16 replies to this topic

[AstroBoyInTheCity]

A couple weeks back during some particularly cloudy nights, I ended up with my first two refractors. An Orion Explorer 90az and a Tasco 7te-5. Thanks Facebook Marketplace!

 

After some side-by-side comparison, It's clear that stars and objects look MUCH better in the Tasco compared to the Orion. The contrast in particular really pops with the Tasco. It's almost like there's a fog across the Orion's lens. Stars appear as dots of light in the Tasco, but a bit spiky in the Orion. The Orion also shows a fair amount of Chromatic Aberration when pointed at bright objects like Jupiter and the Moon, but there is none apparent in the Tasco. I'm using the same Celestron 8-24mm zoom between the two (with a 1.25" adapter in the Tasco).

 

I'm still enjoying them both quite a bit but these being the first two Refractors I've ever looked through, I'm not sure what is "normal". Is there something wrong with the Orion? Does the Tasco just have better optics?? Or is there something else going here that I'm not aware of???

 

 

Attached Thumbnails

• 
• 
• 
• 

[NickWDavis]

The Tasco's higher focal ratio is more forgiving of aberrations. One of that age also probably contains Japanese-made optics, where the Orion's are Chinese.

[John Rose]

You mention using the same eyepiece.  Did you use the same diagonal?  I would not be surprised that the Royal Astro objective is better then the Orion. Your description of the chromatic aberration in the Orion sounds like what I would expect.

[Garyth64]

The 7TE Tasco refractors had an excellent lens, maybe the best out there at that time. I am not surprised at all that the 7TE gave better views.  I've had a few of those scopes.  It is even hard to beat them with the scopes today.

 

The lens was made by Royal Astro Optical (RAO), and can be fund in many other brands back about that time too.  You can see the RAO mark on the focuser label, it looks like a little cross with a circle.

Edited by Garyth64, 26 February 2024 - 08:44 PM.

[Jethro7]

Hello Royce,

Some of these older Japanese scopes have pretty decent optics that are held back by the crummy .965 eyepieces. If I am not mistaking, I believe your Taso's optics were made by Royal Astro Optical (RAO) and are pretty good. The longer focal ratio of the Tasco is more forgiving than the Orion. The views that you are seeing in the Orion 90 Explorer along with the CA, I would expect are normal for this class of scope. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

 

Further reading 

 

https://www.cloudyni...e-series-scope/

 

https://www.cloudyni...m-f10-explorer/

Edited by Jethro7, 26 February 2024 - 09:16 PM.

[RichA]

The Tasco's higher focal ratio is more forgiving of aberrations. One of that age also probably contains Japanese-made optics, where the Orion's are Chinese.

Yes, f15 forgives a lot of optical sins.

[Jon Isaacs]

The Orion is a 90 mm F/10, the Tasco is a 60 mm F/15. The Orion has significantly more chromatic aberration but also 1.5x greater resolution and 2.25x more light gathering..

 

The 60 mm F/15 will be prettier than the Orion but the Orion will be more capable.  

 

Inside every 90 mm F/10 there's a 60 mm F/15, all that's necessary is a 60 mm aperture mask. Comparing the two as 60 mm they will be much closer.

 

Jon

[AstroBoyInTheCity]

You mention using the same eyepiece.  Did you use the same diagonal?  I would not be surprised that the Royal Astro objective is better then the Orion. Your description of the chromatic aberration in the Orion sounds like what I would expect.

I tried both diagonals. To me, it seemed like the Orion 1.24” diagonal worked better and felt like the view was more forgiving with off axis viewing (when my head was not perfectly placed). The .965” diagonal may not have been the original, but I’m not sure. 

[Garyth64]

The Orion is a 90 mm F/10, the Tasco is a 60 mm F/15. The Orion has significantly more chromatic aberration but also 1.5x greater resolution and 2.25x more light gathering..

 

The 60 mm F/15 will be prettier than the Orion but the Orion will be more capable.  

 

Inside every 90 mm F/10 there's a 60 mm F/15, all that's necessary is a 60 mm aperture mask. Comparing the two as 60 mm they will be much closer.

 

Jon

But I really doubt that if you mask that Orion 90mm down to 60mm it will be as good as the 7TE 60mm. 

[Jon Isaacs]

But I really doubt that if you mask that Orion 90mm down to 60mm it will be as good as the 7TE 60mm. 

 

There is only one way to find out... I'm sure they'd be much more alike.

 

But the important thing is realizing that "image quality" depends on s number of factors and we're it possible to increase the aperture of the Tasco to 90 mm, it would also suffer in terms of image quality and exhibit significant chromatic aberration.

 

Jon

[AstroBoyInTheCity]

I setup both scopes this morning to do a really quick side-by-side daytime comparison with the Orion masked down to around 60mm using a quickly taped together piece of cutout paper. I also compared them using the Orion's supplied mask in the dust cover which was about half the diameter of the aperture.

 

Masking the Orion definitely brought out some contrast and a little bit more clarity, and the brightness seemed only minimally effected. Stopping down to the half diameter mask reduced the brightness considerably and increased the contrast similarly.

 

The strange thing was when looking through the 7te this morning it was much foggier than the Orion (both masked and unmasked). The two telescopes had swapped what I had seen in previous attempts to compare them. Granted, the test wasn't under optimal conditions as I had them both inside the house pointing out an open deck door and there was no time to cool them down, but I wouldn't imagine that would produce the foggy haze I was seeing, would it?

 

I think I'll be doing a few more tests tonight...

[John R.]

Shine a flash light up through the eyepiece end of the Tasco. Don’t look directly into the objective but at a 45 degree angle. That should tell you if there is a bit of foreign build up on the lens surfaces. The whole cell should come off the tube to clean the front and rear surfaces if that is necessary. If there is haze between the elements then look up information here at CN to see how to clean the lenses and reassemble the objective. 
 

[AstroBoyInTheCity]

I set the two telescopes up after the sun went down and spent an hour trying them out against a few different objects. I used a Celestron 8-24mm zoom and the same Orion 90 degree mirror diagonal when viewing on both. This was totally unscientific and mainly just for my own curiosity's sake. 

 

also bear in mind, I'm a newbie to telescopes so take these comparisons with that caveat. Any and all comments are welcome! I'm here to learn!

 

 

Full field Moon w/zoom at 24mm - It was pretty clear that the contrast between these two is dramatically different in their native states. The Tasco had a fair amount more contrast which allowed the moon to pop off the background better. In comparison, the Orion was slightly washed out. I noticed no chromatic aberration on the Tasco, but a significant amount with the Orion along the edged of the moon which also caused the image to feel softer overall. Adding the lens cap stop down mask to the Orion brought the comparison between the two much closer, but there was still a slight CA on the Orion. Kidney beaning and blackouts with the Celestron eyepiece were also considerably more of an issue with the Tasco through it's 1.25" adaptor placed in front of the diagonal. 

 

Closeup of Moon terminator at 8mm - Aside from the brightness, both telescopes seemed to look very much in the ballpark with each other. I could not detect much if any chromatic aberration on the Orion for this test, which really surprised me. Focus and contrast felt nearly identical with and without the mask on the Orion. The eyepiece issues that I was having at 24mm did not seem as apparent with the higher magnification.

 

Splitting Castor at 12mm to 8mm - Not surprisingly, in their native state the Tasco performed much better than the Orion. The stars were perfectly circular with no spikes and only minor seeing aberrations which I think were due to the atmosphere. The stars in the Orion were much brighter and very spikey with a few concentric rings around the center. You could see the split at 8mm, but it was not readily apparent through the spikes. However, adding on the lens cap mask brought the brightness down to a level that felt just a touch below the Tasco and the stars were easily split at 12mm.

 

 

Overall, to these newbie eyes who have never peered through a refractor other than these two, the viewing experience of the Tasco 7te-5 and the Orion Explorer 90az were similar but had some significant differences. The chromatic aberration and the lack of contrast of the Orion were the biggest detractors between the two and despite some more precise eye position needed for the Tasco, I would say I had a better experience viewing through the Tasco. It's also just a really pretty telescope! I can imagine the extra light gathering on the Orion would be an asset for planetary viewing, but for these tests, it did not factor in as the Orion seemed to perform better with the lens masked down.

 

Well, I can say this was a wonderful learning experience for me, and thank you everyone for your suggestions and comments. It was all super helpful!

[AstroBoyInTheCity]

Shine a flash light up through the eyepiece end of the Tasco. Don’t look directly into the objective but at a 45 degree angle. That should tell you if there is a bit of foreign build up on the lens surfaces. The whole cell should come off the tube to clean the front and rear surfaces if that is necessary. If there is haze between the elements then look up information here at CN to see how to clean the lenses and reassemble the objective. 
 

Thanks for the suggestion John! I unscrewed the lens cell and checked it as you recommended. It appeared completely clear to me and I saw no haze. Just a very small fingerprint near the center. I'm not sure what happened in my daytime test two days ago, but I replicated it again this morning and the haze was not apparent in the Tasco...

[CHASLX200]

I had a Orion 90mm not long ago and it was good but not freaky sharp.  The diagonal that came with it was bad also.

[Exnihilo]

I had the 7TE-5 as a 12-year old, still one of my favorite scopes 

[Jon Isaacs]

Splitting Castor at 12mm to 8mm - Not surprisingly, in their native state the Tasco performed much better than the Orion. The stars were perfectly circular with no spikes and only minor seeing aberrations which I think were due to the atmosphere. The stars in the Orion were much brighter and very spikey with a few concentric rings around the center. You could see the split at 8mm, but it was not readily apparent through the spikes. However, adding on the lens cap mask brought the brightness down to a level that felt just a touch below the Tasco and the stars were easily split at 12mm.

 

 

Castor is a 5 arc-second double, that is an easy split for a 60mm, I am able to cleanly split Castor in a 50mm F/5 Astro-Tech finder. 

 

The 90mm aperture would show it's advantage on closer doubles.  Zeta Orionis (Alnitak) is a 2.4" double with magnitudes 1.9 and 3.7.  It is doable in a 60mm but difficult.  It should be easier and wider in the 90mm.  Eta Orionis is a 1.8" with magnitudes, 3.7 and 4.9.  It is beyond the Dawes limit for a 60 mm but doable in an 80 mm and maybe doabable in the Orion 90mm at high magnifications.  The zoom plus a 2x Barlow would provide up to 225x which should be sufficient,

 

Jon