Long Perng 90mm F5.5

[cometcatcher]

I've wanted one of these for a while. In fact I wanted one before I got my Kson ED80. The Kson, although it has minor CA, tends to throw bright rainbow flare from bright stars on photos, so saved my money and bought a LP. This is supposed to be the LP ED "APO" scope.

The good. Firstly the build and machine quality is awesome. Miles ahead of the Kson. The focuser is beautifully smooth and I can hit focus point with ease with the reduction.

The bad. So I slip in my trusty Nagler 13mm Type 6 eyepiece and point it around in daylight. What the ... What's with all the blue / magenta fringe I ask myself? This doesn't look like I'm looking through an ED scope.

The ugly. Maybe night time will be better. Eagerly I wait for nightfall and look for a few objects. CA on everything brighter than mag 4. But this was to be my main photo scope. I point to the Orion nebula and take a frame. Well, see the image for yourselves. Blue focus is about 1 light year away from red and green. And it throws rainbow flare from off axis stars as well.

Is this normal for this model or does mine need adjustment?

[AG Hybrid]

From what I read about fast ED doublets. That's about right. The TMB version of 92mm f5.5 is a triplet with a FPL-53 glass element, and every owner describes it as near perfect color correction. I seriously doubt the LP even has a FPL-51 element and has another type that's an equivalent that is almost as good.
A ED doublet "APO" at F5.5 will need to be impossibly well made to give near color free images. Just because it says ED APO doesn't mean it is.

I found that scope on Lee Andrew's website.
"ED APO 90mm f/5.5 doublet achromatic refractor"

Even he can't decide if its an APO or achromat.
No doubt however, I bet that telescope is fantastic at rich field observing. But, I would never put a camera to a ED Doublet that fast.

Get a SW ED80 instead. It has a FPL53 Element and long history of its quality and color correction. A SW ED Doublet will be my next scope purchase in fact.

[cometcatcher]

I realise that a doublet will have "some" CA still, but my little Kson ED80 is a doublet at F5.5 also, like the Long Perng, and it's colour correction while not perfect is far better than the LP. That's why I went for the LP, I thought it would be in the same class as they are both F5.5. I believe the Kson uses a FPL51 glass element.

Kson ED80 pic for comparison.

[AG Hybrid]

Ahh yeah the Kson is much better. A bit of blue balance adjustment and you got a real good shot there.

[brian nordstrom]

:) I had one of these a while ago and it was very good in all ways , especially the build , as you say .
It may be , by the looks of it you have a bad example ?
I have since sold mine and saved my pennies and got a 90mm Takahashi SKY90 and its f 5.5 is the same as the LP but the Flourite lense element is essentially perfect , zero CA or any other problems .
If I was you I would return it and ask for a replacement .
As I said mine was not perfect , but it was good . Better than what I see in your photo , thats for sure . These LP's have FPL51 which is pretty darn good glass .
It boils down to , if you want the best ,, " You gotta be prepaired to pay for it " sad but true :shrug:.
Send it back as it aint right .
Or as said here get an 80ED .( I have had one of these as well , great scopes , abit a little basic but awsome FPL53 optics )
Brian.

[MortonH]

Hi Kevin,

For a given lens design/glass type, CA gets worse as you increase aperture.

Agree with the above suggestion to get one of the Skywatcher/Saxon ED80 scopes with FPL53 element. They are effectively colour-free and lots of people here are using them for photography.

Morton

[cometcatcher]

Looking into my crystal ball I see a Skywatcher ED80 in my future lol. Darn.
Thought I would get away with this one.

In the meantime I discovered a photoshop trick that may get me by.

This is a repo of the Long Perng. I just feel that I shouldn't have to resort to tricks to get a good photo.

[brian nordstrom]

;) Great trick Kevin , wanna share it ? :lol: .
Good descision on the 80ED .
Brian.

[AG Hybrid]

I think I saw one on IS Classifieds right now for about $750.

[MortonH]

That rainbow shouldn't be happening. I'd try to return the LP. Did you buy it new from Andrews? Maybe they'll take a swap for a Skywatcher ED80.

[cometcatcher]

Brian, using photoshop CS2, I use the brush tool and select "saturation" in the mode box. Using the tool over stars takes out the blue but it also leaves a black hole. To fill in the hole I copy the original image, convert it to LAB colour. In the channels box I select "lightness" and copy to clipboard. Going back to the touched up image I paste the layer in from LAB colour selecting "luminance". Basically it's just a luminance overlay from the original.

Morton, yes from Andrews. Perhaps I will ring them next week. The rainbow flare is a worry.

[Larryp]

Hi Kevin
I had a LP 90mm. First one I took back because of the colour error and trying to look at Mars or Saturn was a disaster. The second one had a better lens, but still the same colour error. Nice enough on clusters and nebulae but useless for photography or anything with high magnification. A rubbish scope, in my opinion.
I contacted Long Perng direct about the problems and they sent me a table of glasses used in the lens. It is FPL51 and NBM51, and is the same lens as used in the Megrez 88mm, but the colour problem is probably worsened by squeezing an extra 2mm out of it.
I sold mine.

[cometcatcher]

Thanks for the info Larry.

I'm still trying to decide if mine is normal bad or extra bad lol. Question, I have the LP scope set up side by side with a Skywatcher 120mm F5 Achromat just looking outside. The contrast of the Skywatcher achro is far far superior to the LP. The LP looks smokey to look through by comparison. Now the Skywatcher is a 5 inch scope with an extra 30mm, faster optics at F5, an achro and produces better views? That can't be right.

[Larryp]

I would return it to Andrews and get something else, Kevin. Its a shame-as everyone says, they are beautifully built.

[cometcatcher]

This is probably comparing apples to oranges, but the SW120 achro and LP90 seem to have about the same CA. I think the LP has even more flare on top of the pole.

[Larryp]

It certainly does, Kevin. A really disappointing telescope. I remember looking at power lines with mine, and seeing the same halo around them. Now I have a NG 102 triplet, which of course has no colour error, and a Stellarview Nitehawk 80mm ed f7 doublet, which I have not yet seen any colour error in it.

[cometcatcher]

Okay I think mine might be extra faulty. This is an example of the low contrast views as it flares and looks smokey. I'm sending it back. This is unacceptable.

[Larryp]

Mine was better than that, Kevin. But no matter what you do, you cannot get away from the CA.:(

[cometcatcher]

Yeah I know. I'll get something else.

[Stardrifter_WA]

Kevin, I would check your consumer laws. In WA, if you buy something that doesn't do what it claims or doesn't perform as per specifications, you are protected by law and are entitled to a full refund or replacement. I should imagine consumer laws are similar in each state. IMO the rainbow flare is indicative of a problem with the optics, therefore would probably constitute a defect.

Cheers Peter

[cometcatcher]

Thanks Peter.

[brian nordstrom]

:) Kevin I would not worry at all about this problem ,,,,
Andrews will refund , replace or change this scope to another one of better quality without a second thought , I think they would rather know a problem exists than a customer not being 100% satisified ( bad press ) . :thumbsup: .
As I said it wont be a problem they are a good company to deal with in all ways .
Brian.

[cometcatcher]

No I'm not worried Brian. I think they are good people to deal with.

I'm just very disappointed that a scope so beautifully made has bad optics. The finish on the scope itself looks like a piece of fine polished furniture. I'm not used to that sort of quality. I'm more used to building my own reflectors which are more practical than beautiful. For instance I build my focusers out of PVC pipe fittings from the recycle centre. ;)

Perhaps my next scope should be more ugly and have a sloppy focuser. As long as it has good optics. :)

[casstony]

The Megrez 88 I have has very little CA, no purple fringe visually, but it does have to be carefully focused.

[Larryp]

Maybe LP used the left-over sub standard lenses in the LP90 after the Megrez 88 was discontinued.

[brian nordstrom]

:question: You might have a point there Laurie , as it seems to much of a coincidence that 2 of the last 3 of these were woefully sub standard , yours and Kevins , when mine was quite good and as said its bad that the lenses are so bad when the build quakity is really very good ? .
Luke at Andrews told me they only had 4 of these when I got mine and mine was the 1st one sold .( I wonder who got the 4th one ? )
Food for thought mate :thanx: .
Brian.

[brian nordstrom]

:sadeyes: Its such a shame the optics are not up to scratch in your scope Kevin ( and Laurie's ) when you look at this photo I just found of the LP90 that I had , to see the build quality , baffeling and coatings in this photo and this scope performed very well , this is a real shame .
Brian.

[cometcatcher]

I noticed the same thing Brian. The baffling and coatings look very nice.

Mine was the last one. The demo model actually.

[MrB]

Hi Kevin,

Thought you might like to see the same field through an f6 Stellarvue 70 ED(420mm FL) and Canon 60Da.
Obviously comparing apples and oranges, but the interesting feature I find is the rainbow which is quite pronounced in my image, even though there is very little colour error. There is some voilet halo evident but that is more due to my poor focus(the focuser is fubar, ruined by previous owner)

I think the rainbow is more an internal reflection problem rather than the lens. I am only guessing tho. I will eventually get some Protostar flocking in there to see if it makes a change.

I've made it to the same scale so you can do a side-by-side comparison. Please try to ignore the trailing ;)

[cometcatcher]

Mmm thanks Simon. The Kson rainbow flares there too. If you ever find a way to get rid of the flare let me know.