Looking at getting a 102mm f7 refractor - any reason to choose fpl53 for purely visual use?

[Dave-P]

Hi all, currently looking at getting a lower-end 102mm ED/APO for purely visual use.

Currently using a Skywatcher f5 130P-DS Newtonian, which I got a great deal on used, though I bought mainly for the ccompanying AZ5 mount (whole kit was about the new price of the mount). Actually been really enjoying it as a first proper scope for the last few months, bit finding it doesn't play well with some cheaper eyepieces, so going to move the OTA on. A refractor was always the long-term plan, and I figure that at f7 the 102mm scopes will be a lot easier on the eyepieces.

So I'm aware that most of the current 102mm scopes are closely related, and the two main doublet flavours would be FPL51 ED/semi-APO and FPL53 APO, going for around £600 and £900 respectively. Just wondering if, for purely visual observation, there would be any compelling reason to choose the more expensive glass (apart from knowing it exists). Thinking of things like improved contrast, sharpness, eyepiece compatibility. Figure lower CA is the obvious advantage. It's going to be a long-term purchase as a do-everything scope, though at the moment I'm finding open clusters much more entertaining than the planets.

Any thoughts appreciated.

[Stu]

If it’s a long term purchase, and you can afford it then I say go for the more expensive glass. There is something wonderful about a decent quality 4” apo refractor so why not get the best you can. Definitely noticeable even for visual.

[Mr Spock]

If you are considering the same FPL-53 scope quite a few of us have now (under various labels), then on stars it is completely free of false colour, even at high magnifications. The one I have is also free from spherical aberration, quite important when it comes to sharpness. It can turn its hand to the moon and planets quite nicely, though I did get mine primarily for double stars. It also makes a good solar scope too!

Unless you are on a tight budget the extra few hundred is worth it. These 102mm scopes are really quite versatile and I suspect you'll use it more than you think.

[Ratlet]

Sorry to jump in, but in terms of light gathering how would a 4" compare to the 130pds?  Would the lack of secondary put them on a par?

I also have a 130pds and have been pondering an upgrade path and was wondering abou possibly seeing what the buzz about refractors is about.  My only concern is the 130pds does such a good job on DSO for visual.  That and John Dobson whispering sweet nothings In my ear...

[Dave-P]

I'm figuring that there won't be a lot in it and that, in a funny sort of way, the 130 p-ds and 102 f7 refractor are essentially the same scope with differing optical systems. The PDS has a big secondary for it's 2" focuser, though it will have just a smidge more aperture. Focal length is practically identical, so eyepieces will work the same, just hoping that the hike in optical quality would be fairly staggering.

 Though, all assumption at this stage.

[Stu]

3 minutes ago, Dave-P said:

I'm figuring that there won't be a lot in it and that, in a funny sort of way, the 130 p-ds and 102 f7 refractor are essentially the same scope with differing optical systems. The PDS has a big secondary for it's 2" focuser, though it will have just a smidge more aperture. Focal length is practically identical, so eyepieces will work the same, just hoping that the hike in optical quality would be fairly staggering.

 Though, all assumption at this stage.

True enough. Each will offer you something different. DSO advantage on the 130, lovely star images and contrast on the refractor, plus the flexibility for Solar with a wedge etc.

[Sunshine]

Which 102 refractor did you have in mind? 

[Dave-P]

The 2 most readily available seem to be the TS for FPL51 and the Starfield for the FPL53. Both f7

[mikeDnight]

The FPL53 is fluorite glass, which when combined with the equally important mating element produces virtually no chromatic aberration. This means that pretty much all the light captured by the telescope will be concentrated in the Airy disc, leading to tight stars and bright images, even when viewing DSO's. What they lack in light grasp and resolution, refractors often make up for in fine definition and high contrast.  An FPL53 scope should have a noticable edge over the FPL51, and on a different level of performance to the 130P-DS. 

[Louis D]

FPL-51 in my 72ED pretty obviously shows violet fringing at higher powers.  It's only going to be worse at 102mm even at the same f-ratio.  My 90mm FPL-53 triplet doesn't show violet fringing at pretty much any power, but it has red-green out of focus images on either side of best focus that merge to white in focus.  It's definitely not reflector-like.  I don't know if FPL-53 doublets behave this way as well.  Overall, I much prefer the 90mm to the 72mm scope.

[Stu]

3 hours ago, mikeDnight said:

FPL53 is fluorite glass

That’s not quite true though, is it Mike? Good post from you on this thread explaining the differences. FPL-53 can be used to make excellent scopes, but it isn’t the same as fluorite crystal.

 

[Rob]

The fpl-53 is (as the others have said) going to provide the no CA image on bright objects. Having said that I own 2 x fpl-51 scopes and really enjoy them for visual. It also depends on your eyepiece quality, which can help provide toward a good/very good experience. I'd suggest for me .. 85% of the time I see no CA on various targets. I also use a contrast booster filter for planets, which is awsome in my 102ED F7.

All that being said. As always, see if you can try before you buy?. It may help your choice. Look for local Astro meets etc. 

Rob

[Dave-P]

1 hour ago, Rob said:

All that being said. As always, see if you can try before you buy?. It may help your choice. Look for local Astro meets etc. 

Rob

Not easy up here (Aberdeenshire) no retailers at all, and the local astronomy clubs just seem to do talks. So it'll be a bit of a punt, whatever I do. Though I am picking up from here that:

-Yes, I will notice seriously improved views over the 130 P-DS with either 102, and 

-Yes, there is a noticeable improvement in the FPL53 for visual (concern had been that it would be an "imaging thing")

 

So, thinking both glass types make worthy scopes and with the FPL53 the only thing I'd regret would be the price.

 

Hmmmmm.... 

 

[Mr Spock]

From the Starfield blurb:

"The Starfield ED102 APO doublet offers incredible performance in both imaging a visual astronomy. Using FPL53 (Ohara, Japan) and a lanthanum element, chromatic correction rivals more expensive triplet telescopes with the advantage of faster cool down time.  Taken to the highest magnification, images and views remain colour free."

I've used mine with a SLV 2.5mm - x286 - and can confirm there is no colour. It seems to be the combination of FPL-53 and Lanthanum which does it. As I understand it the Lanthanum removes the last bit of violet the FPL-53 might have had when mated with 'normal' glass.

An FPL-51 doublet isn't even close. The FPL-53 might be expensive and I took a deep breath when I paid out the £900, but, since then, I haven't regretted a single penny of it.

[mikeDnight]

5 hours ago, Stu said:

That’s not quite true though, is it Mike? Good post from you on this thread explaining the differences. FPL-53 can be used to make excellent scopes, but it isn’t the same as fluorite crystal.

 

You're right Stu, but I did say "fluorite glass" and not fluorite crystal. I can't remember the percentages but I'm pretty sure FPL53 is composed of more than 90% fluorite. It doesn't have quite the same properties as the laboratory grown fluorite crystal though. I suppose there's a price to pay either way.

[mikeDnight]

57 minutes ago, Dave-P said:

Not easy up here (Aberdeenshire) no retailers at all, and the local astronomy clubs just seem to do talks. So it'll be a bit of a punt, whatever I do. Though I am picking up from here that:

-Yes, I will notice seriously improved views over the 130 P-DS with either 102, and 

-Yes, there is a noticeable improvement in the FPL53 for visual (concern had been that it would be an "imaging thing")

 

So, thinking both glass types make worthy scopes and with the FPL53 the only thing I'd regret would be the price.

 

Hmmmmm.... 

 

 I know it's a big decision, but when you think that many will pay what I consider a real fortune, just for an annual season ticket to home games, or for annual membership to a golf club; the price of a nice refractor pales into insignificance, especially when you can have it for life. You only pay once, unless you get bitten by the apo bug. 

[badhex]

1 hour ago, Dave-P said:

with the FPL53 the only thing I'd regret would be the price.

When making a rather expensive tripod purchase last year, the wise man that is @Stu who owned the same tripod said to me that my purchase would be "a case of buy once, cry once" and he was absolutely correct.

I can only pass on this same sage advice to you regarding the Starfield; you'll sweat when clicking the purchase button, but once you look through it under a clear sky and see no false colour and pinpoint stars, you will not be thinking about the money you spent 🙂

Edited February 11, 2023 by badhex

[Stu]

1 hour ago, mikeDnight said:

You're right Stu, but I did say "fluorite glass" and not fluorite crystal. I can't remember the percentages but I'm pretty sure FPL53 is composed of more than 90% fluorite. It doesn't have quite the same properties as the laboratory grown fluorite crystal though. I suppose there's a price to pay either way.

True enough Mike. It’s the grown crystal bit that’s important.

Some relevant info here.

http://scopeviews.co.uk/Fluorite.htm
 

I think the general consensus remains that if you can afford it without putting yourself at financial risk then go for the fpl-53 scope, the cost difference over years of ownership will be irrelevant but the performance difference noticeable.

[Dave-P]

2 minutes ago, Stu said:

I think the general consensus remains that if you can afford it without putting yourself at financial risk then go for the fpl-53 scope, the cost difference over years of ownership will be irrelevant but the performance difference noticeable.

I'm fortunate enough that risk of financial ruin isn't a factor. Just performing due diligence to ensure I'm not being "that idiot" who's chasing the most expensive option for no good functional reason.

[paulastro]

44 minutes ago, Dave-P said:

I'm fortunate enough that risk of financial ruin isn't a factor. Just performing due diligence to ensure I'm not being "that idiot" who's chasing the most expensive option for no good functional reason.

Dave, I think there's a few people on SGL who might just come into that category - there, I've said it now! 😅

[Ricochet]

1 hour ago, Dave-P said:

Yes, I will notice seriously improved views over the 130 P-DS with either 102, and 

I think this is a very dangerous idea to take from the thread, especially the word "seriously", which could lead to having expectations which are too high and then disappointment with the real world performance of the scope. 

 

15 hours ago, Dave-P said:

at the moment I'm finding open clusters much more entertaining than the planets.

The question you need to ask yourself here is whether you are not enjoying the planets so much because your preference is for star clusters, or because you are finding the planetary performance of your 130PDS lacking. If you think that your lack of planetary enjoyment is substantially a scope issue, then you may find an increase in enjoyment by viewing the planets with a 102F7ED. Planetary observations tend to take place at high magnification which requires a small exit pupil. At small exit pupils the image quality is dominated by diffraction and so the image quality of the Newtonian will degrade more quickly as you decrease the exit pupil due to the increased diffraction from the secondary mirror and spider support. An f5 parabolic Newtonian is also quite sensitive to proper collimation as the coma free area of the scope is relatively small.

On the other hand, even if you subtract the area of the secondary mirror, account for reflectivity losses and assume that the refractor is used with a prism diagonal and neither have any losses, the 130PDS still has more light gathering capacity than the 102ED. This means that for star clusters the 130PDS will pick up fainter stars than the refractor. Where the refractor will have an edge is in the darkness of the background. The 102F7ED scopes are very well baffled to block out stray light whereas the 130PDS has no baffles and the interior wall is painted in a black paint that can charitably be called light grey. In addition lenses scatter less light than mirrors across the FoV. Adding a primary baffle to mask the edge of the mirror and flocking the inside of the tube will tighten up star images in the Newt and darken the background sky. Doing this will also result in an improvement in planetary views. Where the exact balance lies between brighter stars/brighter background and dimmer stars/darker background I can't say, but I would not count on any improvement by buying the frac. There is very nice presentation to how stars look through a frac but it's not necessarily an improvement of view.

If you really do have a preference for star clusters then I would consider how wide those clusters are. If you're talking <1.5° then you will see a substantial improvement by upgrading to an 8" Dob. An 8" scope has four times the light gathering capacity of a 4" scope and no quality of glass is going to make up for that difference on DSOs. For £900 you can go even larger and buy the 12" Stellalyra dob which is an even bigger step up. Of course, the bigger the scope, the heavier and more difficult it is to get out. A dob is not practical if you have a flight of stairs to negotiate and if you're feeling a bit tired it's a lot easier to pick up a small frac for a quick session. 

16 hours ago, Dave-P said:

finding it doesn't play well with some cheaper eyepieces

I'm also not sure that you will see much of an improvement in this regard. Typically if an eyepiece isn't rated for F5 the next step seems to be around the F8 mark. Sure, a cheaper eyepiece will perform better in a slower scope but it sounds like you will have to upgrade eyepieces whichever scope you have.

I'm not saying not to buy a 4" ED, as they are clearly lifetime scopes, just don't expect one to perform miracles.

[Stu]

1 hour ago, Dave-P said:

I'm fortunate enough that risk of financial ruin isn't a factor. Just performing due diligence to ensure I'm not being "that idiot" who's chasing the most expensive option for no good functional reason.

I think you are fairly safe there, I mean, you could be one of these nutters that spend a fortune on fluorite refractors after all…… 🤪

[Ratlet]

They know who they are..

[paulastro]

7 minutes ago, Ricochet said:

I think this is a very dangerous idea to take from the thread, especially the word "seriously", which could lead to having expectations which are too high and then disappointment with the real world performance of the scope. 

 

The question you need to ask yourself here is whether you are not enjoying the planets so much because your preference is for star clusters, or because you are finding the planetary performance of your 130PDS lacking. If you think that your lack of planetary enjoyment is substantially a scope issue, then you may find an increase in enjoyment by viewing the planets with a 102F7ED. Planetary observations tend to take place at high magnification which requires a small exit pupil. At small exit pupils the image quality is dominated by diffraction and so the image quality of the Newtonian will degrade more quickly as you decrease the exit pupil due to the increased diffraction from the secondary mirror and spider support. An f5 parabolic Newtonian is also quite sensitive to proper collimation as the coma free area of the scope is relatively small.

On the other hand, even if you subtract the area of the secondary mirror, account for reflectivity losses and assume that the refractor is used with a prism diagonal and neither have any losses, the 130PDS still has more light gathering capacity than the 102ED. This means that for star clusters the 130PDS will pick up fainter stars than the refractor. Where the refractor will have an edge is in the darkness of the background. The 102F7ED scopes are very well baffled to block out stray light whereas the 130PDS has no baffles and the interior wall is painted in a black paint that can charitably be called light grey. In addition lenses scatter less light than mirrors across the FoV. Adding a primary baffle to mask the edge of the mirror and flocking the inside of the tube will tighten up star images in the Newt and darken the background sky. Doing this will also result in an improvement in planetary views. Where the exact balance lies between brighter stars/brighter background and dimmer stars/darker background I can't say, but I would not count on any improvement by buying the frac. There is very nice presentation to how stars look through a frac but it's not necessarily an improvement of view.

If you really do have a preference for star clusters then I would consider how wide those clusters are. If you're talking <1.5° then you will see a substantial improvement by upgrading to an 8" Dob. An 8" scope has four times the light gathering capacity of a 4" scope and no quality of glass is going to make up for that difference on DSOs. For £900 you can go even larger and buy the 12" Stellalyra dob which is an even bigger step up. Of course, the bigger the scope, the heavier and more difficult it is to get out. A dob is not practical if you have a flight of stairs to negotiate and if you're feeling a bit tired it's a lot easier to pick up a small frac for a quick session. 

I'm also not sure that you will see much of an improvement in this regard. Typically if an eyepiece isn't rated for F5 the next step seems to be around the F8 mark. Sure, a cheaper eyepiece will perform better in a slower scope but it sounds like you will have to upgrade eyepieces whichever scope you have.

I'm not saying not to buy a 4" ED, as they are clearly lifetime scopes, just don't expect one to perform miracles.

This needed saying. At times   the writers of some posts about how much superior some 4 inch EDs /Fluorite are than most telescopes for just about any object need a reality check.

Despite all the superlatives written about 4 inch apos, the fact is any scope six inches and over (assuming they are of good optical quality and well collimated)  will be better for deep sky on most occassions - and for planetary as well when the seeing plays ball.

Of course, there is some humorous banter and joking about this on various forums - and I've joined in it myself on occassions.  However, for people new to the subject and wanting sound advice, I can't help but think there will be some people who may be misled by some of the hype.

On some occasions more objective advice would be more helpful.

 

 

[dweller25]

19 hours ago, Dave-P said:

Hi all, currently looking at getting a lower-end 102mm ED/APO for purely visual use.

Currently using a Skywatcher f5 130P-DS Newtonian, which I got a great deal on used, though I bought mainly for the ccompanying AZ5 mount (whole kit was about the new price of the mount). Actually been really enjoying it as a first proper scope for the last few months, bit finding it doesn't play well with some cheaper eyepieces, so going to move the OTA on. A refractor was always the long-term plan, and I figure that at f7 the 102mm scopes will be a lot easier on the eyepieces.

So I'm aware that most of the current 102mm scopes are closely related, and the two main doublet flavours would be FPL51 ED/semi-APO and FPL53 APO, going for around £600 and £900 respectively. Just wondering if, for purely visual observation, there would be any compelling reason to choose the more expensive glass (apart from knowing it exists). Thinking of things like improved contrast, sharpness, eyepiece compatibility. Figure lower CA is the obvious advantage. It's going to be a long-term purchase as a do-everything scope, though at the moment I'm finding open clusters much more entertaining than the planets.

Any thoughts appreciated.

I have found that you get what you pay for, so perhaps the FPL51 is cheaper for a reason.

There has been a lot of feedback on SGL about the FPL53 version - all of it very positive.

The choice is yours of course 👍 🙂

But remember a much cheaper 8” Dob will beat even  a 4” Fluorite Takahashi in all areas 😱

Edited February 11, 2023 by dweller25