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Posts Made By: Vahe Sahakian

September 13, 2006 09:40 AM Forum: Eyepieces

Zeiss Abbe orthos 1,25"

Posted By Vahe Sahakian

Not familiar with TMB, UO, but I do have Tak LE’s 5, 7.5,18 & 24mm, also have ZAO’s in 16, 25 & 34mm. The Tak LE 7.5 is my main planetary eyepiece with my 6” AP refractor, just about ideal in a binoviewer for Jupiter, (232x) very sharp, good contrast and extremely comfortable for extended viewing despite its rater short focal length.
I use ZAO’s with long focus Maksutovs, the 25mm is my main planetary eyepiece (250x), unlike Tak LE’s which exhibit an ever so slight warmer tones, ZAO’s are totally neutral in color, great transparency and edge to edge sharpness in long F scopes.
I also have sets of Pentax SMC Orthoscopics in 6, 9, 12 & 18, these eyepieces are equal to ZAO’s in every respect and cost about 1/3 on the used market.
For me Pentax SMC 9mm represents the lower limit for eye relief in Ortho's, anything shorther than that is a pain, the Pentax 6-O is a superb planetary eyepiece but I can not deal with it for more than 5 minutes at a time due to short eye relief.

Thanks,
Vahe

September 17, 2006 11:51 AM Forum: Equipment Talk

Telescope as telephoto

Posted By Vahe Sahakian

"The question is, how well would a telescope be able to compete against a monster like that... or what would it take to compete?"

Take a close look at the arabic letters on the side of the Apo-Sonnar, it was obviously paid by easy money genetared by black gold.

Vahe

December 12, 2006 04:43 PM Forum: Refractors

Geez, how easy it can be!

Posted By Vahe Sahakian

What we have here is pure deception on the part of William Optics, pure and simple. Just how well corrected a lens with FPL53 can be compared to a lens with Fluorite has nothing to do with the facts, the fact is that their lenses were marketd as Fluorites and they did not contain Fluorite in them, that is deception in my books, any way you look at it.

Thanks,
Vahe

December 28, 2006 05:32 AM Forum: Refractors

where to buy a 12-14" achromat?

Posted By Vahe Sahakian

Trying to be a little silly here, if it were me and if I wanted the big eye for the planets instead of a achromat I would definitely consider a lerge Mak Cass with properly ventilated ota.
Advantages: smaller, shorter, lighter, totally color free, can be mounted on AP1200 class mount, portable (relatively on a wheely bar)and very much suitable for imaging with no out of focus color haze. If you can get one designed and built with co. of about 26% images will be essentially refractor like, the contrast will be higher than an achromat due to total lack of out of focus color.

Thanks,
Vahe

January 11, 2007 10:56 AM Forum: Maksutovs

New Orion 7.1" Mak-Cass

Posted By Vahe Sahakian

I do not wish to bad mouth Chinese over their quality control issues, they have both disappointed and also surprised us with their products.
That said I am curious on one issue regarding their 180 Orion Mak, unlike the Meade Mak which came with a huge obstruction this one comes with a very small spot secondary, mechanical issues aside, in the optical department, given the design, a Gregory variant, this particular Mak will require a good bit of aspherization to perform well on planets, that is my $1200 question, can Chinese really pull this one for the $$$ charged? This is not an easy task and I am not sure if machines can do it, let us hear about it.

Thanks,
Vahe

March 23, 2007 09:47 AM Forum: Equipment Talk

The truth about a central obstruction

Posted By Vahe Sahakian

The reason why 16" SCT is somewhat limiting is not the co size but the overall optical quality, you will be lucky to find a Meade 16" SCT that is as good as a 1/2 wave in figure.
RC's with over 40% co. are optimized for imaging, they are not intended for visual applications.
Tak 400 will be your best bet among your choices, although not perfect for all visual, they are clearly better in optical quality than the Meade and have smaller co than the RC.

Vahe

March 25, 2007 06:49 AM Forum: Astro-Physics

AP 152 starfire F9 1992 production.....

Posted By Vahe Sahakian

Read Joe's writeup on several AP refractors including the one you are asking about
http://homepage.mac.com/joebergeron/a-p.html
The 152 F/9 is really not an apo but it is darn close, more importantly it is an AP, it is a vast improvement over the earlier 6" F/8 series but nowhere near the perfection of 155 F/9EDT which in my opinion is the best 6" visual apo that AP has produced to date.

March 27, 2007 04:19 PM Forum: Equipment Talk

Central obstruction size, Round II

Posted By Vahe Sahakian

"The first point would argue for a 20" 40% obstructed scope, while the counterpoint might favor a 16" 30%. Anyone swayed by this, or does everyone still line up on the same side of the fence? "

With due respect to all large Dob owners, I am willing to bet that a 20" is a bit too big for planets, that is if you expect to get its full firepower, the seeing will never allow you to even come close, even if you live in southern Florida, if you live in the snow belt then a 6" apo will certainly outperform a 20" instrument all the time, never mind the laws of physics.
Another thing to consider with a 20" 40% co., since the images are so bright you need to start considering the effects of not only the first ring but also the second and possibly the third, yes they are faint but they will be there and they will affect the contrast.
For planets a high quality 10", IMHO, is the optimum aperture, with 1/4 arcsecond resolution, seeing permitting, you can expect to capture threshold detail, once you go over that aperture the seeing starts to limit its potential.

Vahe

March 28, 2007 04:03 PM Forum: Equipment Talk

Mak-Cass contrast

Posted By Vahe Sahakian

In all spherical Mak Cass., even in the cheaper machine made units, you can expect to find smoother optics than the much complex aspheric SCT’s. With smoother optics more of the light comes to focus giving you tighter and cleaner images than the SCT’s, this also translates in better contrast when comparing the two systems with the similar obstructions.

Vahe

April 1, 2007 10:35 AM Forum: Equipment Talk

What cooling strategy for a big Mak-Cass?

Posted By Vahe Sahakian

"I imagine the primary is the main concert. (But if you disagree, please speak up.)"

I am hoping to do a serious cooling related modification to my TEC250/20 one of these days. I did my studies using a beat up C8 and what I have found is the basis for these mods.

First, I tried to understand what exactly happens when a Cat is exposed to cooler ambient air, we all know what happens when the primary starts to shed its stored heat resulting in a boundary layer at its surface which is considered to be the major source of image degradation, but to me the puzzle is where and how does the primary sense the cooling which triggers its own cooling, after all it sits deep inside the tube and has no contact with the outside world.

Based on my studies the metal tube is where the initial cooling takes place, the metal tube will cool well below the ambient by radiating its heat to the outer space. Once that happens a boundary layer is formed on the inner surface of the tube, this is the cooler and denser air, which will start to flow downward inside the tube by gravity and as it hits the primary surface the primary reacts by releasing its heat.

You have two things happening when your Cat is exposed to the cooler ambient air, inside the tube you have downward flowing cooler air mass formed at the metal tube and the upward flowing warmer air currents started at the primary, these two act as an irregular shaped refracting element in the light path that is constantly changing its shape and screws up the wavefront as it passes through it.

To get an upper hand on the cooling problems of your Maksutov you must take care of both problems and not just the primary mirror.

I will give the details of my modifications when it is done and tested.

Vahe