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Refractors and coma aberration?

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#26 Booster

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Posted 16 September 2020 - 05:13 PM

Thanks guys, I can see someone had exactly the same problem. They replaced a simple, non-adjustable, plastic lens cell with an aluminium one they made on their own. Apparently the issue was gone once they have done it, although doing this is beyond my abilities.

 

The AR-102/1000 seems to have the same kind of lens cell, no adjustment screws so more than likely a retainer ring system. The link in German shows how the whole thing is made out of plastic and more plastic which is great to see. smile.gif

 

Well, it does look like the purchase of my first scope could have gone better. I just want them to take this one back and not sure what to do next. Should I ask for a replacement and get another Bresser surprise? I would have gone for the 102/600 in exchange, but strongly consider spending more money and going with someone else. Saying that everywhere you look the stock is decimated at the moment...

 

Kris


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#27 Redbetter

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Posted 16 September 2020 - 06:26 PM

Kris,

 

Don't give up on it until you have tried adjusting the retainer ring.  Figure out how to get the dew shield out of the way (it will be removable in some fashion).  This will expose the retainer ring. 

 

Loosen the ring.  Point the scope vertical and tap the sides to get the objective settled in, then re-tighten until it is just snug with no rattle.  Take a look through it after that.  If this is simply an element that is loose and out of place, this will clear it up.  If it is something more severe, it will still not give round star diffraction patterns and will need to be returned.

 

These sorts of plastic cells are pretty common on inexpensive refractors.  I've fiddled with about a dozen of them.  What I have learned is that either the objective is good and will "self-align" and center itself properly this way, or the objective is bad and the major observed aberration remains.  Sometimes the spacers appear to be unevenly applied, sometimes the problem seems to be in a particular element.  I've tried some simple internal shimming on others to correct apparent tilt or de-center and even have tried another tube/cell in one case, but have never found that to fix the problems, so in the samples I have I can't say that the cell is to blame.  However a fully collimatable cell would have made it easier to do such adjustments.   


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#28 Booster

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Posted 17 September 2020 - 05:26 AM

Thank you Redbetter, this is some really useful info. I would definitely check the retainer ring if the scope was out of warranty. I will not do it this time in case it causes warranty void, you just never know... I will wait for their response.

 

I have attached a sample of the sound it makes when tilted, it sound like something of a reasonable weight is moving inside. A lens glass? There is another sound which you can hear in the background. This comes from around the focuser, the screws are not loose so maybe a little mouse is living there. smile.gif

 

Kris

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#29 Wildetelescope

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Posted 17 September 2020 - 06:03 AM

Thank you Redbetter, this is some really useful info. I would definitely check the retainer ring if the scope was out of warranty. I will not do it this time in case it causes warranty void, you just never know... I will wait for their response.

 

I have attached a sample of the sound it makes when tilted, it sound like something of a reasonable weight is moving inside. A lens glass? There is another sound which you can hear in the background. This comes from around the focuser, the screws are not loose so maybe a little mouse is living there. smile.gif

 

Kris

Send it back!  Refractors should not rattle!

 

jmd



#30 gnowellsct

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Posted 17 September 2020 - 10:26 AM

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.

This is sort of distressing news.  I wonder how nice the Vixen cells are for example my SD81s.  

 

The thing is when the discussions come up is it "worth it" to spend $2000 on an 80mm when you can get one for under $500 and someone says well, I set up next to such and such and the inexpensive scope did just as good, well that might be true.  But the problem is cheap.  Cheap works until it doesn't.  Which I suppose is true of everything.  

 

But what I'm getting at is that we are sold the illusion of a telescope at these low prices.  The 8x50 $60 finder that I have looks like the $300 Tak 7x50.  

But after using it a year or two the cross hairs pop out and in the field the thing becomes worthless until you take it home and repair it.  Crosshairs don't pop out on Tak finders (I hope).   So I had a perfectly good finder up until the moment it failed.

 

It sounds like these ED doublets are sort of the same thing.  The owner might be vulnerable to failure if the telescope suffers a shock in transport.  And it might even fail out of the box which after all is a transport situation.  It works until it doesn't, sometimes right out of the box, sometimes later.

 

But it's very difficult to upsell a scope by telling people this thing has a wicked good lens cell.

 

Greg N



#31 gnowellsct

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Posted 17 September 2020 - 11:04 AM

Well since you are in London (I sort of envy that) you have access to most of the UK.  This is good because you should buy from a UK vendor that you can go see and physically shake a fist at if they sell you a piece of junk telescope.  But they were trying to send you a message by not carrying this brand to begin with. 

 

ANYHOW, please consider saving your nickels and dimes and getting a Vixen telescope.  You might consider their made in China achromat 105mm

 

https://www.vixenopt...ope-p/26071.htm  

 

I'm suggesting this because of my personal view, not held by everyone, that Japanese companies have an advantage in maintaining quality control of their products coming out of mainland China.  It's easier for them to have someone over there checking the production and if issues arise they are much closer for getting on to the scene to deal with it.   So were I to buy a Chinese refractor I would want it to be a Vixen.  It costs more but as you have discovered their are hidden prices to buying an inexpensive rock bottom scope.   Of course there is some mighty bad Japanese actions in the history in the region but still the economic integration and cultural affinities are there.  

 

You'll get a better lens cell a better focuser and a better tube out of the deal, IMO.  They also have an 80 mm (made in China) and an 81mm (made in Japan) which is apochromatic and more expensive.  Many people here will say that an 81 mm apo will beat the pants off a 102 mm achromat in planet viewing.  I have read it here more than once but not run the test myself.  To do that I would have to buy an achromat (I did own one in the past).

 

I would also recommend, if these prices seem too high, the Vixen R130 SF with mount .   That I think is a really nice beginner's set up because it raises the tube to a more adult friendly height than many of the dobsonians recommended in the beginners' forum.  And it's a 130mm aperture instead of 100 and because it is a mirrored optic the color correction will be terrific.

 

NONETHELESS, this is the refractor section, you seem to like refractors, and your first order was an achromat that fell apart.   So I'm trying to recommend an achromat that won't fall apart and that will do you good service.  For me that would be the Vixen A105M.

 

Greg N 


Edited by gnowellsct, 17 September 2020 - 11:06 AM.


#32 Hesiod

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Posted 17 September 2020 - 12:06 PM

The A105M is made in Japan, as is the 81M.

There are also cheaper models they outsource, which are marked by the letter "f": e.g A80Mf, ED80Sf (I have no proof, but this could be a close kinsman of others 80mm f/7.5 "CHinese" doublets).

The R130Sf is also outsourced.

The only models I could remember that are outsourced but not labeled with the "f" are the VMC95 and 110.

My experience with their outsourced stuff is that should not expect a dramatic upgrade from Skywatcher/etc...as an example I have both the Skywatcher 90/1250 MCT and the VMC95, and both sail the same boat (not an especially nice boat).

 

As for the 105M, it never struck me as an especially good deal. The day I wanted a mid-focus achromat ended with the purchase of the TAL100 which, while not so fancy as the Vixen, is in any case made to last and, unlike the A105M, came with really nice accessories (TAL eyepieces are as good as the Volcano-top orthos, the finderscope is excellent and I prefer it to those of the Taks due to better ergonomics, and even the diagonal is very good, to the point that I keep using it even with other telescopes. To get a meaningful upgrade had to go for the BBHS mirror).

I got mine brand new for ca 300€ (299+shipping) and developed an healthy respect for the brand.

In truth, were to pay over 1000€ for a 100mm, I would look into one of those rebranded "APOs".

 

My first "serious" telescope was a Tecnosky 66/400ED, made by LongPerng and rebranded by a reputable Italian vendor: the reasons behind it being sold at almost one third of the price I paid for the FL55 are quite obvious, but despite a feeling of false opulence, there is nothing really bad about it (I even pried its lens through a Ronchi eyepiece, and the result was a very decent correction of SA, even if nobody could mistake it for a Tak).

When I made the purchase I was just lucky (Tecnosky was the shop closest to my home), but had I asked to more experienced stargazers they would have likely pointed me toward a trusty vendor as the safest course of action. If the vendor selling you the stuff check and allows only to those reaching the threshold of decency to leave the shop, then are somewhat shielded by the worst.

 

Obviously nothing could be done about design issues, and my impression is that a fair share of the difference between the prices of FL55 and 66ED is due to unsavoury savings on mechanical parts, so the fact that am using the 66ED as guidescope and take the picture with the FL55 could suggest something, as the fact that I have still the 66ED after eleven years.



#33 Booster

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Posted 17 September 2020 - 03:11 PM

You are absolutely right. When it comes to a hobby I do not tend to do things on the cheap as the result is always the same. I have been into music for quite some time and still have a pair of studio monitors I bought nearly 20 years ago. Their sound simply can't be matched by anything that is currently being released, full stop. This is just one example, I have other bits of gear (some discontinued) I would not swap for anything else. I have gone for the Bresser scope knowing it would not be the high end of the market, although my expectation was it would provide a reasonable experience for someone who was starting.

 

Lesson learned...

 

I am glad you mentioned the A105M as I have never spotted it at any of the UK retailers. Talking Vixen, I could mainly see the pricey APOs so I did not dig into it. I can now see one retailer I can put an order with:

 

https://www.rotherva...ractor-ota.html

 

The AR-102/1000 I bought only became an option as I could not get the shorter focal length AR-102/600. Have any of you had experience with Altair Starwave ASCENT 102ED?

 

https://www.altairas...cuser-468-p.asp

 

It looks promising and Altair claim it is guaranteed to arrive in perfect collimation.



#34 Booster

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Posted 17 September 2020 - 03:21 PM

By the way... Bresser asked me another question... 'Were the photos were taken through the star diagonal or with the second photographic extension tube inserted?'

 

Clearly trying to make sure it is not my fault that what I can see through the scope is... well... what I can see.

 

I have provided them with all the info I posted in this thread, so... well done Bresser smile.gif



#35 Redbetter

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Posted 17 September 2020 - 03:39 PM

Thank you Redbetter, this is some really useful info. I would definitely check the retainer ring if the scope was out of warranty. I will not do it this time in case it causes warranty void, you just never know... I will wait for their response.

 

I have attached a sample of the sound it makes when tilted, it sound like something of a reasonable weight is moving inside. A lens glass? There is another sound which you can hear in the background. This comes from around the focuser, the screws are not loose so maybe a little mouse is living there. smile.gif

 

Kris

Generally when I have had problems with gear and I might need to do something that would be noticeable, I have asked whether or not the vendor/manufacturer rep was okay with me taking a shot at fixing it before exchanging.  I do this for the reason you mention, and they have said no problem.  This allowed me to fix simple adjustment issues with what turned out to be fine optics.  It is a shame to reject good optics that simply need adjustment.

 

With typical inexpensive achros, there is no need to even ask.  The dew shield on most slip off (they can be extremely tight, but they are just friction fits.)  Once the dew shield is out of the way the retainer ring can be turned.  Usually, these are screw on way too tight (possibly so they don't end up loose and rattling after a long boat ride, then other transport on the way to the customer.)  So I end up loosening them slightly, then checking the image.  With new ones I often do this before I even look through the scope...because when I haven't the optics are usually pinched.  

 

It isn't just refractors.  My Mak was badly out of collimation when it arrived (and folks erroneously believe Maks never need collimation...sheesh.)  My son's Z10 Dob's secondary screws could not be moved...until I got medieval on it with power tools, then modified the secondary holder slightly.  Zhumell gave me $50 credit on the Dob for the trouble, and that thing had a perfect star test!  Both it and the Mak put up fine images.   It is very doubtful I would have gotten optics as good in an exchange...objective figure is still luck of the draw, so I am very glad I adjusted them before doing a knee-jerk return.  


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#36 Redbetter

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Posted 17 September 2020 - 03:44 PM

By the way... Bresser asked me another question... 'Were the photos were taken through the star diagonal or with the second photographic extension tube inserted?'

 

Clearly trying to make sure it is not my fault that what I can see through the scope is... well... what I can see.

 

I have provided them with all the info I posted in this thread, so... well done Bresser smile.gif

No, they asked because many folks do not realize that the diagonal itself can be the problem.  Cheap diagonals are frequently badly misaligned (or astigmatic.)  The average newbie taking images through an inexpensive refractor has little idea what they are doing or is unaware that could be the case.  You had already accounted for this, but most do not, so it is best to ask this question.  It is Troubleshooting 101.



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Posted 27 October 2020 - 09:17 AM

I finally replaced the faulty scope and this time went with a different vendor. I have to say I had to add quite a bit to the Bresser refund, but wooow, what a difference! I went for Altair Starwave 102ED-R FPL53, I am honestly really happy with it, it should serve me for years. I would like to thank you for all your help again guys! smile.gif

 

Kris

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#38 Haydon

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Posted 28 October 2020 - 04:10 PM

I had this same issue on a nice scope once.  It was the result of a de-centered element.  Coma in one direction all across the field.  A grossly miscollimated objective In a refractor results in astigmatism, not coma I believe.  



#39 FedericoB

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Posted 20 November 2020 - 03:45 AM

Hello, I just happened to land on this recent thread trying to understand if doublet refractors suffer from intrinsic coma or I happened to have bought a telescope with misfigured lenses.

My setups is a Kasai Trading Blanca 72SED which is a 72mm F6 doublet with an FPL-53 ED element (pretty sure same OEM as TS optics).

In order to get into astrophotography, I also bought the William optics FLAT-6AIII. Which depending on the adjustment distance you can change the level of correction.

I've been trying to carefully adjust the distance between the camera and the flattener; however for the life of me I am unable to get round stars in the corners. The center seems somewhat fine.

I can't even get round stars when using an APS-C sensor.

I have attached a photo I took using a full frame camera and you can see the aberration.

I'm curious if it's this coma you guys are talking about or an unflat field.

By the way in order to find the optimal adjustment distance, I used a Bahtinov mask to find the distance at which the center and corners show the focused pattern. I hope this method is okay.
Even after doing that the non round stars persist.
I also tried a whole range of distances and was unable to achieve pinpoint stars.

Finally, during a star test the on axis star image is composed of concentric circles so I do not think it is miscollimation of either the lens cell or the focuser.


Thanks for reading!

BEwVF4D.jpg

Edited by FedericoB, 20 November 2020 - 04:14 AM.


#40 dan_h

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Posted 20 November 2020 - 10:44 PM

FredericoB,

 

Your pic does not show the coma that this thread has been discussing. To me it looks like your pic indicates a tilt in the camera alignment. The full left side and right side have issues but the center is good all the way top to bottom. 

I don't do AP so I am not good to judge this. Have you tried a post with your question down in the Astro Photo forums? I am sure someone there has experience to shed light on your problem. 

 

dan



#41 FedericoB

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Posted 21 November 2020 - 02:12 AM

Thank you very much for your help! 

Maybe I should try to post this there, but I was worried about double posting and this seemed like recent enough thread that I could get help on.

 

I do wonder if it's tilt tho, since it feels like camera tilt should import non-symmetric aberrations on the image, I can't imaging the angle that would correspond to the results I have.

 

Also would flat frames be able to show sensor tilt? 
I have taken those and they are actually quite smooth, you can even see it in the above picture since it is not flat corrected.

 

Thanks for your help again!



#42 SeattleScott

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Posted 21 November 2020 - 02:40 AM

I agree it don’t look like no coma I’ve ever seen. That being said I can’t say what it is.

#43 FedericoB

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Posted 21 November 2020 - 05:41 AM

I've been trying to get rid of it for so long... tried different cameras and all sorts of distance adjustments.

I have a feeling after reading this thread and finding on twitter another folk with a similar issue that maybe it's just bad optics.

 

 

Which sucks because the on-axis star image is very round and with concentric circles when defocused. Maybe a little bit of spherical aberration.

 

Then off-axis the stars look pretty bad even in APS-C format...



#44 Booster

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Posted 21 November 2020 - 05:05 PM

Federico, the Bresser scope I returned had pinched optics, lens cell elements (most likely the lens glass) were loose. I did not write about it, but I got to the bottom of it just before sending it back. I tilted it down, heard a knock originating from somewhere around the lens cell, then very gently mounted the scope and pointed it up (without hearing the knock again) and the stars looked sharp when focused, suddenly no coma-like tails.

 

I saw your picture and these coma-like tails seem to be pointing away from the center of the image while in my case they were pointing in the same direction (left to wright) across the whole field. You seem to be having a different kind of issue. Does it look the same if you use an eyepiece or only the camera and the flattener setup? I could see it using either.



#45 FedericoB

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Posted 23 November 2020 - 06:30 PM

Booster, thanks for the advice!.

 

So what I ended up doing was to take a photo with a full frame DSLR and look at the edges.

The pictures were taken during the day of the suns' reflections onto some electric insulators that were 500m away (1/3 of a mile).

 

The images were taken without the flattener.

 

I focused the image at two points, either the center of the image or the edge of the image.

 

I assume that if the main aberration of a refractor is field curvature then it stands to reason that when I focus the image at the edge then the stars should look round.

 

That's not what I am finding... the stars have a very strange shape, it does not look like any of the aberrations that I've read about... 

 

Here are the corner and center images:HJHe0zm.png

 

 

If you guys agree that this is definitely something more than just field curvature then I would like to give up on this telescope.

 

Thanks again guys.


Edited by FedericoB, 23 November 2020 - 06:31 PM.



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