12 replies to this topic

[RichA]

80mm f5 Celestron Power Seeker

 

 

100mm f6 Apogee Widestar

 

[abby_normal]

ok, I'll take one for the team.  neither one looks great, but can someone interpret those for us?

[PKDfan]

Psychedelic !!

Thats called CHROMATIC Aberration.

Taking a close look at the first image and besides CA its not half bad.

A Low power telescope for sure.

I'm not sure about whats happening with the second optic though.


CSS
Lance

[RichA]

ok, I'll take one for the team.  neither one looks great, but can someone interpret those for us?

Actually, the figure on both scopes is good, no issues.  But colour is inescapable.  The two images were done with two different types of grating and two different kinds of exposures.

[mikeDnight]

Don't blame the scope, it's the nature of the beast! It would be equivalent to someone blaming me for being ugly, (I blame testosterone!).

If that's a Ronchi grating, the figure on both lenses looks pretty good and the refractors will most likely perform admirably within their design limitations. I doubt the CA would be much of an issue visually in such rich field scopes. Just don't expect wonders on the planet's. 

Edited by mikeDnight, 27 March 2025 - 03:19 AM.

[KerryR]

Just curious: What grating, single or double pass, real or artificial star star (if single)?

[RichA]

Just curious: What grating, single or double pass, real or artificial star star (if single)?

Single pass, artificial star.  The two gratings were a basic polyester printed grating and a specialized tape that has lines and used to index the motions of a high-end printing device.  Also, I'm beginning to wonder about the ronchi screen positioning, relative to the sensor.  Notice in the first image how obvious  the three aluminum foil spacers are in the image?

Edited by RichA, 27 March 2025 - 09:56 AM.

[kmparsons]

A short-tube achromat will have noticeable CA. No escaping that. That does not mean that they are worthless. On the contrary, one of the most-loved scopes here on CN was the Orion 120mm short tube. Many put on a better focuser and enjoyed it for years. Yet for deep sweeps it was terrific, and the CA tolerant could use it for anything. 

[Lagrange]

Those images are a really good illustration of how achromats predominantly have longitudinal colour error (especially in fast designs), while spherochromatism is negligible.

 

Compare that to an apo where the lines will overlap a lot more, but they'll also bend a lot more.

[saemark30]

The Ronchi screen test is quite insensitive to fast telescopes of F/5 and in single pass.

Better to have 3 lines rather than 4.

A close artificial star should produce some SA as refractor lenses are corrected for infinity.

Edited by saemark30, 27 March 2025 - 10:25 AM.

[Refractor6]

A short-tube achromat will have noticeable CA. No escaping that. That does not mean that they are worthless. On the contrary, one of the most-loved scopes here on CN was the Orion 120mm short tube. Many put on a better focuser and enjoyed it for years. Yet for deep sweeps it was terrific, and the CA tolerant could use it for anything. 

  I had the Skywatcher 120 F/5 version for a few years. Most used scope at that time and yes I did a focuser upgrade too. Took it everywhere inc. star parties across BC and it did a couple of years at International Astronomy Day in Vancouver for public outreach since I had a solar filter for it for daytime viewing then the lines to look at objects at night...great scope and memories   

[RichA]

The Ronchi screen test is quite insensitive to fast telescopes of F/5 and in single pass.

Better to have 3 lines rather than 4.

A close artificial star should produce some SA as refractor lenses are corrected for infinity.

I often wonder if it would be better to do video, with essentially one line allowed to pan over the star.  I think that would graphically illustrate any optical issues, compared to 3-5 lines used statically.  I think the 40ft distance I used should mean no noticeable SA would show up.  I think Questar said around 30ft for their 90mm Mak and if closer, stopping it down with the (then) included stop down ring to around 80mm.

[Phillip Creed]

A 120mm f/5 achromat is admittedly a one-trick pony.  It just happens to do that one trick amazingly well.

General rule of thumb is keep your eyepieces in the double digits with one, as it sported more purple than a Prince concert around bright stars >60X, but a 120-ST is an absolute blast under dark skies with a low-power, widefield eyepiece.  Easily framing the Veil Nebula or Sagittarius Star Cloud with room to spare is an experience.

I think I had more fun in a night with a 120-ST at Cherry Springs at the 2009 Black Forest Star Party than I've had visually with any other telescope.  It was the first time I saw the Sculptor Dwarf Galaxy (not bad for transiting at just 15° up) and I could even make out -- without filters -- both IC434 and the Orion Nebula.  Sure, when you're surrounded by well-heeled observers with APs and Taks you feel like you're driving a rusted out pickup truck through The Hamptons but after a while instead of being embarrassed by it I embraced it.  And when the dew hit hard, I went into the restroom and used the wall-mounted hand drier to remove the dew and carried on from there.

Clear Skies,

Phil