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Advice On Buying Used Dob

Beginner Dob Reflector
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#1 Chicken

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Posted 26 May 2024 - 11:56 PM

Hey guys,

Pretty much what the topic title says. I’m buying my first telescope and I have the opportunity to get a used 8” dobsonian for a great price. If anyone can tell me red flags I should look out for in a used scope that would be incredible. The guy said he only used it a few times and that he doesn’t know much about the telescope. It looks like a skywatcher 8” flextube but I can’t find any online that have that color of base so any info on that would help too. I have the listing’s photos below!

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#2 gnowellsct

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Posted 27 May 2024 - 12:04 AM

Buy it. Great entry instrument. You can figure out what you do like and don't like as you go along. That's what happens no matter what scope you buy.
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#3 Taosmath

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Posted 27 May 2024 - 12:59 AM

These are a good scope for a beginner if the price is right and they can take you a LONG way in the hobby.  The one I used had a black base, so don't stress about that.

 

Things to look for are to take the covers off (both the fabric secondary tube cover and the hard plastic cover over the mirror box.  Look at the mirrors to make sure that there are no significant scratches/gouges/chips in the front of the primary mirror or the smaller diagonal secondary mirror.  (don't worry if the mirrors are dusty, you can easily clean that and there are lots of threads here on CN  telling you how to do that.)

 

Also check that the secondary tube slides out the full length reasonably easily and can be locked into a position that you can focus the eyepiece (The third picture shows an 1.25 Extension tube that you will probably need to permit visual focussing.  That's normal for these scopes).

 

At some point you will need to check the collimation of the mirror, but if the on site test shows a clear terrestrial image of something at least 100' away, you should be fine.

 

Enjoy it !


Edited by Taosmath, 27 May 2024 - 01:00 AM.

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#4 terrain_inconnu

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Posted 27 May 2024 - 01:46 AM

Hey guys,

Pretty much what the topic title says. I’m buying my first telescope and I have the opportunity to get a used 8” dobsonian for a great price. If anyone can tell me red flags I should look out for in a used scope that would be incredible. The guy said he only used it a few times and that he doesn’t know much about the telescope. It looks like a skywatcher 8” flextube but I can’t find any online that have that color of base so any info on that would help too. I have the listing’s photos below!

Astronomics has this exact item on their website (incl. user reviews): https://www.astronom...-dobsonian.html

 

Regarding red flags: as already mentioned check the primary and secondary mirrors (dust is ok on both mirrors, a reasonable amount of pinholes on the primary mirror or even some minor scratches do not noticeable degrade the experience in most cases). Also check that all moving parts are operating as expected (smoothness of alt/az movement and stability once in position, focuser, truss mechanism etc).


Edited by terrain_inconnu, 27 May 2024 - 01:50 AM.

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#5 Ranger Tim

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Posted 27 May 2024 - 07:46 AM

The finder scope is in its holder backwards.


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#6 Echolight

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Posted 27 May 2024 - 07:47 AM

That looks pretty new, so I wouldn't expect any issues.

 

But, you might take a close look at both mirrors to see that there is no damage and that the mirror surface isn't coming off.

Some mirror coatings last for decades. But the coatings can be easily damaged from over aggressive cleaning.


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#7 Echolight

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Posted 27 May 2024 - 07:51 AM

The finder scope is in its holder backwards.

Doesn't look like it to me.

 

Edit:

Oh, I see what you mean now . The adjustment screws should be towards the middle of the finder instead of the end.

 

Easily corrected though. Not a reason to not get the scope.

 

Edit again:

Actually, it looks like the finderscope is missing the objective lens.

 

Still, if the price is right, not a reason to not buy the scope. Finderscopes can be easily replaced.


Edited by Echolight, 27 May 2024 - 08:02 AM.

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#8 Paul Sweeney

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Posted 27 May 2024 - 12:54 PM

Check the base. The base is made of particle board covered with plastic. Particle board will absorb water and expand, breaking open the edges. This does not happen when used normally and stored in a dry place. It will happen if left in a damp cellar or outside for extended periods. So if the base is bloated, the scope has not been properly cared for.
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#9 Vic Menard

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Posted 27 May 2024 - 01:20 PM

...Actually, it looks like the finderscope is missing the objective lens.

The finderscope is definitely messed up--the white mounting part needs to be reversed at the dovetail, and then the (RACI?) star diagonal looks like it needs to be removed from the objective end of the finder scope (hopefully it hasn't damaged anything) and screwed in to the opposite end where it belongs...


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#10 sevenofnine

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Posted 27 May 2024 - 01:36 PM

No bad red flags other than possibly replacing the finder scope that I can see. As others have said just make sure the primary mirror looks very shiny like your bathroom mirror. Ignoring the dust, that easily washes off, your own image should look sharp and detailed. If the mirror looks cloudy or otherwise damaged then the scope is a fail. Sometimes improper storage in a garage can do damage to both mirrors. The outside of the scope looks very good so best of luck to you! borg.gif

 

 https://www.youtube....Y8xFnXFVGQ&t=3s.

 

https://www.youtube....G98RTP6jbY&t=2s.


Edited by sevenofnine, 27 May 2024 - 02:17 PM.

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#11 Chicken

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Posted 28 May 2024 - 11:48 AM

Thanks for the help from everyone, I'm definitely going to try and snag this before anyone else can. Even if I have to replace some things on the scope I think it's well worth it. He's selling it for less than $300 and I guess has no clue what he has or just really has to get rid of it. I'll check out the mirrors as that's the only reason I could imagine someone selling this nice of a scope for such a low price. Given the fact the finder scope is backwards and possibly missing the objective lens I'm assuming he just has no idea what he has.


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#12 Echolight

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Posted 29 May 2024 - 09:14 PM

He probably bought it, used it once or twice, couldn't find anything hubble-like, stuck it in the garage for a couple or five years, and now wants to get rid of it.


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#13 Jon Isaacs

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Posted 30 May 2024 - 07:18 AM

The finderscope is definitely messed up--the white mounting part needs to be reversed at the dovetail, and then the (RACI?) star diagonal looks like it needs to be removed from the objective end of the finder scope (hopefully it hasn't damaged anything) and screwed in to the opposite end where it belongs...

 

For what it's worth:

 

It is possible and even desirable to mount the finder  bracket backwards..  There is a tab on the bracket that mates with the finder shoe.  If the bracket is reversed, that tab is on the top side and prevents the finder from slipping out of the shoe. Stock, the tab is on the bottom side and does not prevent the finder from accidentally falling. 

 

It is best to also reverse the shoe but it's not necessary.

 

It is possible that the finder can be aligned as it is but when I have reversed the bracket, I have removed the objective and mounted it as Vic suggests.  I think it's better to remove the objective rather than the diagonal because the objective cell is metal and it threads onto the metal tube.  The diagonal adapter and the diagonal body are plastic and it's possible to damage the threads.. 

 

It looks like a nice scope, clean.. 

 

Jon


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#14 vtornado

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Posted 30 May 2024 - 10:13 AM

It looks fairly clean.  A lot of dust, you may have to wash the mirrors if the dust was able to infiltrate the tube.

 

check for spider damage to the mirrors.

 

I see what looks like water in the floor next to the scope.  That could cause the base to warp if the base got wet too.  Check that by spinning the the scope 360 degrees and see if there are any big sticking points.  Also watch the base board as it rotates and see if they maintain a fixed distance.  If the base is warped and you have simple carpentry skills you could rebuild it.

 

Water on the floor could also mean a high humidity which could affect the mirror coatings check those.   The mirror should look bright and shiny.  Dust can be removed, don't let that freak you out.  Other discolorations might give me pause.

 

In general the scope looks good.  The above are just things to check.

In my area used good shape dobs go for around 60% of retail, but the price varies alot from "get this junk out of my house" to "I think my used telescope can be sold for retail"


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#15 Echolight

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Posted 30 May 2024 - 10:21 AM

For what it's worth:

 

It is possible and even desirable to mount the finder  bracket backwards..  There is a tab on the bracket that mates with the finder shoe.  If the bracket is reversed, that tab is on the top side and prevents the finder from slipping out of the shoe. Stock, the tab is on the bottom side and does not prevent the finder from accidentally falling. 

 

It is best to also reverse the shoe but it's not necessary.

 

It is possible that the finder can be aligned as it is but when I have reversed the bracket, I have removed the objective and mounted it as Vic suggests.  I think it's better to remove the objective rather than the diagonal because the objective cell is metal and it threads onto the metal tube.  The diagonal adapter and the diagonal body are plastic and it's possible to damage the threads.. 

 

It looks like a nice scope, clean.. 

 

Jon

I think his main point, the one that the op would benefit most from knowing, was that the diagonal is stuck into the dewshield instead of being screwed into the ocular end. laugh.gif


Edited by Echolight, 30 May 2024 - 10:25 AM.

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#16 Chicken

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Posted 31 May 2024 - 11:13 AM

I see what looks like water in the floor next to the scope.  That could cause the base to warp if the base got wet too.  Check that by spinning the the scope 360 degrees and see if there are any big sticking points.  Also watch the base board as it rotates and see if they maintain a fixed distance.  If the base is warped and you have simple carpentry skills you could rebuild it.

 

Water on the floor could also mean a high humidity which could affect the mirror coatings check those.   The mirror should look bright and shiny.  Dust can be removed, don't let that freak you out.  Other discolorations might give me pause.

Honestly the water is what I'm most worried about. I'm hoping I don't find rust and that the scope and base itself did not get wet. I assume that the water is just from snow/ice melting near the door of the shed or storage unit they were keeping it in. For the most part Montana is a pretty dry environment. Even if there was some water on the floor I doubt it would increase the humidity enough in the shed to damage anything (my knowledge about mirror coatings is very limited so I will certainly still look). I've stored books in similar situations to this and they didn't get damaged at all.

 

Another worry I have; could dramatic changes in ambient temperature cause damage to the mirrors? That shed probably isn't temperature controlled and some days we have temperature changes of 40F or more. I know some people store their scopes outside under a cloth cover so I would assume the mirrors are quite resilient to temp changes.



#17 Chicken

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Posted 31 May 2024 - 11:24 AM

He probably bought it, used it once or twice, couldn't find anything hubble-like, stuck it in the garage for a couple or five years, and now wants to get rid of it.

I have a hunch it's something like this. Or he bought the scope, used a couple times, and after sitting in storage or a room for years collecting dust his SO decided it had to go.

 

Lucky for me, even if I could only see white specks in funny shapes I'll still go out every night to look. Just the idea that there are near infinite things like our sun unfathomably far away is enough to keep me interested. Don't even get me started on galaxies, nebulae, the planets and their own moons. Just the concept leaves me awestruck.



#18 Chicken

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Posted 31 May 2024 - 11:48 AM

These are a good scope for a beginner if the price is right and they can take you a LONG way in the hobby.  The one I used had a black base, so don't stress about that.

 

 

Astronomics has this exact item on their website (incl. user reviews): https://www.astronom...-dobsonian.html

This is actually much more of a relief than you would expect. I don't know why I was so worried about the base being a different color. I guess I've been duped by too many fake Chinese look-alikes of quality products.



#19 star acres

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Posted 31 May 2024 - 08:27 PM

The red flags are high shipping costs and guys that

break things and force things. If you can see and touch the telescope, take a cha cha cha chance.


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#20 Echolight

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Posted 31 May 2024 - 09:04 PM

Probably gone by now.



#21 Bearcub

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Posted 02 June 2024 - 07:39 PM

Probably the smartest way into astronomy is buying a used dob. After you find out if you really like looking at stars and stuff you can then understand if you want to go premium and different size dob and so on.. And if you find out sky is not for you then - it was used dob.


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#22 star acres

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Posted 06 June 2024 - 05:53 PM

I use my small Sky Watcher more for terrestrial curiosity than anything else, and I often get to share it. It's not just for looking at your favorite stars.

#23 EricSi

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Posted 07 June 2024 - 12:34 AM

Probably the smartest way into astronomy is buying a used dob. After you find out if you really like looking at stars and stuff you can then understand if you want to go premium and different size dob and so on.. And if you find out sky is not for you then - it was used dob.

Another great way to start is to find a local astronomy club and go to their star parties and get a chance to see different scopes in action without having to pay anything. Also a good way to set realistic expectations of what things look like through a scope vs. long exposure photographs.


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#24 Chicken

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Posted 21 June 2024 - 10:19 AM

Update: I finally got the scope. It took a lot of patience with the guy, and I sometimes had to wait a couple weeks for a response, but it'll almost certainly be worth it. The mirrors look great, the finder scope definitely has some issues, looks like a plastic part was forced on and got stuck because it was skewed. Along with this it's missing the objective lens for sure. Seller knocked the price down a bit for the finder scope issues. Hopefully this weekend I'll collimate and get my first views.


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#25 Chicken

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Posted 21 June 2024 - 10:21 AM

Another great way to start is to find a local astronomy club and go to their star parties and get a chance to see different scopes in action without having to pay anything. Also a good way to set realistic expectations of what things look like through a scope vs. long exposure photographs.

My local astronomy club only meets once a month and their meeting time is while I'm at work :( maybe I should reach out via email to see about star parties though.




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