First light: 90/500mm Long Perng SD APO

[grjsk]

I got the 90/500 mm Long Perng SD Apo November 11^th. For more than a month there were clouds every single night. Just before Christmas the clouds lifted, so I got to try it out.

I purchased it to replace my 72/432 mm TS-Optics doublet. I was hoping the Long Perng would be the biggest scope my az-gti and carbon fiber tripod could carry within my tolerance for vibrations, and therefore render my 72mm obsolete. I took them both out this evening to compare them. The 90mm has more aperture, is faster (f5,5 vs f6), has worse glass (fpl-51 vs fpl 53) and is about 1,1 kg heavier. I was hoping that the added amount of aperture would make sure it could go a little deeper, without the CA increasing too much. I compared the two scopes on a range of targets that are of interest to me.

 I used a Skywatcher dielectric star diagonal, a 32mm Celestron Plossl and the Tele Vue 3-6mm zoom on both scopes.

I conducted a simple star-test on both scopes, and they both seemed well collimated.

 

Jupiter:

The 90mm showed slightly more details on the main bands. They were a tad more refined. There was however a bit more CA. In-focus the amount was negligible, but it was obvious out of focus. I was surprised how narrow the “focus window” was. Even a slight touch on the focuser brought out the colors, even though the image still looked in focus. The 72mm showed no color in focus, and even out of focus I struggled to see it. Overall, the 90mm won the Jupiter-test.

 

Mars:

After the positive experience on Jupiter, I had high hopes for Mars. Unfortunately, CA was overwhelming. I was really surprised how much difference there were between the two scopes. The 72mm only showed color out of focus and remained color-free in focus. That was not the case with the 90mm, that showed color all the time. I could not see any more details with the 90mm. The 72mm wins on Mars.

 

The Moon:

I could not see any big difference here. The atmospheric conditions seemed to matter more than the extra 18mm. There was more color around the edges with the 90mm, but it did not really bother me. But since that was the only real difference I could see; I’ll give the victory to the 72mm.

 

Open clusters in Orion (CR69, CR70, NGC1980, NGC1981):

The 90mm showed a little bit more. The faintest stars were a liiiiittle bit easier to spot, and the brighter ones stood out a little bit more. But the difference was very small. Had I not been going back and forth between them; I am not sure I would have spotted the difference. Still, a 90mm win.

 

Double stars (Sigma Ori, Meissa, Castor, Polaris)

This is the real test. Double stars are my preferred objects. Since I was already in Orion, I started with Sigma Ori and Meissa. The results were about the same as with the open clusters; the 90mm showed a smidge more. Barely noticeable to my eyes. But on Castor and Polaris a small amount of CA was creeping in when I passed the 120-130x range. The 72mm was color free.

Not many years ago when I started out in this hobby, I was using a 70/500 achromat. It had a bit of color, but it did not really bother me. Now, after having lived with the virtually color free 72mm, suddenly it bothered me a lot.

The 90mm could also take magnification a little better, as was expected. But again, not a massive difference. Overall, I preferred the 72mm.

 

Other differences:

The 90mm is 1,1 kg heavier. The difference was noticeably. Dampening times where still less than a second, but that felt like a lot since dampening time on the 72mm is close to instant. Also, the glossy black tube on the 90mm felt a lot colder than the rugged white 72mm (-10 degrees Celsius outside).

 

Conclusion:

I was hoping to do several comparisons over the next months, but I do not think I need it. The 72mm is the keeper. The extra 18mm gave a slight boost to the 90mm, but the added color and weight is bigger drawbacks to me. Especially since it was visible already at 120x on Polaris. Color on the planets I could probably have filtered away, but I prefer double stars and open clusters without any filters. The added weight was also a bigger drawback than I envisioned.

I try not to be disappointed with the Long Perng. It was perhaps wishful thinking that an extra 18mm would matter a lot to me. Doing the math, and readying on the forums, I assumed the difference would be bigger. Several years ago, I compared the 72mm with a 102mm, and the differences were not great. But I was unexperienced, and I have many nights under the stars since then. I assumed that a better trained eye would be more sensitive to a slight aperture increase. But 18mm is still only 18mm. And I have been spoiled owning the 72mm. It is really a fantastic little scope. I have forgotten how CA looks like, that is how good it is.

There is a lot of subjectivity in this hobby. I am sure there are people out there that would prefer the benefits of the 90mm. But I do not think I am one of them.

Edited January 2, 2023 by grjsk

[JeremyS]

Sorry to hear of your disappointment. A (much) higher focal ratio will help a lot, at the expense of weight, of course.

[Franklin]

A good honest appraisal.

Scopes that use FPL-51, as opposed to the more expensive FPL-53, are a great budget option for those wanting to try ED type glass for the first time, perhaps upgrading from a standard achromat. But, if you are looking at an FPL-51 scope, I think you need one with a longer, rather than shorter, focal length.

A good example of this are the 102ED f/11 scopes available from Altair, TS and others. I had the opportunity to use one of these and the CA was very well controlled and the contrast was superb. But of course you then have the task of mounting such a very long ota.

[Louis D]

In my own 72->90 journey, I went from an FPL-51 f/6 doublet to an FPL-53 f/6.7 triplet and have had no regrets.  That, and the mechanics of the 90mm were much better.  The 2.5" R&P focuser and camera angle adjuster make for a much better experience.  False color went from barely noticeable to nonexistent.  You're right though, even with the FPL-53 triplet, out of focus stars show red/green on either side of best focus.  The 90mm is quite a bit heavier than the 72mm as well; but it's nothing my DSV-2B mount can't handle.

If you've already got a 72mm FPL-53 doublet, you're going to need to jump up quite a bit in aperture to see a significant improvement in resolution performance.  Something like a 125mm FPL-53 doublet or even triplet might do the trick.  Of course, you'll probably need a much beefier mount.

Along these lines, I bought a 6" f/5 GSO Newtonian to see how it performs.  So far, the jury is out.  I think I much prefer my 8" Dob in every respect.  Aside from astrophotography, I'm not sure what the advantage is to a Newt on an alt-az mount.  Sure it's smaller to store, but not all that much, especially when you figure in the mount head and tripod.  That, and it has a massive secondary to favor astrophotography which does nothing to improve visual contrast.

[Ags]

Hi, this doesn't match my experience - I see very little false color with my SS 500 GD. The only differences would be my eyes and the diagonal I am using. Expecting the Long Perng would be less well corrected in red, I had purchased a cheap Celestron prism diagonal, which can cancel some of the chromatism from the scope. If you have one of these very cheap diagonals to hand, why not give it a try?

Note that I have been using my Long Perng for many Mars observations this opposition. I've been getting clear views, particularly of dark albedo features. One area the scope has performed less well has been in showing the polar hood. Overall, I would say I prefer the views in the Long Perng to those I have had with a 102 mm Mak for example. 

On Jupiter, the 90 mm gives far better views than my C6.

For me the 90 mm makes a big difference for doubles in comparison to my 66 mm -- for example, Polaris' companion is impossible with the smaller scope due to my local light pollution, while it is easy with the 90 mm. The Long Perng also (with the Celestron prism) seems to show authentic colors for the components. 

I will try my Long Perng on Mars with my spare mirror diagonal and let you know what I see.

Also, I will add my obligatory note that the scope since 2019 uses two special glasses, Lanthanum and FPL-51. So we should expect it to perform somewhat better than the previous Megrez 88 incarnation that featured only FPL-51. Contemporary reports of the Megrez 88 generally regarded it as a good visual instrument, although very sensitive to misfocus.

I agree on the weight/bulk issue -- the scope is right at the limit of what I would consider suitable for travel. As I have said elsewhere, the Long Perng was exactly the size I was told it would be, but it was much bigger than I expected! It works quite well on my AZ-GTi - but I am using a Berlebach Report 112 tripod.

Edited January 3, 2023 by Ags

[grjsk]

10 hours ago, Ags said:

Hi, this doesn't match my experience - I see very little false color with my SS 500 GD. The only differences would be my eyes and the diagonal I am using. Expecting the Long Perng would be less well corrected in red, I had purchased a cheap Celestron prism diagonal, which can cancel some of the chromatism from the scope. If you have one of these very cheap diagonals to hand, why not give it a try?

Note that I have been using my Long Perng for many Mars observations this opposition. I've been getting clear views, particularly of dark albedo features. One area the scope has performed less well has been in showing the polar hood. Overall, I would say I prefer the views in the Long Perng to those I have had with a 102 mm Mak for example. 

On Jupiter, the 90 mm gives far better views than my C6.

For me the 90 mm makes a big difference for doubles in comparison to my 66 mm -- for example, Polaris' companion is impossible with the smaller scope due to my local light pollution, while it is easy with the 90 mm. The Long Perng also (with the Celestron prism) seems to show authentic colors for the components. 

I will try my Long Perng on Mars with my spare mirror diagonal and let you know what I see.

Also, I will add my obligatory note that the scope since 2019 uses two special glasses, Lanthanum and FPL-51. So we should expect it to perform somewhat better than the previous Megrez 88 incarnation that featured only FPL-51. Contemporary reports of the Megrez 88 generally regarded it as a good visual instrument, although very sensitive to misfocus.

I agree on the weight/bulk issue -- the scope is right at the limit of what I would consider suitable for travel. As I have said elsewhere, the Long Perng was exactly the size I was told it would be, but it was much bigger than I expected! It works quite well on my AZ-GTi - but I am using a Berlebach Report 112 tripod.

Yes, I have read your reports, so I was surprised as well. The atmosphere, the eyepiece or the diagonal could of course have been contributing factors, but those would also have impacted the 72mm. I have not yet been able to take the scopes out together a second time. I do have a cheap Celestron prism, so I might give it a go. But again, I might be able to deal with the color in some way, but that would not change the fact that I do not feel the extra 18mm gave me a whole lot more. We'll see. I have double-checked: I do have the same version of the scope as you. The 66mm: is it fpl-51 as well? 

[grjsk]

20 hours ago, Louis D said:

Along these lines, I bought a 6" f/5 GSO Newtonian to see how it performs.  

I have a C6 SCT that occupies the same "role" as your 6" Newt. The difference is of course that the small C6 sits well on the same mount/tripod as the smaller refractors. 

[Ags]

Snap! I've got a C6 too 😀

I think the ZS66 is FPL51 but not 100% sure. There is a little CA, which seems cleaned up with the Celestron prism. I didn't know there was any CA until I tried the Celestron prism...

Agreed the LP90 is quite a lump. If money was no issue I would get a Borg 90.

Edited January 3, 2023 by Ags

[grjsk]

1 hour ago, Ags said:

Snap! I've got a C6 too 😀

I think the ZS66 is FPL51 but not 100% sure. There is a little CA, which seems cleaned up with the Celestron prism. I didn't know there was any CA until I tried the Celestron prism...

Agreed the LP90 is quite a lump. If money was no issue I would get a Borg 90.

Yeah, the Borg 90 seems like the obvious dream scope among lightweight and compact refractors, but I have never really seriously considered it because of the cost. If anything, my findings has actually made me want it less, even though it would solve my two biggest problems with the LP90, color and weight: the small increase in aperture simply isnt worth it to me at the moment. And it would’nt beat the 72mm on color anyway.  
 

there is a slight chance for clear skies tonight, perhaps I’ll try the LP90 with the Celestron prism.
 

 

[grjsk]

14 hours ago, grjsk said:

 

there is a slight chance for clear skies tonight, perhaps I’ll try the LP90 with the Celestron prism.
 

 

Clouds came of course. And the forecast for the next 7-10 days is also horrible. This has been a rather poor season so far. 1 (!) session in 2 months...