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Old 20-10-2008, 08:37 AM
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Low-cost lens alternatves: MC Rubinar 8/500 test

Just a test, before glass relaxing, from Mt Waverley light polluted skies... The first image is stack of 8x1min exposures at 1600ASA, on EQ6, unguided.
The lens performs quite OK as it is now, but the fine details (on the full resolution image) are missing, of course .
The 200% detail from Upper Right corner (from un-processed RAW) is attached, to illustrate the field flatness performance (field is very flat, as there is no difference in stars shape from those in the centre).
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Last edited by bojan; 20-10-2008 at 05:59 PM. Reason: clarity and more data
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Old 21-10-2008, 09:39 AM
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Not much interest shown here so far for cost-effective solutions.. Hmmmm

As a proof that I am not talking nonsense here among Canon club members (btw, I also belong here as I own Canon 400D)... .. have a look at this website below, and search for "russentonne" or "MTO" or "Spiegeltele" (those lenses are very popular in Europe, and referred to as Russentonne - just try to google a bit and you will see what I mean..)

http://www.mhm-ka.de/CANON.html

There are some amazing images taken with those low-cost lenses.. for 1/10th of Canon price. And they are not THAT much worse, aren't they? :-)

Last edited by bojan; 21-10-2008 at 10:08 AM.
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Old 21-10-2008, 10:12 AM
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Ta for the link.
I'm always looking for less expensive alternatives.
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Old 31-10-2008, 11:58 PM
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Here is one that got a good review and seems quiet affordable.
http://www.adorama.com/catalog.tpl?a...op=academy_new
There is a thread at CN about it.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1

I like these lens, you get color free performance for a fraction of the cost. Might have to save a few pennies and get one.

Regards
Fahim
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Old 01-11-2008, 09:04 AM
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Fahim, thanks for the link..
Yes, there are a number of models available out there..
The most common problem with all of them it seems to be astigmatism, in most cases caused by mirror too tightly fastened and therefore deformed ("pinched").
Relaxing of glass (by loosening the retaining rings that holds the mirror in place) generally helps a lot to get rid of those "tear-shaped" stars.
Those procedures on cheaper lenses are very popular in Europe (especially Germany.. people there are known to use their money very wisely :-)
But, the field flatness appears to be excellent in almost all models.
Sometimes, the mirror is simply not good enough... so no help here.

Well, if final image is resampled, those little flaws are not visible anyway.
In my opinion, what we really have to consider here is the price/performance ratio, and what we want to do with the lens.
Personally, I am not prepared to pay 10-20x more for only 1.5-2x better image (visible at sub-pixel level). Some of those "flaws" can be taken care of by post-processing (de-convolution) relatively easily and at no additional cost.
Quite a wide variety of very "serious" work can be done with those, medium-quality lenses (that means photometry etc, not just pretty pictures).

BTW, I still haven't done mirror relaxing on this lens (Rubinar 8/500), the retaining ring thread seems to be glued with brown stuff (shelack?).. so I gave up for the time being - I will try again with stronger tool in the next couple od days when I am on my annual leave.
But I have good outcome with MTO_10/1000A, the resolution apparently doubled after procedure (or, the star shape is now half the size compared to what was before the procedure).

And, finally, if the lens is not good for photography, it will be perfect as a guide scope :-)
Or as a visual telescope.. and a very transportable one :-)

Last edited by bojan; 01-11-2008 at 09:19 AM.
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Old 01-11-2008, 09:49 AM
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Bojan,

The Rubinar's are well respected as being amongst the best there are. I also believe there is a rare Gem made by Questar, but have not really looked into this as it would be quiet expensive route. I am on the lookout for a used rubinar but with a shorter focal length. The one i lined seems a good low cost option which seems to have tested well for terestrial use. I wonder how it would perform for Astro use.

The other question that has to be asked is the value proposition compared to other low cost ED80 refractors. With such a small apperture on these mirror lens's one has to wonder how it would compare against the light gathering power of a non-obstructed ED telescope. I would like to see some direct comparrisons. I have the WO 80mm Triplet now and am looking to get a mirror lens to compare. A 500mm F5 would be nice to compare with.

Regards
Fahim



Regards
Fahim
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Old 01-11-2008, 11:48 AM
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Well, take a photo with your triplet so we can compare directly with those at the beginning of this thread. They are taken at prime focus, and the detail is from one of the corners... You have all the necessary parameters listed so there should not be a problem for you to repeat the exposure :-)

Last edited by bojan; 01-11-2008 at 12:11 PM.
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Old 01-11-2008, 01:34 PM
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I will give it a whirl once the weather clears. I have not had good weather at the right time to try the WO yet.
Btw i found a Questar 700/8 on Ebay US if anyone wants one this is probably the best you can get. If i had the cash I would surely get this one.
I found this information on it.
http://www.cameraquest.com/questar.htm
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Old 01-11-2008, 02:03 PM
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Questar is a Legend...
However, not cheap.

Back in late '70s, when I was looking to buy something, it occurred to me that Questar 1000 and MTO10/1000 must be very similar to size and design.. and I bought MTO for $400 (versus $2200 or more for Questar, if my memory serves me well about those prices). I still own that MTO today, and I am happy with it. I had to build the mount for it, though.. but hey, I am amateur :-)
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Old 01-11-2008, 06:17 PM
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Bojan,

I am just also wondering if it would not be better to buy a Skywatcher / Celestron / Meade 3-4" SCT or MAK. I wonder how they would rate against the MTO 1000/10 which i guess is a 4" apperture. Should be similar to a Meade ETX-105 or C5 SCT or Mak from Celestron. The MTO however probably does not suffer from Mirror flop as it focus mechanism is more like a lens.


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Fahim
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  #11  
Old 01-11-2008, 07:21 PM
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Yes, it would be interesting to see test images, like those I posted here and on another thread (just a crop of a star, from centre and from corner of the frame, not overexposed so that it is possible to see the problems like astigmatism, coma etc).
I never sow them though.. people are hiding those pictures like snakes are hiding their legs (this is expression where I am from, I apologise if you do not get it :-) )
However, I would expect MAK to perform better that MTO in terms of image quality.
And yes, MTO (and other Rubinars) is designed like a lens (it has M42 thread) for full frame cameras (so it performs better at frame corners when used on camera with smaller sensors, like one in Canon 400D), so it does not have mirrors or anything like that.
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