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BB Reviews the Bresser Messier AR-102L

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#101 PatrickVt

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Posted 14 April 2019 - 10:50 AM

I have to admit that I also have difficulty understanding the purpose of DPAC bars especially since it is difficult to tell if straight lines going through a circle are truly straight.  

 

That being said, I've been following along anyway because I've always been interested in finding a long focal length refractor.  It is nice to know a few are becoming available.

 

What would be more effective for me to check the quality of views through that scope would be to see images of various targets captured through the scope.  The lunar views, above, look good for a cheap achro even if they are showing some blown highlights (which has nothing to do with the scope) so that is good news for a cheap scope. 

 

As far as all the info pertaining to DPAC bars, I don't understand the purpose nor importance of the DPAC bars so that is a bit wasted on me.  I can make some guesses on how these DPAC bars could be used in calibration of gross distortion though. 

 

I do appreciate, however, the effort that anyone puts into any sort of review and I appreciate the effort here.

 

Patrick


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#102 Bomber Bob

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Posted 14 April 2019 - 10:53 AM

No mention of the obvious undercorrection...

 

From Post #13:

 

Star Test:  Lens is slightly under-corrected.

 

From Post #14:

 

Your startest confirms the same as I've observed with my AR90L. Slightly uncorrected but a nice!y performing lens nonetheless.

 

------------------------------------------

 

I discussed it again in Post #16. 

 

In Post #25, I described that 1st test, and showed sample photos of the patterns I look for, as I'm aware not everyone has a DPAC rig and/or has used one to test telescope lenses.

 

It appears to me that your criticisms of the content / flow of the thread are subjective rather than objective.  You're free to buy an AR-102L, and create your own thread based on your own standards.


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#103 Bomber Bob

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Posted 14 April 2019 - 10:58 AM

I don't understand the purpose nor importance of the DPAC bars so that is a bit wasted on me.

 

I post the DPAC results because it's an objective measurement versus my star / sky reports.  For this thread, the fact that DPAC confirmed my star test results means I don't have to repeat these tests.

 

I will make Jupiter images with the Bresser when the skies cooperate, and y'all can compare those with my other 4" refractors.


Edited by Bomber Bob, 14 April 2019 - 11:00 AM.


#104 memento

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Posted 14 April 2019 - 11:45 AM

Terra, I've seen that from Xavier, too.  In fact, I bought this set from Xavier, and it included 4 .965" color filters, and the 35mm to 1.25" adapter. I've also read what I posted on another vendor's site.  What I haven't found - yet - is authentic spectros documentation on these eyepieces.

Maybe ask the guys who made them. You can apparently still get new old stock ones here:

 

https://www.haag-str...te/astro-optik/

 

(It says: We still have eyepieces and other accessories in stock, various focal lengths of the types Kellner and Plössl, some secondary mirrors, various color filters in 24.5 and 35mm diameter.)


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#105 Bomber Bob

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Posted 14 April 2019 - 11:52 AM

Thanks Thomas!  I sent them a message via their web form, and attached a photo of my set.


Edited by Bomber Bob, 14 April 2019 - 11:56 AM.

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#106 memento

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Posted 14 April 2019 - 12:10 PM

As to the scope that's being reviewed here, I noticed there were some discussions when these Bressers hit the market originally, and always there were issues with the lens cell and – as BB explicitly says – especially with the too-flimsy retaining ring and that ill-fated non-circumferential threading.

 

However, I also notice that once these issues have been dealt with, the Bresser 102L seems to be capable of very nice high-power views. What BB describes on the Moon, sounds very promising to me.

 

I am still curious how false color is compared to other Achromats. My personal experience is that I don't bother about any false color in a 60mm f/15 and also an 80mm f/15. I do find it a bit disturbing in my 90mm f/11 sometimes, but still like the pinpoint sharp stars and also the crisp lunar details in this scope a lot. Huygens eyepieces are much better with achromats as they seem to cancel out some of the false color of an Achromat, but I don't have many quality ones yet.

 

Theoretical calculations tell me that the 102mm f/10 should be considerably worse than the 90mm f/11, while a 102mm f/13.4 should be very similar.

 

So my current impression is I wouldn't want to buy the Bresser

(1) cause I also dislike the flimsy lens cell. I repeat again I would not mind if the focuser would be much simpler. What's the point of that rotating feature and generally more elaborate design at this price point? Especially if it comes at the expense of the much more important lens cell.

 

(2) in my opinion the false color is maybe just a bit too much (if what BB said so far is true, that it kind of matches the 4" f/10). I'd rather want a 90mm f/14 then, or jump to an ED scope / other scope design instead. Maybe the Bresser 90/1200 would be more "my" purchase. It's even more cheap and maybe even better in color correction. (t's maybe too close to my 90/1000.) The problem is that the Bresser is no classic. You love classics because of all kinds of other reasons than just their "perfect" performance. The Bresser, though, is a new scope. I see that with different eyes.

 

(3) I'm not sure if buying a classic achromat means more hassle than buying this thing brand new, if it still involves all that fuzz about sorting out the lens cell and collimation before you can put it up to good use. But, YMMV. It surely is nice that Bresser cares for that niche despite all these issues, and let's hope they maybe get some stuff right if they ever happen to modify their offerings.

 

I fully understand that mirror optics can also be excellent, I have (had) some myself. But that's an entirely different discussion which is off-topic here.


Edited by memento, 14 April 2019 - 12:16 PM.

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#107 AstroKerr

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Posted 14 April 2019 - 08:56 PM

Whatever. Have it your way. To me, that's an optimistic description that soft-shoes the issues. No mention of the obvious undercorrection, which is terminology most here can relate to, as compared to "bloated DPAC bars" which goes right past the reader, including me...

I don't think it's a matter of 'have it your way' - he's giving a very thorough review. I have one - it's nice, it ain't a Tak, etc. etc. If 'bloated DPAC bars' goes right past you, then read more, ask more questions - it's what I've had to do. What's with the 'demands' on a reviewer anyways? A review is a gift, for goodness sake, a gift from someone who knows their way round scopes fairly well in this case. I bought one, Bob bought one, if you want to have an opinion on the bloody thing or write a 'better' review, then by all means purchase one and do so - gee whiz! But really, demands/dissatisfaction on/with the review - I'm childish enough to just say 'Oh, go have a piece of fruit..."

 

My opinion about things I don't own or use - like when I say "Questar slag" or some other witty phrase - is just rubbish, yes? It should be understood when seen that it is rubbish based on nothing - I know that when I post it and I expect everyone should know I am speaking out of my tailpipe or in jest - that's what an 'opinion from a distance' is - rubbish, at least mine are grin.gif . A thorough, thoughtful review is not that - so you don't nitpick "I'd review differently" , you don't ballyhoo other things "who would buy one of these when they could makmakmak" - you read the review, digest it, and ask questions and so forth - but you do not kick at the reviewer.

 

I've got my AR102L on mount and pointed at the moon, just to spite all the "I don't have one, but..." naysayers who will not and cannot enjoy the views (with monor flaws) I shall. 


Edited by AstroKerr, 14 April 2019 - 10:13 PM.

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#108 barbie

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Posted 14 April 2019 - 09:37 PM

After testing my Bresser AR90L, I have no plans to do anything but observe with it!!  No plans to take it apart, do any flocking or modifications, etc.  For what I paid for it, it performs very well and I'm quite happy with it the way it is.  If I were Bresser, I would at least change the plastic finder stalk to a metal one because there is quite a bit of flexure present with the stock unit but other than that, I have no quibbles with the scope and its overall performance so far.  As for further testing, I'll have to wait for the planets to make themselves available in the evening sky to get a completely accurate assessment on the scope's total performance but from what I observed on the moon at high power the other night, I don't think there will be any issues.


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#109 precaud

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Posted 14 April 2019 - 09:53 PM

I don't think it's a matter of 'have it your way' - he's giving a very thorough review. I have one - it's nice, it ain't a Tak, etc. etc. If 'bloated DPAC bars' goes right past you, then read more, ask more questions - it's what I've had to do. What's with the 'demands' on a reviewer anyways? A review is a gift, for goodness sake, a gift from someone who knows their way round scopes fairly well in this case. I bought one, Bob bought one, if you want to have an opinion on the bloody thing or write a 'better' review, then by all means purchase one and do so - gee whiz! But really, demands/dissatisfaction on/with the review - I'm childish enough to just say 'Oh, go have a piece of fruit..."

 

Opinions about things I don't own or use - like when I say "Questar slag" or some other witty phrase - is just rubbish, yes? It should be understood when seen that it is rubbish based on nothing - I know that when I post it and I expect everyone should know I am speaking out of my tailpipe or in jest - that's what an opinion from a distance is - rubbish, at least mine are grin.gif . A thorough, thoughtful review is not that - so you don't nitpick, you don't ballyhoo other things "who would buy one of these when they could makmakmak" or "I'd review differently" - you read the review, digest it, and ask questions and so forth - but you do not kick at the reviewer

 

I didn't "kick the reviewer"; I thanked him. But had issues with some things.

 

When I was a kid, I had to eat everything that was put on my plate, and act like was good, whether I liked it or not.

 

I'm not a kid anymore.


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#110 AstroKerr

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Posted 14 April 2019 - 11:43 PM

Neither are any of us - and I do love you dearly =) but this isn't a plate of food we're being force fed - this is a freebie courtesy of someone's considerable effort and it should be appreciated. Bob does wonderful reviews, as do many of the longer term CNers - they go the extra mile for us, they draw upon considerable experience. My cataract inhibited reviews are left eye only til late next week, and I tend to keep it / them short and quick - devoid of the fine detail or real experiential depth we find here. Because of Bob and the DPACers, I ordered the materials to setup my own DPAC and FL tester - because of the quality of their reviews and the info they've shared, it inspired me to do better, try to be more factual & pictorial - to give back a little of what I've received from all of them. Bob, in this review, said everything I didn't and said it better than I would have - these folks are just 'good' - and I just don't want this marvelous well, or any of the reviewer wells, to dry up. I worry when even the equipment donaters don't get a thank you and the reviewers get mucked with. 

 

meh. Clear Skies, Sharp Eyes and Sorry if I'm pissy - I'll be in 7DTD setting up a new outpost bunker. 


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#111 AstroKerr

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Posted 15 April 2019 - 01:37 AM

After testing my Bresser AR90L, I have no plans to do anything but observe with it!!  No plans to take it apart, do any flocking or modifications, etc.  For what I paid for it, it performs very well and I'm quite happy with it the way it is.  If I were Bresser, I would at least change the plastic finder stalk to a metal one because there is quite a bit of flexure present with the stock unit but other than that, I have no quibbles with the scope and its overall performance so far.  As for further testing, I'll have to wait for the planets to make themselves available in the evening sky to get a completely accurate assessment on the scope's total performance but from what I observed on the moon at high power the other night, I don't think there will be any issues.

Agreed =) Tempted to get a 90 just because. These are Blueberry Peaches for the price~!

 

Here's a pic representing the CA - this one was fom the BLU studio 5.0 with generic adapter w/ ~10mm lens: 

 

AR102L_ph_10mm.jpg

 

I can't get a better pic at the moment to show the CA exactly as I see it, but to clarify - the thin blue-ish line is phone/low-end adapter 10mm EP artifact, because I don't see any CA (visually) @ 6~10mm with good EPs, and if it shows up under 5mm visually it is whispy violet, and just a bit denser / less whispy when I've the 2.3 ~ 4mm EPs in. It's worse by a factor of 1~2 perhaps with the included diagonal. Any yellow is as Bob reported.

The Celes zoom shows that line or slightly less in purple @ 8mm. Nothing on the Baader zoom @ 8mm

The Baader Hyps and Celes Xcel LXs - that line doesn't show at all with the good EPs until under 5mm.

The view in general is crisp and contrasty, of quality along the lines Bob, Barbie, Thomas and others have put forth; it's got a 'wide' or 'slow' focus, but it does pop.

If you ignore the cost of the diagonal and finder provided and load the cost on the OTA alone, well, what a peach for ~$300. And that's as delivered, without tweaking the objective (which might be worth the effort). I've got a Crescent 60/910 that has less colour, and the SV80 of course. Gol-ly Ma! 

 

Took the Apogee 150 out for 'compare' - lotsa yellow, hardly any purple - once you leave the limb, it's okay-ish 'juicy kiwi' =) makes the AR102L seem 'colourless'.


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#112 Magnetic Field

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Posted 15 April 2019 - 02:10 AM

I don't understand the purpose nor importance of the DPAC bars so that is a bit wasted on me.

 

I post the DPAC results because it's an objective measurement versus my star / sky reports.  For this thread, the fact that DPAC confirmed my star test results means I don't have to repeat these tests.

 

I will make Jupiter images with the Bresser when the skies cooperate, and y'all can compare those with my other 4" refractors.

I pretty much disagree. The DPAC test is practically useless and doesn't give you numbers. Also when someone says "the optics is undercorrected according to the star test" it means nothing. If you see someone on the street who is overweight all you know is he is overweight but you don't know by how much.

 

I don't use the intra- and extra focal star test any more (it cannot be used except for rough collimation issues). All that counts it the focused star and even then you haven't got numbers. You have got a f/14 achromat and better colour correction but if memory serves me right it was nearly impossible to do the star test with my Vixen 4" f/10 achromat because on one side of the focus the image was very blurred (due to chromatism) and I was never able to easily see the diffraction rings (never tried to use filters though).

 

In the end an interferometer test would tell you the true story.



#113 Magnetic Field

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Posted 15 April 2019 - 02:17 AM

Agreed =) Tempted to get a 90 just because. These are Blueberry Peaches for the price~!

 

Here's a pic representing the CA - this one was fom the BLU studio 5.0 with generic adapter w/ ~10mm lens: 

 

AR102L_ph_10mm.jpg

 

I can't get a better pic at the moment to show the CA exactly as I see it, but to clarify - the thin blue-ish line is phone/low-end adapter 10mm EP artifact, because I don't see any CA (visually) @ 6~10mm with good EPs, and if it shows up under 5mm visually it is whispy violet, and just a bit denser / less whispy when I've the 2.3 ~ 4mm EPs in. It's worse by a factor of 1~2 perhaps with the included diagonal. Any yellow is as Bob reported.

The Celes zoom shows that line or slightly less in purple @ 8mm. Nothing on the Baader zoom @ 8mm

The Baader Hyps and Celes Xcel LXs - that line doesn't show at all with the good EPs until under 5mm.

The view in general is crisp and contrasty, of quality along the lines Bob, Barbie, Thomas and others have put forth; it's got a 'wide' or 'slow' focus, but it does pop.

If you ignore the cost of the diagonal and finder provided and load the cost on the OTA alone, well, what a peach for ~$300. And that's as delivered, without tweaking the objective (which might be worth the effort). I've got a Crescent 60/910 that has less colour, and the SV80 of course. Gol-ly Ma! 

 

Took the Apogee 150 out for 'compare' - lotsa yellow, hardly any purple - once you leave the limb, it's okay-ish 'juicy kiwi' =) makes the AR102L seem 'colourless'.

I have got a 5 mm Sky-Watcher TMB type planetary eyepiece.

 

It doesn't shoe a blue fringe around objects though but it shows a very noticeable blue fringe around the edge of the eyepiece (similar to your moon photo but around the edge of the field of view). By visual comparison my 6 mm Vixen NPL shows none.

 

Does you eyepiece also shoe a blue fringe around the edge of the eyepiece.


Edited by Magnetic Field, 15 April 2019 - 02:18 AM.


#114 memento

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Posted 15 April 2019 - 05:11 AM

I watched the Moon yesterday with my 90mm f/11 (=comparable false color to 102mm f/13) refractor but had some issues with bad seeing (out of the city). The blue/purple color fringe around the limb was obvious at all magnifications (I tried various eyepieces from 20mm to 7.5mm). But in moments of steady seeing it was much reduced. Same goes for some false color that showed in dark shadows of craters close to the terminator – the better the seeing, the less I saw them. Again, my H.M. eyepieces reduced the false color significantly, still I preferred the view through my Plössl and Baader Eudiascopics yesterday.

I have the impression that it's hard to really "nail down" how much false color you see, as it is a function of not only the scope, the individual eyes and the eyepieces, but then also even of the seeing, the positioning of your eye in front of the eyepiece, etc.

 

The bottom line though is that if I were not looking for false color, I'd pretty much forget about it after a few seconds and just indulge in all those beautifully crisp and contrasty details of the craters and formations on the Moon ... if I wouldn't feel and see that way, I'd not have an achromat at all :)

As I am a lover of simple eyepiece designs, I am very attracted by the fact that an f/14 refractor would allow me to go to .7mm exit pupil with a 10mm eyepiece, the Bresser f/13's would then need a 9mm and my own f/11 needs a 7.5mm. The difference in eye relief between a 9/10mm or a 7.5mm Ortho or Plössl is maybe just a millimeter but still it is very noticeable.


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#115 precaud

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Posted 15 April 2019 - 07:40 AM

Neither are any of us -  but this isn't a plate of food we're being force fed - this is a freebie courtesy of someone's considerable effort and it should be appreciated.

 

I think you're making too much of this. My point was; we're adults, and we can express our differences respectfully and without trashing each other. As far as I can see, that's what has happened here.

 

I ordered the materials to setup my own DPAC and FL tester - because of the quality of their reviews and the info they've shared, it inspired me to do better, try to be more factual & pictorial

 

That's a good move. I built my DPAC several years ago and it is an amazing tool to have access to. A lie detector for optical systems. Setting it up accurately and photographing the results is a challenge, as we can see in the low-quality images in this thread. If you've seen any of the ones I've posted, you'd know that I solved those problems and they are of excellent quality. And so I offered advice to the reviewer to improve his. Twice. That's what we do here to be helpful, right? But no response. Now that's bad manners...

 

The DPAC test is practically useless and doesn't give you numbers.

 

I completely disagree. One doesn't need numbers to make a good qualitative assessment. I'd much rather have a set of good DPAC photos than one number like Strehl, which is way too simplistic. If you've ever used one, you'd know that DPAC is an extremely sensitive tool, and we amateurs are fortunate to have access to it. I look forward to interferometers being as accessible and inexpensive to put together. But I'm not holding my breath.


Edited by precaud, 15 April 2019 - 04:39 PM.

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#116 precaud

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Posted 15 April 2019 - 08:33 AM

After testing my Bresser AR90L, I have no plans to do anything but observe with it!!  No plans to take it apart, do any flocking or modifications, etc.  For what I paid for it, it performs very well and I'm quite happy with it the way it is.  If I were Bresser, I would at least change the plastic finder stalk to a metal one because there is quite a bit of flexure present with the stock unit but other than that, I have no quibbles with the scope and its overall performance so far.  As for further testing, I'll have to wait for the planets to make themselves available in the evening sky to get a completely accurate assessment on the scope's total performance but from what I observed on the moon at high power the other night, I don't think there will be any issues.

 

Barbie, if you ever decide to open the cell of your AR90L, it would be of interest to see if it was a Baker or a Fraunhofer design...



#117 Magnetic Field

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Posted 15 April 2019 - 09:04 AM

 

 

 

That's a good move. I built my DPAC several years ago and it is an amazing tool to have access to. A lie detector for optical systems. Setting it up accurately and photographing the results is a challenge, as we can see in the low-quality images in this thread. If you've seen any of the ones I've posted, you'd know that I solved those problems and they are of excellent quality. And so I offered advice to the reviewer to improve his. Twice. That's what we do here to be helpful, right? But no response. Now that's bad manners...

 

 

I completely disagree. One doesn't need numbers to make a good qualitative assessment. I'd much rather have a set of good DPAC photos than one number like Strehl, which is way too simplistic. If you've ever used one, you'd know that DPAC an extremely sensitive tool, and we amateurs are fortunate to have access to it. I look forward to interferometers being as accessible and inexpensive to put together. But I'm not holding my breath.

(Not sure if this "no response" was directed to me).

 

Anyway, I see it like this: how would you feel or trust your doctor if you developed cancer and the doctor would base his cancer treatment solely on looking at a computer aided tomographic image of your tumor. Yes of course testing telescope optics will have no fatalities and no one outside of cloudynights cares.

 

There is definitely a correlation between good optics and DPAC tests (e.g. Takahashi optics always look very good and one could assume you get what you pay for) but that's it. No one here knows how the refractor of the OP would perform in the interferomtric operating theatre. Good luck with your star test and DPAC test to make any meaningful prior propositions with numbers attached that will later be confirmed by the interferometer test.

 

Edit: Lets do an experiment here

 

I think the OP mentioned the optics is undercorrected. Maybe he feels confident to give a ptv/rms number and posts it here. Also if someone feels brave enough to give a number to the DPAC test and posts it. We then would need to find an optics tester who would be willing to interferometrically test the telescope. Also the interferometer test would reveal how close the 4" f/14 actually is to a semi-apo.


Edited by Magnetic Field, 15 April 2019 - 09:12 AM.


#118 precaud

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Posted 15 April 2019 - 09:34 AM

(Not sure if this "no response" was directed to me).

 

It was directed toward the post quoted above it.

 

    Edit: Lets do an experiment here

    I think the OP mentioned the optics is undercorrected. Maybe he feels confident to give a ptv/rms number and posts it here. Also if someone feels brave enough to give a number to the DPAC test and posts it. We then would need to find an optics tester who would be willing to interferometrically test the telescope. Also the interferometer test would reveal how close the 4" f/14 actually is to a semi-apo.

 

Unless you're offering to fund said testing, I think you're being WAY too demanding. And I'd go so far as to say, you'd be better off dropping it and learning more about DPAC, because chances are very good you'll be seeing more of them around here. It's something that is within the reach of us amateurs, whereas interferometers are not at present. Or maybe you could lead the way and build an IF and offer said service to your fellow stargazers. At no charge, of course.



#119 terraclarke

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Posted 15 April 2019 - 09:49 AM

Edit: Lets do an experiment here
 
I think the OP mentioned the optics is undercorrected. Maybe he feels confident to give a ptv/rms number and posts it here. Also if someone feels brave enough to give a number to the DPAC test and posts it. We then would need to find an optics tester who would be willing to interferometrically test the telescope. Also the interferometer test would reveal how close the 4" f/14 actually is to a semi-apo.


Estimate a PVT/RMS number from the ‘data’ presented here? Really! That would indeed be a rather painful exercise. One needs to realize the the DPAC test images presented here occur in ONLY two posts. The post (#25), titled The First DPAC Test shows two images of a rather good test! The problem is that NEITHER of these two images are from the scope features in this thread! One is from a Takahashi FC50, the other from an early Japanese 60mm Fraunhofer refractor. I can’t for the life of me understand why they are even included? Even tho, when one reads the ‘fine print’ the provenance of the tests is in fact indicated, for the person skimming the thread, they give a rather erroneous impression that they are derived from the Bresser objective. It’s a ‘bait and switch’ argument all to familiar in today’s politics.

The only other DPAC images in the post (#25), titled The Second DPAC Test. These are indeed derived from the Bresser objective’s testing. They are pretty dreadful, blame on the camera withstanding (both the images, and presumably the test results):

https://www.cloudyni...102l/?p=9286563

(One has to wonder why the camera images in #25 are good and the camera is performing so badly in the later post’s images?)

So, with regards to the question you posed, do you really want (or need) to go there? I for one do not, but, as they say, one person’s poison is another’s pudding .

Edited by terraclarke, 15 April 2019 - 09:57 AM.

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#120 Magnetic Field

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Posted 15 April 2019 - 10:11 AM

Estimate a PTV/RMS number from the ‘data’ presented here? Really! That would indeed be a rather painful exercise. One needs to realize the the DPAC test images presented here occur in ONLY two posts. The post titled The First DPAC Test shows two images of a rather good test! The problem is that NEITHER of these two images are from the scope features in this thread! One is from a Takahashi FC50, the other from an early Japanese 60mm Fraunhofer refractor. I can’t for the life of me understand why they are even included? Even tho, when one reads the ‘fine print’ the provenance of the tests is in fact indicated, for the person skimming the thread, they give a rather erroneous impression that they are derived from the Bresser objective. It’s a ‘bait and switch’ argument all to familiar in today’s politics.

The only other DPAC images are indeed derived from the Bresser objective’s testing. They are pretty dreadful, (both the images, and presumably the test results):

https://www.cloudyni...102l/?p=9286563

So, with regards to the question you posed, do you really want (or need) to go there? I for one do not, but, as they say, one person’s poison is another’s pudding .

I have seen mediocre RONCHI images but the corresponding interferometer test wasn't that bad (way better than the diffraction limited threshold often cited). I cannot find that link anymore but there is  German site which is quite revealing (they listed dozens or even hundreds of test reports with RONCHI gratings and interferometer tests etc).

 

I would even go so far as to pitch the Bresser against a 5" MCT in an interferometer test. It would really be interesting to see what comes out at the other end in terms of MTF. The implicit assumption for most: a refractor does not suffer from the effect of obstruction and is a-priori better; disowning the fact that a $300 achromat will most likely suffer from spherical aberrations of all kinds and  sphero-chromatism in one form or the other.



#121 Magnetic Field

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Posted 15 April 2019 - 10:20 AM

It was directed toward the post quoted above it.

 

 

Unless you're offering to fund said testing, I think you're being WAY too demanding. And I'd go so far as to say, you'd be better off dropping it and learning more about DPAC, because chances are very good you'll be seeing more of them around here. It's something that is within the reach of us amateurs, whereas interferometers are not at present. Or maybe you could lead the way and build an IF and offer said service to your fellow stargazers. At no charge, of course.

Croud funding (would have to be a tester close to where the OP lives).

 

All that we know know: DPAC is the way to go although no one knows if DPAC can give us a number or anything else in terms of how to gauge the performance of a telescope.

 

You can use a DPAC test to cross-check and cross-evaluate a telescope a-posteriori once it has undergone an interferometric test.

 

On a slightly related note:

 

I personally lost faith in the Suiter star test (caveat: I haven't bought the second edition). So can anyone please tell me what it means in practical terms if as the OP stated the optics is undercorrected?


Edited by Magnetic Field, 15 April 2019 - 10:21 AM.


#122 precaud

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Posted 15 April 2019 - 10:21 AM

This is a false equivalence. DPAC (Double-Pass AutoCollimation) is not the same as Ronchi. You can use a Ronchi grating with DPAC to double its sensitivity. But they are not equivalent.



#123 precaud

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Posted 15 April 2019 - 10:24 AM

Croud funding (would have to be a tester close to where the OP lives).

 

All that we know know: DPAC is the way to go although no one knows if DPAC can give us a number or anything else in terms of how to gauge the performance of a telescope.

 

You can use a DPAC test to cross-check and cross-evaluate a telescope a-posteriori once it has undergone an interferometric test.

 

On a slightly related note:

 

I personally lost faith in the Suiter star test (caveat: I haven't bought the second edition). So can anyone please tell me what it means in practical terms if as the OP stated the optics is undercorrected?

 

We're dragging this thread way off topic now. I suggest you post your questions in the ATM forum, where the guys who understand DPAC in depth hang out.



#124 Bomber Bob

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Posted 15 April 2019 - 10:24 AM

Terra, fine print?  Really?

 

Here's the separate caption for the first sample photo:  "I use 4-5 bars to check for edge curvature, and 3 bars to check for central bowing / curving of the bars.  Like this one from my Takahashi FC-50:"

 

emboldened & color-coded the scope name to make it stand out

 

IF readers are skimming that much, then they may as well ignore the thread entirely.


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#125 Bomber Bob

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Posted 15 April 2019 - 10:54 AM

Way back in my USAF days, I lived in the small town outside the base.  The wife of the local newspaper's owner did a weekly "restaurant review" column... in a small town... with very few "fancy" restaurants.  She actually reviewed the local Dairy Queen.  That's right, the DQ.

 

This Bresser is the DQ of refractors.  If anyone expects haute cuisine from a $250 scope, they're gonna go away disappointed.  Might I recommend the Takahashi, the TEC, or the AP reviews...  You can be as nit-noid as you like, and you'll be in very good company.

 

For the Burger Lovers:  Is the Bresser Special edible?   That's what I'm reporting on.


Edited by Bomber Bob, 15 April 2019 - 11:04 AM.

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