54 replies to this topic

[Marcsabb]

 

Taken from the sales pitch at Omegon websie:

 

"The two element objective lens is constructed of a clever combination of OHARA FPL-53 and S-NBM51 glass. These are special types of ED glass with high and low refractive indices and superior light transmission characteristics. A particularly special feature is the use of S-NBM51 glass, which is usually employed in the optics of apochromatic fluorescence imaging microscopes. The benefit to you is that the optics provide a particularly sharp and high-contrast image!"

 

 

BTW, it is "clever"  marketing. Statements are correct but notice there is no statement about "superior color correction". Yet, the use of FPL-53 invokes in many people that the lens will have excellent color correction.

 

I'm not bashing the scope. I think, this could be nice visual instrument, the color factor of SN2~2.5 is similar to my 63/840mm achromat where the CA is very, very small. But definitely people should not expect from it the apochromatic performance, especially for imaging.

 

Thanks for the clear explanation. Looking at the Omegon site, I got the impression that this instrument is sold mainly as an imaging device. The name of the scope, also, "ED-APO 100/600" clearly hints at some kind of good color correction performance. Which it has - most of the time. Clever marketing, as you said.

 

Visually I find the images produced at the eyepiece sharp and well defined. But when I take pictures... it's hard to describe, they just look a bit 'dull', for the lack of a better term. So. I'll use it a bit more in the weekend and next week I'll decide its fate. 

Edited by Marcsabb, 05 January 2017 - 08:11 AM.

[Mark9473]

 

 

when I take pictures... it's hard to describe, they just look a bit 'dull', for the lack of a better term. 

 

Check if there is perhaps a shiny surface on the inside of the focuser drawtube or an extension tube you may be using. That could noticeably reduce contrast.

[Sasa]

Nice update.

 

Likely the long (over 2hrs) cooldown period allowed and the much better seeing remedied most of your issues.

 

The cleaning of a dirty lens surface is not something a new lens should need, nor is it a known cure for chromatic aberration and the likes. Plus there is no guarantee that the residue on the inner lens will not form again. So I would still seriously consider returning it.

Indeed, it is hard to understand how cleaning a surface can change color correction of the lens. My former experience with astro imaging with colorful achromats (I started with 80/480mm Stellarvue AT1010) was that amount of the blue/violet halo around the stars was very sensitive to the focusing. My bet is that the first Marcsabb image was slightly out of focus (may be the fact that the optics was not clean was making the focusing more difficult - hard to tell without having the lens in hand for playing).

 

BTW, even with this fast 80/480mm achromat you can take nice images - my best is this one using LRGB filters and CCD camera (no special treatment for blue channel):

 

http://www-hep2.fzu....rig/m27_031.jpg

 

The color correction of Omegon 100/600 ED is around 3 times better and I can imagine that one can use it to produce nice images even though it is not perfectly apochromatic. It is all about expectations and what one wants.

Edited by Sasa, 05 January 2017 - 10:53 AM.

[Sasa]

 

 

Visually I find the images produced at the eyepiece sharp and well defined. But when I take pictures... it's hard to describe, they just look a bit 'dull', for the lack of a better term. So. I'll use it a bit more in the weekend and next week I'll decide its fate. 

 

I don't know how much experience you have with imaging. What makes the result through this lens dull? Producing not-dull "breath-keeping"  images is an art and requires a lot of post-processing experience. Obviously, the data has to be of good quality in the first place. Light pollution in RGB imaging is large handicap and can lead to dull results even with perfect optics.

 

In the next night, try to get as much precise focus as you can, then try to measure FWHM of stars using short expositions (to eliminate guiding issues; but long enough you have well exposed stars in the FOV) - if the number is unreasonably high, there might be a problem. Use L-filter (or IR/UV cut filter) for this purpose, this is not an apo and far blue and IR ends can make the numbers to look bad. Then try some longer expositions with some brighter stars, like you did and see if the blue halo is objectionable to you or not. If the two tests pass your expectations than you will be probably happy with the lens. The result you already got on Rigel looks quite good (at least to my standard).

 

There is one more important test I would do for new lens for astroimaging. You need to check the whole optical train, including the flattener and see if you are happy with off-axis performance in your camera.

[Marcsabb]

I think I've quite some experience with photography and with visual astronomy. When it comes to astrophotography, however, I'm a neophyte as I approached it only 3 years ago - with modest effort and equally modest results, I should add.

 

For 'dull' I mean a combination of poor contrast, flat tints and a certain lack of sharpness. I always use a Bahtinov mask to focus my subjects and I've tried with both my SLRs and my astrocamera, so it's either the photographer or the telescope to blame 

 

This is however the first time I try to do some photography with a refractor. Aside from some achromats I've owned in my youth, and which I used only visually, I'm mostly a newt/cat/mak guy so I'm sure part on the blame falls on me. 

 

BY the way, thanks for all the support and assistance I got on this topic. It's great to be able to listen to the advice and the experience of so many experienced hobbyists. 

Edited by Marcsabb, 05 January 2017 - 11:16 AM.