapes it, but that's just me. However, salvation may be just a rock-thrown-across-the-pond away...

http://www.ebay.com/...HsAAOSw7W5XNsIK

...an all-metal 1.25" Newtonian focusser, although I know nothing about it, as to its precision and quality. Its drawtube and the rack thereof might have to be cut and shortened a bit to ensure that the tip of the drawtube does not jut out into the light path.

Now, that's not to say that the telescope itself is bad, for the rest is very good, especially the primary mirror and its cell. If it wasn't, I wouldn't be replacing the focusser, and would move on to another branding of a 6" f/5. The only other 6" f/5 is this one, but it comes with a 2" focusser...

http://www.highpoint...iYzEaAmkI8P8HAQ

...which results in a larger secondary obstruction, and thereby reducing brightness and contrast a bit further. The image within that listing illustrates the slightly older model which originally came with a 1.25" focusser, but no more...

http://i48.tinypic.com/14j9ufr.jpg

The Orion 6" f/5 appears to be the last of the 6" f/5 instruments throughout the world with a 1.25" focusser, and a smaller secondary mirror.

There is the Celestron C6-N, and with a 1.25" focusser, but its primary-mirror cell is practically the same as that of the Sky-Watcher 6" f/8 and the Zhumell Z130...

https://stargazerslo...-1416380156.jpg

...and the telescope itself... http://firstlightopt...mages/C6NGT.jpg

Understand that I did not want you to decide before knowing the particulars and the truth about the focusser of the Orion 6" f/5 Newtonian...




Steven Morrison

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #19 on: January 20, 2018, 09:02:01 PM »
Quote
The 6" dob is an easy choice. Go for it!

+1 That. Its hard to beat simplicity of a 6" Dob and the killer planetary views it provides at F/8. Ease of collimation is another plus.

subhymerlo

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #20 on: January 23, 2018, 12:03:05 AM »
Quote
As I had stated earlier, the focuser is very important, and a most essential part of the optical train. A successful collimation is dependent upon the focusser to quite an extent. Unfortunately, Synta, the manufacturer of these and other telescopes, does not take 1.25" focussers too seriously.
To amplify on this: the plastic focusers found on most inexpensive telescopes are fine for deep-sky observing, which is typically done at magnifications of 10X to 20X per inch of aperture. But they get touchy at the magnifications used for the Moon, planets, and tight double stars, which typically range from 25X per inch of aperture on up. They're not hopeless, but I do tend to spend a lot of time overshooting perfect focus in one direction, then overshooting in the other, as I try to fine-tune the precise position of the eyepiece.

This also depends a lot on the focal ratio of the telescope in question. Inexpensive focusers are a really bad problem at f/4, a significant issue at f/5, and hardly ever a problem at f/8. That's because a tiny change in eyepiece position has a much larger effect at short focal ratios than at long focal ratios.

Good focusers are a joy to use, a critical component of any truly high-quality telescope. Unfortunately, first-rate focusers all on their own cost more than the entire telescope packages that you're considering. Precision machining doesn't come cheap.

Unlike SkyMuse, I'm not allergic to the idea of plastic. There are places where it's the perfect material to use, including some places in a telescope. Focusers, however, are not one of those places.

dustsungline

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #21 on: January 25, 2018, 03:12:02 PM »
You are spending $200.00 on a scope, There will be some warts. You though are in the cat bird seat.
You have two of the best scopes out there in the price range to choose from. You could have
wasted $200.00 on some department store piece of junk.

I don't have the 6" dob, I have 8" dob and it does collimate.
They do good on all objects for their size and price, the dob does some
things better than the EQ, and the EQ does some things better than the DOB.
I have a scope that has a plastic focuser and it is OK.; It actually has less slop
than my all metal synta focuser. I know one day it might stop working and on that
day, I will get a new focuser and put it on. Newts are simple. BTW it is a $200 scope.

Since these are used just make sure the optics are good. Use the flash light test for the primary.
Some dust might look scarry, but it can be removed. Just make sure there is no hazing,
or scratching of the primary mirror. Same with the secondary.

Some others here with a digital camera might send you a pic of either issues
with mirrors or what a good dusty mirror might look like

Jay Garcia

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #22 on: January 25, 2018, 10:41:15 PM »
Quote
You are spending $200.00 on a scope, There will be some warts. You though are in the cat bird seat.
You have two of the best scopes out there in the price range to choose from. You could have
wasted $200.00 on some department store piece of junk.


The 6" F8 Dobsonian is a very user friendly scope that will put up really nice images of DSOs and planets. That would be my choice, but on a local pickup I think $200 is on the high side. I would be more comfortable $150-$175.

malphandrafsadd

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #23 on: January 29, 2018, 09:41:46 PM »
Quote
Some others here with a digital camera might send you a pic of either issues with mirrors or what a good dusty mirror might look like

Does anyone here have some pics of these issues they could share or a link to a good thread on buying used scopes with pictures etc?<p class="citation">vtornado saidYou are spending $200.00 on a scope,  There will be some warts.[/quote]

Yes, I need to keep this in mind, can't have everything for $200.  At this point I'm now thinking I'll go with the dob.  The rubber grommet issue on the collimation screws bothers me but I likely won't have the scope long enough for that to be a problem.  If I decide to stick with the hobby I'll drop some coin on good equipment before they begin to break down.
I appreciate the input, especially from those of you that have these scopes.  I'll go look at both this weekend and either buy one of them or order one and stop looking at them on the internet.

Mark Patterson

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #24 on: January 30, 2018, 02:54:34 AM »
You could ask the seller of these to send you pics of the mirrors.
I usually do when someone is selling a scope. It saves my and their time if there is an issue.
You can post them here and we can have a look at them.
You can also gauge the eagerness of the seller and if they are polite.

artufanchess

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #25 on: January 31, 2018, 04:30:55 AM »
Quote
Quote

Quote
Some others here with a digital camera might send you a pic of either issues with mirrors or what a good dusty mirror might look like

Does anyone here have some pics of these issues they could share or a link to a good thread on buying used scopes with pictures etc?<p class="citation">vtornado said
You are spending $200.00 on a scope, There will be some warts.

Yes, I need to keep this in mind, can't have everything for $200. At this point I'm now thinking I'll go with the dob. The rubber grommet issue on the collimation screws bothers me but I likely won't have the scope long enough for that to be a problem. If I decide to stick with the hobby I'll drop some coin on good equipment before they begin to break down.
I appreciate the input, especially from those of you that have these scopes. I'll go look at both this weekend and either buy one of them or order one and stop looking at them on the internet.

I've not read about any Skywatcher dob's that could not be easily collimated. If the 6" dob has a 2" focuser it is the easy best choice IMO.

incojukam

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #26 on: January 31, 2018, 11:04:15 AM »
Quote
Yes, I need to keep this in mind, can't have everything for $200. At this point I'm now thinking I'll go with the dob. The rubber grommet issue on the collimation screws bothers me but I likely won't have the scope long enough for that to be a problem. If I decide to stick with the hobby I'll drop some coin on good equipment before they begin to break down.
I think that's a wise decision. The Dob is what it is -- a high-quality example of a very simple design. There's a lot less to go wrong than there is with an equatorial-mounted Newtonian. And I doubt you'll be getting rid of this scope ever. If you buy a more expensive one, you will probably find that the 6-inch Dob complements it beautifully. I very much doubt you'll ever have problems with the collimation. Besides, an f/8 scope works flawlessly even when the collimation is pretty far out of whack. F/8 has 2.5 times as much room for error in collimation as f/5.

tenewbandhams

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #27 on: January 31, 2018, 01:08:04 PM »
"Besides, an f/8 scope works flawlessly even when the collimation is pretty far out of whack."

I'd wager that that's a stretch of one's imagination.

"F/8 has 2.5 times as much room for error in collimation as f/5."

If it wasn't for the mediocre Synta focusser, the 6" f/5 would be far and away the very best choice; more versatile in observing the gamut, compact and easily transported.

planvelsynchcest

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #28 on: February 02, 2018, 01:34:52 PM »
The Skywatcher primary cell design does not prevent collimation but it makes the process more difficult than it has to be.

Loosen a collimation screw and instead of the mirror moving up into the tube, the screw will back out of the mirror cell holder while the mirror itself remains stationary. Tighten a collimation screw, and instead of the screw resisting the applied torque in a relatively predictable manner provided by a coil spring under tension, it will tighten with no resistance until the mirror cell contacts and binds up the O ring, which at this point the colllimation screw becomes very difficult to turn.

The collimation screws themselves are incapable of properly locating the mirror. The tiny locking screws actually do most of the work locating and supporting the mirror.

The mirror does not respond in a predictable or linear fashion to collimation screw adjustments.

I also feel a straight through finder is almost impossible to comfortably use on a Newtonian but that is another topic.

akbrevecop

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Re: Considering 2 (used) reflectors for 1st scope
« Reply #29 on: February 02, 2018, 09:42:15 PM »
Dropping in for an update. 
Ended up with the Orion skyview.  The family who was selling the dob decided to keep it and try to use it - the young daughter thought it would be to big to wrangle which is why they thought of selling it in the first place.  I'm happy they decided to keep it, they will probably have lots of fun with it once they get it out of the house.
I ended up with the scope/mount (which was very clean) for $200 and a set of celestron plossl eyepieces (4,6,9,15,25,32mm, 2x barlow, ~10 filters) for $50.  I'm happy overall.
Kinda inadvertently stumbled upon venus and mars (i think) already, so already having some fun.