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Refractors and coma aberration?

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#1 Booster

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Posted 14 September 2020 - 12:06 PM

Hi, 

 

So I bought my first telescope a few weeks ago and seem to be having a slight issue. Stars and planets appear to have a comet shaped tail to an extent that trying to get a good view of any planet is rather pointless. I did some reading and followed all the advice I found. I used the telescope on different nights with seeing ranging from regular to very good and good transparency getting the same result every time. My eyes do not seem to be at fault as my wife can see exactly the same thing and the comet shaped tails (all facing in the same direction) are visible through different eyepieces (Bresser, Vixen).

 

I then attached a dslr to take a picture and find out what the result would get using prime focus and no diagonal / eyepiece. It came out the same. There is no difference whether I use an eyepiece or a camera, the view goes in and out of focus when adjusting it, thought the tails always remain. Please forgive me the artistic value of the pictures, they could be sharper. lol.gif  They should though give a rough idea of what I am trying to describe.

 

Is this a common thing you get with refractors? It looks similar to coma aberration, or maybe even is. From what I have read reflectors are the ones more prone to this kind of aberration. Could there be something wrong with the OTA? The telescope in question is Bresser Messier AR-102/1000.

 

Thanks a lot!

 

 

 

 

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#2 MikiSJ

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Posted 14 September 2020 - 12:45 PM

I bought a ZWO guide scope a couple of months ago and it had such bad coma I immediately sent it back. This was with $70 refractor and I suspect it might have been a very simple convex lens.

 

If your coma radiates from the center you are stuck with a bad set of lenses. If it is in one direction, left to right or up to down, then you likely have a collimation problem.

 

Is this something you can send back for a refund?



#3 junomike

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Posted 14 September 2020 - 12:50 PM

Looks to be grossly out of collimation.


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#4 sg6

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Posted 14 September 2020 - 12:51 PM

For some reason would have thought coma was radial, not in one direction.

Brings up ideas of astigmatism or tilt.

 

If the scope follows my 102 then it came with 2 extension tubes - are you using 1 of them?

Thought being the focuser can still manage to deliver an in focus image if wound fully out and the diagonal and eyepiece weight is causing tilt.

 

Other is - are there screws to retain the lens?

Thought here is that the lens have moved and need releasing and allowed to come to rest in their correct position. Likely best done with the scope vertical.

 

If the lens is out of collimation possible then returning it is an option. Especially as on mine there is no screws that I can get at. Not sure if the front objective is separated or cemented. Would expect cemented.


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#5 gnowellsct

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Posted 14 September 2020 - 01:27 PM

That's not coma.  That my friends is collimation.  It's either just out of collimation or there is something bad that jarred the cell, which is a more-than-collimation issue.  It needs to go back whence it came.  That is not going to be fixed by waiting for thermals to go away.  That is not "a slight issue."  That is ghastly.  The scope is worthless until fixed or replaced.  It might even be a simple fix for them that has the knowin'.  

 

You can tell it's collimation because of the comet like appearance.  You can attempt to collimate it yourself if it is out of warranty, but for me personally, I don't have the best capabilities when it comes to using screws with glass.

 

As I posted recently in EQUIPMENT, I have been pretty content with my 8x50 Meade finder scope which I bought because I wanted a finder with rear focus.  It goes for about sixty bucks.  The Tak finders that I have go new for about $300 (mine are used).  Different finders on different rigs.

 

Anyhow I've had a couple of those Tak finders for 10+ years.  The Meade I've had maybe, I dunno, somewhere between two and four.  So this past weekend I heard it rattling when I was out in the field and I couldn't see through it, it was 70% vignetted.   Next day I take it out and open it up.  The lenses are OK, I was worried the internal one had broken.  What was rattling around was a little module with the crosshairs in it.   The modul is just pressure fitted into a recessed area.  What was going on was that the pressure was no longer enough.  Who knows why.  I took some aluminum foil, wrapped it around the module, squished the whole thing in to the recessed area, and there you go, a fixed $60 finder.

 

The greater lesson here is the difference between expensive and cheap--at least one hopes.  Cheap works right up until it figures out a way not to (sometimes out of the box!).  Durable gear is--durable.  I don't think I'll be finding crosshairs rattling around in the OTA of my Tak finders.   I could be wrong.  I've been wrong before.  I have a Meade 8x50, blue, made in Taiwan back around 2000.  I don't use it because no rear focus.  It was cheap, and it is a tank.  So durability is not always related to cost.  But it often is, and as you climb up the ladder of refractors, the lens cells get more robust and accurate.  The idea is not to fly to Zanzibar for an eclipse and find your refractor lenses are dislodged.

 

Does it matter? most of the time probably not.  But I could have lost a night to that malfunction with the finder (it didn't matter, because we were soon clouded out in any case).  Where it can be a problem is if you pack this scope up for a long vacation driving over sometimes rough roads, or for airline travel, hoping to catch some views at the destination.  There you might have a problem.  Or not.  It will work until it doesn't.

 

Greg N


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#6 Redbetter

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Posted 14 September 2020 - 01:42 PM

Since you see it without the diagonal, this is in the optics.  Either it is somehow tilted, or wedged in the cell, or there is something major wrong with the figure of one of the elements or they are decentered to one another. 

 

I am not familiar with how these refractors' objective cells work, but if you are lucky you might be able to loosen the retainer while pointed vertical, then tap the side to get things centered and settled into lace before retightening the retainer to just barely snug.  If that doesn't do the trick and make the star test nice and uniform with no flaring to one side or the other, then send it back as defective.  Don't hang onto it thinking you can tweak it to make it right.  I have held onto a few achros trying to tweak milder problems, and regretted not returning ones that didn't clear up with the simple adjustment described above.


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#7 markb

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Posted 14 September 2020 - 01:43 PM

+1 (really +10) on sending it back to get a replacement, if you bought it new.

 

I should have done this with my first purchased new telescope nearly 30 years ago but didn't because I was not yet experienced enough to understand the irreperable defect in manufacture. So it has lived the last 30 years in my closet.

 

I would only attempt to fix things that are long out of warranty. If bought used, you're stuck with either attempting to resolve it yourself, or returning it as unusable to the seller if the defect was not disclosed presale.

 

That assumes of course that you are capable of fixing things like this yourself, my guess would be the 70% of the folks here could not make such a repair on a refractor, 95% if it did not come with a three screw collimatable cell. My closet queen it will be given one last attempt at usefulness after I move and can install a meniscus bought a decade ago, and after I can 3D print a cell after I relocate next month.

 

Otherwise it would have gone out to the curb with the rest of the trash when I cleared my house 2 months ago.

 

Good luck.


Edited by markb, 14 September 2020 - 01:46 PM.

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#8 Hesiod

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Posted 14 September 2020 - 01:53 PM

That's a very interesting topic as it tears the veil about a very common misconception.
There are tons of topics about people messing with the collimation of their reflectors from the very moment the box landed at home, but very few about someone questioning the notion that his refractor came all the way from the other side of the globe perfectly collimated.
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#9 Star_Shooter

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Posted 14 September 2020 - 02:15 PM

I assume this is an air spaced doublet. On axis coma means one of the element is wedged or not centered. It is NOT because of the mechanical mis-alignment between objective and eyepiece. The problem lies within the two elements of the objective itself. I would return it if possible.  

 

Lei


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#10 gnowellsct

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Posted 14 September 2020 - 02:29 PM

That's a very interesting topic as it tears the veil about a very common misconception.
There are tons of topics about people messing with the collimation of their reflectors from the very moment the box landed at home, but very few about someone questioning the notion that his refractor came all the way from the other side of the globe perfectly collimated.

Refractors are supposed to be good at going half way round the world and staying collimated.  That is sort of the point.  One of them anyhow.  My refractor that came from Poland was and is collimated.

 

It might be different.  They could just ship the refractor with collimation instructions.  But adjusting collimation on screws that push a glass lens is difficult compared to adjusting tip and tilt on the mirrors in a Newt, or the secondary of an SCT.  There is much greater probability of messing things up.

 

I do like the idea of giving the thing a whack and seeing if the lens resets, but I'm not sure I would do it or recommend it.  I would do it before tossing my out-of-warranty Bresser.  But not my newly arrived Bresser.

 

 

 

Greg N


Edited by gnowellsct, 14 September 2020 - 02:29 PM.


#11 Booster

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Posted 14 September 2020 - 03:02 PM

Guys,

 

Thanks you for your responses, really appreciate it. I have not done anything about it yet, as I thought it could have something to do with the quality of air in London, light pollution etc. I have used it in a reasonably dark place, though I have not taken it out of town yet. I have just been outside, best conditions in over a month, the issue still there.

 

Cosmos, it came with 2 extension rings. I have been using the shorter one with a diagonal and both with the camera to get it into focus. I don't seem to be able to see any screws around the lens itself (please see the picture).

 

Greg's viewfinder story has but a smile on my face. Guess why... The viewfinder which came with this telescope was also rattling, it came from somewhere around the lens. The lens itself would also fall out of the viewfinder. I did not make much of it as I thought I would just glue it again and it would make a trick (yes, it was only held by glue...). When I decided to do it I realised the crosshair was gone, so the manufacturer sent me a replacement viewfinder. This time all good.

 

What I found just now though while having a look around the lens was the damage to the rim of the dew shield. It's on the second picture. It looks like it was covered using some touch up furniture chip repair kit, pro job. smile.gif  It is really close to the lens. If the tube got a hit after the lens had been fitted, then I guess we may be looking at the answer.

 

Honestly, I think they just sold me some b-grade scope without even telling me. Not looking too good on Bresser, bashed up scope and disintegrating viewfinder. Well, I will give them a call tomorrow, I will ask for a replacement.

 

Guys, again thank you very much for your help, I will soon let you know how it all went.

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#12 junomike

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Posted 14 September 2020 - 03:08 PM

Few thoughts:

 

1) Send it back

2) Get a replacement elsewhere

3) Avoid the first Vendor at all costs.


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#13 gnowellsct

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Posted 14 September 2020 - 03:13 PM

Few thoughts:

 

1) Send it back

2) Get a replacement elsewhere

3) Avoid the first Vendor at all costs.

Yeah yeah yeah.  And he's a moderator.  You HAVE to do what he says.  GN


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#14 gnowellsct

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Posted 14 September 2020 - 03:17 PM

+1 (really +10) on sending it back to get a replacement, if you bought it new.

 

I should have done this with my first purchased new telescope nearly 30 years ago but didn't because I was not yet experienced enough to understand the irreperable defect in manufacture. So it has lived the last 30 years in my closet.

 

 

 

Good luck.

This gets me so frustrated.  OEM ships total junk and user thinks it is his fault for "not understanding" and that's the end of the person's interest.  

 

A mom came to a star party with her daughter some ten years ago and asked us to show her how to use their 4.5 inch Newt (not in itself a bad thing) on some kind of hideous imitation of a GEM.  It took TWO grown men with years of experience with scopes to get that thing pointed at the Double Cluster in Perseus.   

 

It was virtually unusable.  I wish there were enforceable laws about this stuff.

 

Greg N


Edited by gnowellsct, 14 September 2020 - 03:18 PM.

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#15 Hesiod

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Posted 14 September 2020 - 03:28 PM

Refractors are supposed to be good at going half way round the world and staying collimated.  That is sort of the point.  One of them anyhow.  My refractor that came from Poland was and is collimated.

 

That may, or may not, happen; however this is not the point. Many kind of telescopes are supposed to stay collimated, but I suspect that the habit of checking the refractor's collimation is not a very common one.

The last three refractors I received were indeed properly collimated after a lot of travel and, I believe, a lot of abuses from the day they left the factory somewhere in Japan, but nonetheless felt convenient to put the trusty Cheshire eyepiece in their focusers before anything else.

What could be done after this check is wholly another matter, sadly sending the refractor back being the most convenient choice.

Interestingly the TAL 100 (which I had to send back due to major damages to the focuser) came with suggestions about the collimation: but as can be clearly seen here I am not a native English speaker, and the ersatz English spoken at TAL was too much for me to understand, beside the fact that apparently the manual suggested to verify the collimation of the refractor and then use it to collimate the diagonal as well



#16 Booster

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Posted 14 September 2020 - 03:28 PM

I don't think I have a choice of a vendor. The telescope was shipped directly from Bresser in Germany, I can confirm it as per DHL tracking information. Vast majority of UK retailers do not stock Bresser scopes/mounts etc. and place an order with Bresser directly. This then gets sent out to the customer. I had the same thing with the mount even though I bought it through a different retailer. It was sent from Bresser in Germany.

 

Makes you wonder, huh?



#17 gnowellsct

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Posted 14 September 2020 - 03:35 PM

I don't think I have a choice of a vendor. The telescope was shipped directly from Bresser in Germany, I can confirm it as per DHL tracking information. Vast majority of UK retailers do not stock Bresser scopes/mounts etc. and place an order with Bresser directly. This then gets sent out to the customer. I had the same thing with the mount even though I bought it through a different retailer. It was sent from Bresser in Germany.

 

Makes you wonder, huh?

Yup.  Them thar vendors know something you don't.  Or rather didn't.  Greg N



#18 Star_Shooter

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Posted 14 September 2020 - 03:58 PM

I would return it and choose some different brands, maybe a skywatcher 80ED, never heard that scope had a lens problem.

Lei

#19 sixela

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Posted 14 September 2020 - 04:21 PM

That's not coma. 

It is coma, but not collimation. Usually it's one of the lenses in a doublet that slid and is now at an angle with the other. I've fixed that on a number of doublets (all out of warranty). The hard part is that usually when you have this, extracting the lenses out of the cell might be hard (especially if you don't want to chip them).


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#20 sixela

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Posted 14 September 2020 - 04:23 PM

I would return it and choose some different brands, maybe a skywatcher 80ED, never heard that scope had a lens problem.

Lei

I hate to break it to you, but I've already had to pry open three ED80 doublets with a misaligned crown element; the lens cell and spacer are not exactly high tech in these. Granted, some of the owners might not have been extremely careful with them -- the owner that asked me to star test them was usually not the first owner anyway.

 

And yes, many vendors do not verify the state the refractors are in, and you get them just like they were when they were put in a box in China, plus whatever nasy changes that happened in transport.

 

Some companies do verify the refractor internal collimation before shipping scopes to you (and these also offer maintenance on refractors). They aren't the cheapest one.


Edited by sixela, 14 September 2020 - 04:27 PM.

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#21 Star_Shooter

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Posted 14 September 2020 - 04:26 PM

Wow, they really got abused, I mean, used.

Lei

#22 Uwe Pilz

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Posted 15 September 2020 - 12:28 PM

This decolimation is so awesome that I think it could be a displaced distance platelet. As you can see there are three sucht platelets between the lenses. If one of them slips out you get such an effect. In this case, it is not the fault of the sender (ind a strong sense), because it could had been ok before.



#23 Booster

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Posted 16 September 2020 - 03:10 PM

Bresser asked me to take some pictures of the focuser for investigation. When I was taking the scope out of the box I tilted it up and down. I then noticed a slight knock coming out of where the optics is located, the sound repeats every time I tilt the scope up and down (45 degrees in each direction from horizontal). It's not the dew shield, I checked, it's coming from inside the tube.

 

I guess one of the elements is loose and it is something I should not be able to hear, am I right?

 

Thanks!

 

Kris



#24 SeattleScott

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Posted 16 September 2020 - 03:56 PM

Correct, that is NOT something you should hear!

Had that issue once. Fortunately just a washer that came loose and was rattling around between the baffles.

Scott

#25 Redbetter

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Posted 16 September 2020 - 04:06 PM

If it rattling it is probably a loose element (or elements) that is tilting and causing this.  I have seen that with smaller achros with the simple screw-in retainer ring.  Many folks say to leave the ring just a little loose, but I have found that leads to miscollimation over time.  I find a little snug works better.  There is a compromise to be had between a somewhat snug fit with no rattle and good collimation, versus being so tight that astigmatism or triangular pinch is seen in the star test.

 

The question is how is the cell constructed on this scope?  Is it a retainer ring system, and/or does it have adjustment screws that are hidden by the dew shield?  A discussion of the AR-102/1000 is here:  link.  Within is a description of a similar problem that links to a longer tube version and how they addressed it (in German.)  Link.  I haven't tried to translate/read the German but their are some images that might prove helpful.  


Edited by Redbetter, 16 September 2020 - 04:07 PM.



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