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Vixen VMC200L

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#26 bottlecrusher

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Posted 02 January 2016 - 05:40 PM

A reflex site is definitely on the list, thanks.  And thanks for the link to the bag.  The Tal is cool design.  Any thoughts on a low power eyepiece for a scope of this FL?



#27 iKMN

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Posted 03 January 2016 - 12:31 AM

Nice scope bottlecrusher.  I am jealous. ;)   I do have a C8 and I love it but I suffer from battles with dew and I do have heater and shroud but still the dew.  Always I been interested in this alternative design from Vixen glad to hear you like it and looking forward to more reports.  Enjoy the new scope you got a deal.  cheers

 

k


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#28 samuelpkco

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Posted 06 January 2016 - 11:23 PM

I have a VC200L and use it primarily for DSO imaging. For visual use, however, it is not ideal due to contrast depletion with the OC. But, its weakness in visual comes ALSO from 3 other things:

1) thick vanes causing unnecessary diffraction further reducing contrast;

2) inadequate stray light control within the OTA, baffle tube;

3) less than perfect collimation.

 

My experience with the VC200L suggests that when these factors are rectified accordingly, the well made optics CAN perform beyond what most people would expect. Inherent strengths of the VC200L include:

- having a flat field,

- well corrected aberration,

- oversize baffle tube permitting use of eg N31T5 without vignetting,

- fast cool down,

- nearly dew free,

- no mirror shift,

- light weight compact for 200/1800.....

 

Therefore, in 2012, I felt that it could also be a good visual instrument IF I could take steps to tackle the 3 weaknesses above.

 

1) spider vanes milled down from 5.5mm to 1.5mm

image.jpg

 

image of NGC253 showing stars are rounder and tighter: (Nikon D810 urban sky)

image.jpg

 

2) 100% flocking with Protostar non-adhesive material, all internal surfaces along light path to the eyepiece

image.jpg

 

3) ensure dead on collimation and finally verified by a star test, fine tune if necessary

image.jpg

 

image.jpg

 

It's a lot of work, yes. Most people would perhaps prefer getting a better scope or an APO to save the trouble. But to me, going through the modification also taught me a lot. And in the end, I am able to enjoy the inherent design benefits of the VC200L and also getting the best out of what it can delivery. I really feel that its optics are visually superb, except for hardcore planetary observations. I know, love could be bias in the eyes of a beholder! ; )

 

Clear skies!

Samuel


Edited by samuelpkco, 07 January 2016 - 02:18 AM.

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#29 fred1871

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Posted 07 January 2016 - 01:42 AM

Regarding the VC 200L modifications noted above and the resulting improvements - it's well to remember that the VMC 200L has a different optical configuration and some mechanical details, such as spider arms, are different too. So the VMC version would need to be assessed and modified to take the significant differences into account.


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#30 bottlecrusher

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Posted 07 January 2016 - 10:03 AM

Samuel,

 

That is an excellent writeup on the scope and its advantages and shortcomings and those are exactly the reasons I got interested in the VMC aside from the ridiculously good deal I got on it.  The VC is supposed to be a little better corrected and that VMC, but I have not noticed any difference in correction to full aperature Maks side by side aside from the diffraction spikes you well covered.  I would love to see more pictures of the spider vane and CO assembly when its off the scope.  The spiders look very similar on the VMC vs the VC.  The optical difference between the two is one is a field Mak (VMC) and the other is a Klevtsov (VC), but it sure seems like the spider assembly is shared and from a manufacturer's standpoint that would make sense.  I think you could water jet out this part and with a little machining to get a curved spider vane to eliminate the diffraction spikes altogether like on the TAL series of subaperature Klevtsovs.  That is probably what Vixen should have done initially and their smaller VMCs have curved spiders. 

 

Also could you please share information on the material used to line the light path and how big an improvement it made?  I'm very interested in making that improvement. 

 

I've had it out 4 nights now and I believe the collmination to be in order.  It has been very enjoyable to use and I'm for sure having better deep sky results than the 4" TMB that I owned previously.  No dew problems, but I have noticed some stray light on bright objects.  Thanks for sharing. 



#31 Andreas-TAL

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Posted 08 January 2016 - 01:23 PM

I own a TAL-250K telescope and the curved spiders are a really gain, despite they are rather thick. Their thickness is needed due to the rather heavy corrector unit of the 10" Klevtsov. But only around the brightest stars about 0 mag. (Vega, Capella ...) a very faint "halo" (created by the curved spiders) could be seen - under darkest skies. In general, the star images of the TAL-250K are pinpoint, crisp and "refractor like".

 

One difference between the TAL-250K and the Vixen VMC is that the corrector unit of the TAL has a two element design (the backside of the meniscus lens has a reflective coating) and (as far as I know) the VMC 260 corrector uses three elements (the third element is an additional mirror). Due to this the TAL-250K might have a little benefit in thermal stabilization of the corrector unit, but on the other side the VMC has (as far as I know) a dielectric coating which is (if it's a high precision coating all over the visual spectrum) a valuable und expensive surplus.

 

image.jpeg

 

But what I'm writing for is another topic:
I had the chance to get some calculations investigating the optical behavior of a TAL-250K and its optical performance in relation to different aberrations. One result was, that the design is very sensitive to a lateral offset of the corrector unit. As far as I remember even 0.1mm lateral offset decreases the performance dramatically. Depending from the other aberrations even below diffraction limited performance. I think that's the reason why no lateral offset adjustments are provided from TAL. Having the possibility to do this, many of the users might "disimprove" their telescope to a better umbrella stand.

 

The tube itself (made of 3mm aluminium) is turned on a lathe (inside with numerous small grooves to prevent light scatter) and the ring of the curved spider is rotation symmetric (turned on a lathe) too. Due to this manufacturing procedures the mechanical tolerances are still minimized. Afterwards the best possible optical correction is achieved by minimal lateral shiftings of the mangin lens under interferometric control. Reaching the optimum the corrector unit of the TAL-250K is finally sealed and it's highly recommended not to open it.

 

I don't know if the Vixen VMC has the same optomechanical requirements, but their optical designs are definitely related. Therefore they might behave "in the same direction". I think you'll need narrow tolerances by replacing a "VMC spider vain system" with a "curved spider system" to avoid offsetting the corrector unit (in relation to its "original" position) and an interferometric control afterwards might be highly recommend, too.

 

Andreas


Edited by Andreas-TAL, 09 January 2016 - 02:53 AM.

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#32 samuelpkco

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Posted 08 January 2016 - 08:54 PM

Brian,

sorry for the delayed reply.



1) Vanes
Reduction of the vanes to 1.5mm was the thinnest I would go WITHOUT weakening structure. In the VC, and VCM even more so, I think rigidity in secondary mirror support is crucial. The VC's spherical secondary is less sensitive to lateral displacement than the VCM which employs a corrector in front of the secondary. So my feeling is that you might be safer to just keep using the 4-spoke vane, reduce it, than go about making a curved one for the VCM. I can see you want to get rid of the diffraction spikes but you will need to keep the secondary PRECISELY dead center in the VCM. Making a curve vanes might be a real engineering challenge to ensure that. In fact, when i was putting back the spider vane onto the VC, I was so careful to tighten the 4 screws one by one progressively so as try not to throw it off center by any small amount. A bit paranoidal in the VC case, but your VCM's secondary unit is more sensitive so maybe you need to take this into account.

 

In my VC, the 1.5mm vanes result in much tighter stars and way shorter diffraction spikes BOTH visually and photographically so I m happy with it. I also find a little spikes help in confirming focus. As to adopting a curved vane, I doubt if it could improve overall contrast apart from hiding the spikes. Diffraction is still there, only being smeared out. 

 

As others have mentioned above, inherent design differences, especially in the secondary mirror/corrector, between the VC & VCM might need to be noted and carefully thought through. But, I think the spider vanes of VC200L & VCM200L are identical parts.

 

2) Collimation setup
It had been a real challenge for me. The VC has collimatable focuser, secondary and primary. So I choose to use laser method to get to a reasonable mechanical AND optical alignment first and then star test to fine tune for ultimate precision. The setup you see in the picture was a Hotech:
http://www.hotechusa...tegory-s/23.htm (it is designed for a usual SCT however, not the Vixens)
But I DIY a bench to put OTA at one end and the Hotech target board on the other, both ends are position-adjustable to facilitate quick and precise alignment between the OTA and the laser board. If using a tripod for the board and OTA on a mount as suggested by Hotech, it is workable but might take lots of patience and time comparing to the DIY bench I use. I made the bench only because I was collimating other people's VC through a dealer here.

 

i) It is a very important foundation to the whole process to exactly align the Hotech target board with the OTA. I put a circular grating on the 2"Glatter to shine a circular pattern onto the Hotech target board. Adjust to square the Hotech target board with the OTA by making the patterns centering on the board.

image.jpg

 

ii) To square the focuser, I remove the secondary unit (not the vanes). Put a 2" Glatter laser in the eyepiece position to shine through the spider center hole. A DIY crosshairs is placed at the hole. This ensure the focuser is mechanically aligned with the OTA. This is my starting point in collimation to square the focuser first before aligning the rest of the optics.

image.jpg

 

iii) Then to align the primary, with the secondary still removed, I cover the center laser on the Hotech board withsemi-transparent material. Adjust the primary to center the cross-pattern and circular pattern ouncing back from the primary on the target board.

image.jpg

 

Up to now, the focuser and the primary are mechanically aligned with the OTA tube.

 

iv) Put back the secondary onto the spider. Remove the Glatter laser. Collimate the secondary mirror as per Hotech's instruction.

image.jpg

image.jpg

 

The focuser, secondary & primary should be in close-to-perfectly alignment with each other AND the OTA.

 

v) Do a star test. Adjust only the secondary for any final correction. In my experience, only very minor adjustments are needed if i) to iv) above were done with care.

 

3) Flocking liner
http://www.fpi-proto.../flockboard.htm
This is without adhesive backing. I would like to avoid putting any adhesive inside an OTA so no chance of gassing. It's also cleaner easier to flock. This material is optically flat at glazing angle. It also has less material-come-off than a normal felt.
My other post here:
http://www.cloudynig...ticularly-flat/
Installation is just about cutting the right size and curl it up insider the OTA, baffle tube, draw tube... etc. A bit of measuring and patience to obtain the right sizes (internal diameter of the tubes) so that the tight circumferential fit would secure the liner in place. 

The resulting contrast improvement is subtle but nevertheless significant to experienced observers especially under light polluted situations. 

 

An extra bonus of flocking the VC/VCM OTA, I find, is that if you cut the liner that lengthwise making a tight compression fit internally, it helps with the rigidity of the entire OTA thus reducing chance of flexure which some say a potential problem of the Vixens. However, I don't experience flexure from the VC OTA even imaging at 1800mm with 4.8um pixels. I have seen such problem with one other VC and it was due to screws on both ends of the OTA not being tighten properly after the owner took it apart.

 


Clear skies,

Samuel

 

 

In a general sense, while I am sharing my rewarding experience in taking a scope apart, it would be more responsible to also mention that HORRIBLE THINGS DID HAPPEN I have seen in some case people open up their scopes!! In one instance a VC came in for collimation with owner's complaint being image blur. After collimation it was still giving a blurry image. A lot of fault finding was done to reveal that one lens in the 3-element corrector was installed front side back! Obviously this suggested someone had taken the scope apart right down to elements of the corrector and did not pay attention to the configuration. In fact as part of a prudential practice, rotational orientation of all optics should also be marked before disassembling as in some designs these are also optimized in the factory.

So rather than seemingly advocate anyone doing it, I would also say proceed with caution. If you are not 100% know what you are doing , please DON'T.


Edited by samuelpkco, 10 January 2016 - 01:13 AM.

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#33 fred1871

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Posted 08 January 2016 - 09:02 PM

Andreas, some useful thoughts there. Good to hear from you again. It seems to be difficult to get much information about Vixen optical systems and how sensitive they might be to miscollimation. 

 

I've realised since writing my recent not here that the VMC name is used for differing optical designs. The diagrams for the VMC 200L show a 2-element secondary, a mirror plus a meniscus lens, so it's different from the VMC 260L which has 2 lenses in the secondary as well as a mirror. And the larger 260 model has different support vanes - they're thin from the front, but wide in section.

 

The secondary housing of the VMC 260 is large - not so much in diameter, at about 33%, but long, at near 10cm long. This results in a large thermal mass in the light path, certainly bigger in proportion than the one in the VMC 200. The result is that cooling of the secondary assembly in the 260 will be slower than in the VMC 200. From experience with a 260, the cooling is a major problem for achieving thermal stability, with the resulting difficulty of thermal interference within the light path, made worse by the closed bottom end of the tube. 

 

The VMC 200 should be less affected than the VMC 260 in this respect, but it might still in some environments (where the temperatures are not falling slowly at night) have problems with tube currents from the secondary assembly which is larger and more complex than in an SCT, DK, or Newtonian. So, you gain compared to an SCT by not having dewing problems, but in some environments you lose because of more significant thermals/tube current problems.



#34 bottlecrusher

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Posted 08 January 2016 - 11:27 PM

Those are all interesting thoughts. I have access to a Zeiss measuring machine and Catia software which is like solid works bigger brother.  Pretty sure they could design and machine a part better than Vixen did.  I think I would have to take it off to install the flocking so it would be ready to get looked at by the ZMM.  

 

The VMC 200 is a field mak with secondary mirror and corrector between primary and secondary.  My understanding is the Klevtsov with a mangin mirror has a better potential for correction, but in terms of off set of the secondary and how it would degrade the performance I have no idea but obviously it would hurt.  I appreciate everyone's assistance and input.

 

I've had it out 4 or 5 times now and just got today a ES 40mm eyepiece for low magnification viewing.  The Orion Nebula was very nice and then I moved to sigma Orionis it still showed four stars at the low magnification.  Pretty soon clouds rolled in.  My views of Comet Catalina the other morning were good as well.  For what I paid and as mobile as it is for an 8" scope I'm very pleased with the purchase.  I treat it like a grab and go scope, leaving it setup and bringing it in and out ready to go.  


Edited by bottlecrusher, 08 January 2016 - 11:29 PM.


#35 Andreas-TAL

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Posted 09 January 2016 - 10:09 AM

Yes, it looks like that Vixen VMC design has different optical schemes, depending from the aperture of the telescope. The 200mm VMC might have a two element corrector unit, the 260mm VMC should have three elements. Don't know why, cause very few information to the optic design is available.

 

The mangin lens in the TAL-250K offers the ability for correcting aberrations with this optical element too (spherical aberration and longitudinal chromatism). But the radiuses of both lenses (meniscus, mangin) are very ambitious and their tolerances are narrow. It's a delicate manufacturing process and it may take a long time to find a good performing TAL-250K, which is not suffering from severe spherical aberration, the most common error of the TAL-Klevtsovs.

 

I add a study from Yuri A. Klevtsov calculating the best design of curved spiders. Each element in the light path will cause diffraction images. Straight lined spiders (mostly used in Newtonian telescopes) create the well known "crosses" at bright stars (with some esthetic appeal - for many of us). Curved spiders will do the same but they create a halo (a circular, faint and dimmish "lucency")  around a bright star. But not every curved spider produces a "perfect" halo. Depending from the position, angle, curvature, radius the halo may have blurs, asymmetric brightness, spots or other unesthetic shapes.

 

curved spider.jpg

 

Unfortunately the technical drawing isn't dimensioned, so you have to adapt the sketch to your needs manually (they may work as a rough guide). The drawings itself are explained by Yuri A. Klevtsov, but I can't speak or read Russian language (may be someone here will translate the explanation). Of course the technical drawing is related to the TAL-Klevtsov design, but this shouldn´t matter (may be the specific obstruction (=green) of the telescope doesn´t fit perfectly, but that´s easy to adjust).

 

BTW (1): Fred, nice to hear ("read" :) ) from you again. How about your VMC?

 

BTW (2): TAL deciced to realize the three-armed spider vains. The four-armed study should be used with the planned 12"-14" TAL-Klevtsov telescope. It was never manufactured.


Edited by Andreas-TAL, 09 January 2016 - 01:03 PM.

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#36 samuelpkco

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Posted 10 January 2016 - 01:30 AM

I have corrected some errors which I made in the collimation steps writeup.

 

The reason I explained the setup I used step by step was to share the methodology that worked for me. By no mean it was the only correct way to go about it. The DIY collimation bench was also for my convenience. I guess anyone can do without it by using just lasers in a similar way as I did. Maybe someone will come up with a better method than mine. I used mine only because it was very repeatable with consistent result.

 

Wishing Best of luck to all owners of VCM & VC scopes. I hope you enjoy reading the methodology and principles I am sharing, and someone will even improve on that or suggest alternatives.

 

Clear skies!

Samuel



#37 jjack's

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Posted 10 January 2016 - 06:14 AM

A reflex site is definitely on the list, thanks.  And thanks for the link to the bag.  The Tal is cool design.  Any thoughts on a low power eyepiece for a scope of this FL?

Hi Bottlecrusher. The VMC 200L i have seems to have the same aberrations than a SC8. Field curvature and coma.

I don't know about other eyepieces but with my nagler 4.8, 7, 9T6, 13T6, LV 25, i see the field sharp to the edge.

The televue 32 plossl is 95% sharp.

Not so with the panoptic 19 and 24 but good enough you forget quickly the problem.

same way with my UWAN 28.

Overall i saw better edge correction with eyepieces with integraed Barlow (Smith) lens curved convex.

For example : the UWAN 16 with a concav Smith lens is not very good. But i never tried the Nagler 16 or 20...so...?

With the reducer corrector, the field is very flat and well corrected @ F6 and not vigneted like the SC8 and is own reducer, but it's only a photo tool.



#38 jjack's

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Posted 10 January 2016 - 06:24 AM

In fact, all my vixen LV are sharp to the edge ( 6, 9, 15, 25). But they have 45-50° field only. I use the televue's more often.

I collimate it with my eye only and have never got a problem. the only thing to take care : Don't tighten a lot the primary Mirror push-pull screws or you can get astigmatism.


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#39 fred1871

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Posted 11 January 2016 - 04:55 AM

The VMC is an interesting design, but after spending quite a lot of time with one (the larger 260) I'm completely unconvinced that it's a step forward compared to the more usual designs. Wolfgang Rohr's reports on a couple of VMC 200L that he tested are not encouraging. A test of a VMC 260L that Andreas provided in a thread here about a year ago was also discouraging. The example that I have under-performs significantly for a telescope of that size. It's at its best used at lower powers for sight-seeing - plenty of light, good contrast, but not good definition and thermals very hard to tame. I'd not recommend it for planets or double stars.

 

The 200 version, having a less massive secondary, and overall less material to act as a heat sink, might do comparatively better. At least if you're lucky enough to get one that's optically good and accurately assembled (the examples tested by Rohr were not like that). However, personally I'd suggest a good SCT or Mak or DK instead. The choice of which is better for you will be based on the kind of observing you'll be doing. Horses for courses.


Edited by fred1871, 11 January 2016 - 07:05 AM.


#40 bottlecrusher

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Posted 11 January 2016 - 12:34 PM

I looked up Wolfgang Rohr's report and I did some star testing of mine and saw nothing as bad as he saw in the tests I found.  If I read it right he did say once collminated it wasn't bad.  Again, my intention was for a lightweight, simple, compact, easy to travel setup and I got it for 600 bucks with a GP2 mount.  No doubt that a good SCT, Mak, or DK would outperform.  I do think I will flock it, grind the vanes, use the ZMM to examine the spider assembly and check collmination more accurately.  I like DSOs and have to travel to a dark site to really do well so this works pretty good for me.



#41 Andreas-TAL

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Posted 11 January 2016 - 03:18 PM

I just scanned the articles from W.Rohr concerning the VMC telescopes (this language is a little bit easier for me).  ;)

 

Yes, he wrote that the VMC/VC he got were slightly miscollimated. The VMC telescopes are (in general) better performing compared with the VC telescopes, both suffering from undercorrection and the VMC is at its best at a back focus of 135mm (or a little bit more). He noticed a big influence of thermal issues to the optical performance, writing that in evening the telescope was perfectly collimated (at the optical bench in the basement) and the other morning (with higher temperatures) there was a significant astigmatism.

One further thing he supposed is that the optical system is sensible to "change of position".

 

I think the VMC200L you described is really a very good performing individual scope, fortunately having none of the above  mentioned issues (or only in a negligible dimension). Fred wrote about his VMC260 - unfortunately it's a world of difference.


Edited by Andreas-TAL, 11 January 2016 - 03:19 PM.


#42 samuelpkco

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Posted 12 January 2016 - 08:11 PM

This is a post on the VMC200L but I guess my sharing on experience gained from owning a VC200L could be of some use. Both Vixen designs are equally controversial and share some similarities.

Sorry bottlecrusher, hope you didn't mind. ;)

 

I can confirm, as long time user of a VC200L, that ever so slight collimation error degrades the image especially in terms of contrast, introduce astigmatism and field curvature. Very VERY collimation sensitive than most other CAT designs! Also sensitive to tube current when not in thermal equilibrium.

 

Regarding undercorrection/overcorrection of the VC200L, it is important to keep the primary secondary mirror separation at 450mm or some residual spherical aberration is evidential. It is difficult to measure the separation for adjustment, however. The alternative is to insert a Vixen NLV4 or NLV5 into the Vixen flip-diagonal straight through, focus at infinite, and when the separation distance is at 450mm the draw-tube should rack out 13.4mm.

 

On a recent night with average seeing, I put side by side the perfectly collimated VC200L( Delos10, 180x) with the Esprit 120ED(Delos 4.5, 187x) aiming both on Jupiter. While the Esprit gave a more contrasty overall feeling, the VC put a smile on my face when I saw Jupiter in the Delos10. They were that close!

 

image.jpg

 

image.jpg

 

IMHO, a less than perfectly collimated VC200L (I can't say VMC but guess similar) doesn't show any of its true potential. Sadly, a lot of opinions were drawn from samples like that. The problem lies in the VC being a non-professional grade scope but having its primary, secondary & corrector all collimate-able. The push-pull adjustment screws are not ideal for smooth fine adjustments. The die-cast focuser body sitting in the die-cast primary mirror cell may not be absolutely centered. It could have been better if machined parts were used and some sort of lateral adjustment was provided. That means in 3 places you can throw its sensitive optics out of collimation. Varying the primary to secondary separation would also offset the corrector from its optimal position which can only amplify any aberration caused. Specific number of spacers for each sample of VC200L were put between the secondary holder and the spider during factory collimation in order to keep separation distance correct. In some VC200Ls I inspected, apparently the owner had taken the scope apart but failed to put any spacers back. This wouldn't have been of any issue, of course, if the person somehow move the primary and/or secondary knowingly to compensate for the missing spacers. 

 

I can see why the VC200L (and maybe the VMCs too) is doomed and will continue to carry a bad reputation amongst fellow amateur astronomers.

 

Only if the user can address those issues, the VC's superb optical character can then be realized. And unfortunately it is rare. I have seen VCs professionally collimated but no attentions were paid to the primary secondary separation distance. The usual argument I got was that a normal SCT focus by a moving primary anyway so it didn't matter in the VC. The fact is that the VC is secretively different in optical correction done in the factory, namely VISAC....who knows how Vixen came up with that proprietary coating on the primary. The special coating, Vixen says, determines the shape of the primary to correct for all aberrations. I guess you either sneeze at such claim or believe it. I went the latter and enjoy the VC more, visually and photographically.

 

Clear skies!

Samuel

 

PS, as reference, an email reply from Vixen Japan regarding my query in collimation of the VC200L.

 

image.jpg


Edited by samuelpkco, 13 January 2016 - 01:44 AM.

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#43 bottlecrusher

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Posted 12 January 2016 - 09:45 PM

that is great information.  the VC is a very cool telescope and your results speak for themselves.  



#44 Live_Steam_Mad

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Posted 13 January 2016 - 12:31 AM

I see that there are a LOT of TAL 200K's and 250K's that were tested at ;-

 

http://translate.goo...xbMWpY4n02AYNxQ

 

...and they were nearly always disappointing, and some very bad indeed.

 

Wolfgang Rohr tested VC's and VMC's here by Vixen ;-

 

https://translate.go...DBbDQMViWJnXwAA

 

...he also tested TAL 200K's and found quality problems, but the optics can give Strehls above 0.9 when they are made correctly in nicely made and properly adjusted mechanical assemblies.

 

I have used an 8" Meade ACF F10 OTA for 3 years that was "perfect" mechanically (text book Airy disc and diffraction rings at 500x), same with my Meade 7" Mak,  but my Celestron C11 is not mechanically up to the standard that I am happy with. Optically all these 3 that I have used have been very good, my C11 even outperforms my 7" Mak on Jupiter.

 

I find the TAL / Vixen VC / VMC designs interesting, and when executed properly (preferably with smaller Central Obstrucions and slightly longer tube) and with an optical window I'll bet they could easily rival SCT's. Unfortunately TAL and Vixen don't seem to give a toss about the quality control (if they are supposed to all be beyond diffraction limited when bought new from the factory, since most are not up to standard). A great pity, an opportunity wasted.

 

I find the C11 to be the best compromise between cost / weight / performance / cool down / mounting ease / convenience   so far. Maybe I would be interested in an 11" TAL / VC / VMC if they fixed the QC and thermal issues etc.

 

Regards,

 

Alistair G.



#45 Live_Steam_Mad

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Posted 13 January 2016 - 02:46 AM

no attentions were paid to the primary secondary separation distance. The usual argument I got was that a normal SCT focus by a moving primary anyway so it didn't matter in the VC. The fact is that the VC is secretively different in optical correction done in the factory, namely VISAC....who knows how Vixen came up with that proprietary coating on the primary. The special coating, Vixen says, determines the shape of the primary to correct for all aberrations. I guess you either sneeze at such claim or believe it. I went the latter and enjoy the VC more, visually and photographically.

 

The distance of the secondary from the primary mirror isn't necessarily going to be the same in all VC's in order to achieve best Spherical Aberration correction (SA = the planetary image contrast destroyer, more so even than TDE), since each individual VC will have different levels of SA and sign (under or over- corrected), so you can't set the back focus to be a certain specific amount that the factory tells you to.

 

Instead you have to Star Test the item and pay attention to the intra and extra-focal images (and especially the size of the black disc either side of focus at the center of the diffraction pattern until you get it the same size either side of focus, when moving the focusser in and out by an identical amount).

 

I was hoping that Wolfgang Rohr and also the Russian guys at the Moscow Astronomy Club have realised that, and from what I read in some tests Wolfgang did on these instruments, he does seem to realise this, and I assume he adjusted the primary to secondary distance to get the lowest SA when testing interferometrically.

 

Thanks for the excellent info from your personal experience of a VC 200L. Interesting stuff.

 

Regards,

 

Alistair G.


Edited by Live_Steam_Mad, 13 January 2016 - 02:48 AM.

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#46 bottlecrusher

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Posted 13 January 2016 - 10:06 AM

I think it unfair to say that Vixen doesn't give a toss about quality control.  I have spoken to Brian Deis the owner of the US representation here in the US.  They check each scope from Japan and ensure performance and collmination.  Of course collmination can change some.  

 

I got up early this morning to take a look at Comet Catalina.  It was 18 degrees outside and was windy when I went to bed so I didn't want to leave it out.  I brought the scope out at 4:40 in my skivvies.  I got dressed in warm clothing, got all my accessories together and was back at the scope by 5:00 ready to go.  By 5:10 when my night vision was getting good it was thermally stable and I enjoyed a good hour of the Comet.  I don't think any Mak or SCT could have done that.  I'm more impressed the more I use it.


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#47 fred1871

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Posted 13 January 2016 - 08:06 PM

bottlecrusher, surely quality control should happen at the factory, and not be merely a means of checking how the scopes have travelled on their way to the buyer. Vixen USA might check each scope before releasing it - although in what respect ensuring "performance" isn't clear. But in some countries the importer/distributor does not provide any checks or inspection. The box arrives at the buyer's sealed from the factory.

 

Viewing a comet, presumably at lower powers, does not tell you anything useful about thermal stability of the scope. It might or might not have thermally stabilised, depending on conditions. Even if it hasn't you're unlikely to have issues at lower powers and on a fuzzy object. The C8 I used to have showed no thermal problems under such conditions and type of usage. The C9.25 I now have, and my Tak Mewlon 210, likewise are not problems for such low power viewing of fuzzy objects. Any of the scopes would need to be used for planets or double stars (preferably) and at higher powers to evaluate whether thermal stability was adequate.

 

It's good that you're happy with the scope you got. It may well be a good instrument. Sample variability allows for good as well as bad. But beware of making claims beyond the evidence.



#48 bottlecrusher

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Posted 13 January 2016 - 09:05 PM

The scope was pointed at Jupiter when it was brought outside, and left there until stable.  Surely quality control should happen at the factory; however, I said I talked to the US distributor.  I have not audited their factory and I'm sure you have not either.  Vixen has been in the astronomy business as long as anyone.  Unless you have conducted an audit yourself of their QA/QC procedures then it is unfair to say they don't give a toss about quality.  Have they released bad instruments?  I'm sure they may have.  But your statement simply not fair.



#49 samuelpkco

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Posted 14 January 2016 - 02:24 AM

 

no attentions were paid to the primary secondary separation distance. The usual argument I got was that a normal SCT focus by a moving primary anyway so it didn't matter in the VC. The fact is that the VC is secretively different in optical correction done in the factory, namely VISAC....who knows how Vixen came up with that proprietary coating on the primary. The special coating, Vixen says, determines the shape of the primary to correct for all aberrations. I guess you either sneeze at such claim or believe it. I went the latter and enjoy the VC more, visually and photographically.

 

The distance of the secondary from the primary mirror isn't necessarily going to be the same in all VC's in order to achieve best Spherical Aberration correction (SA = the planetary image contrast destroyer, more so even than TDE), since each individual VC will have different levels of SA and sign (under or over- corrected), so you can't set the back focus to be a certain specific amount that the factory tells you to.

 

Instead you have to Star Test the item and pay attention to the intra and extra-focal images (and especially the size of the black disc either side of focus at the center of the diffraction pattern until you get it the same size either side of focus, when moving the focusser in and out by an identical amount).

 

I was hoping that Wolfgang Rohr and also the Russian guys at the Moscow Astronomy Club have realised that, and from what I read in some tests Wolfgang did on these instruments, he does seem to realise this, and I assume he adjusted the primary to secondary distance to get the lowest SA when testing interferometrically.

 

Thanks for the excellent info from your personal experience of a VC 200L. Interesting stuff.

 

Regards,

 

Alistair G.

 

Thanks Alistair for pointing out the correct procedures in star testing for SA.

Yes, I did that on my own VC and one other one. But other time I skipped it.

 

It requires good weather, a steady sky and ideally a CCD not any eyepiece which could introduce its own SA.

 

So I often use the factory recommended way as a short-cut to set the back focus roughly first. And after doing the laser, I star test to fine tune. Most cases I find little sign of undercorrection/overcorrection; well, at least acceptable to me.

 

So yes, I did cut corners and you got me there!

And frankly, I go from an amateur's point of view and without proper lab equipment, I'd say collimating something complex like a VC seems to be a job never felt accomplished but only abandoned. That's how I feel. It was because of this I looked for consistency in my procedures and strived for repeatability.

 

Your input completed the whole picture because that was supposed to be the most correct procedure after all. Very H.R.Suiter! ;)

 

Clear skies,

Samuel


Edited by samuelpkco, 14 January 2016 - 02:32 AM.


#50 SeattleScott

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Posted 20 January 2016 - 10:46 PM

The scope was pointed at Jupiter when it was brought outside, and left there until stable.  Surely quality control should happen at the factory; however, I said I talked to the US distributor.  I have not audited their factory and I'm sure you have not either.  Vixen has been in the astronomy business as long as anyone.  Unless you have conducted an audit yourself of their QA/QC procedures then it is unfaiconthey don't give a toss about quality.  Have they released bad instruments?  I'm sure they may have.  But your statement simply not fair.

Agreed. My new ED103 is clearly better quality than my 100ED. A post early in the thread compared a VMC200 to a C8 and said the VMC star tested better and had better build quality. Not saying Vixen is Tak. Comparing to Chinese stuff doesn't set a real high bar, but clearly they do care about quality. Ever hear any complaints about LVW eyepieces other than it being possibly overpriced for an older design? Their refractors, eyepieces and finderscopes are consistly high quality. Their VMC model doesn't have quite the reputation but certainly the company as a whole produces some very high quality gear. I hear their mounts are pretty good too. Are they perfect? Neither are the Chinese. I had to take a Dremel to my CG5 just to be able to plug in the power supply. I bought the mount new in 2012. Probably the 4 millionth CG5 Celestron sold and they still can't include a power supply cable that fits the mount? If you want perfection get a Mewlon 210. If you can't afford perfect then a VMC or C8 is a good alternative. They are built to a price point so they won't be perfect. Doesn't mean they aren't good. I still have my CG5 after all. Overall it is a pretty good mount, once I was able to power it up.

 

Scott


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