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


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4 minutes ago, paulastro said:

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.

 

 

You are right, of course, Paul. To be fair I have written on a number of occasions, to the point of being boring perhaps, about the giant slaying abilities of the humble Heritage 130 and 150p scopes, particularly where resolution is concerned eg splitting Zeta Herc more clearly than an FL102S.

In the four inch camp I would put the arguments of convenience, consistency, stability of image and that hard to define aesthetic quality to the views.

If I had to bet on a particular night which scope would give the best planetary views regardless of seeing, the 4” is likely to be the one. They don’t require heavy mounts, are quick to cool, pretty bomb proof in terms of collimation and capable of anything from 3 or 4 degree widefields to well over x200 planetary, lunar and solar views. I think it’s the versatility and convenience which make them so appealing, as much as anything else.

But yes, it should be made clear that they cannot defy the laws of physics, however much we think they can.

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17 minutes ago, Stu said:

You are right, of course, Paul. To be fair I have written on a number of occasions, to the point of being boring perhaps, about the giant slaying abilities of the humble Heritage 130 and 150p scopes, particularly where resolution is concerned eg splitting Zeta Herc more clearly than an FL102S.

In the four inch camp I would put the arguments of convenience, consistency, stability of image and that hard to define aesthetic quality to the views.

If I had to bet on a particular night which scope would give the best planetary views regardless of seeing, the 4” is likely to be the one. They don’t require heavy mounts, are quick to cool, pretty bomb proof in terms of collimation and capable of anything from 3 or 4 degree widefields to well over x200 planetary, lunar and solar views. I think it’s the versatility and convenience which make them so appealing, as much as anything else.

But yes, it should be made clear that they cannot defy the laws of physics, however much we think they can.

To be fair to the 130pds, it mounts really well on an azgti (I find it more stable than the az5) and if you get the collimation good you can easily get above x200.

I do find that without filters on planets it can be a bit washed out, but I spent more on flocking my scope than for the yellow and moon glow that sorted that out.

The key is though the collimation which can be a bit of a mare, especially with the 130pds because the screws have a habit of cutting into the aluminium on the back of the secondary which gives it a preference of sorts to where it wants to collimate.  This can be fixed by putting a bit of plastic from a milk bottle between the screws and the aluminium though.  Just hope it doesn't lose it.

Over on cloudy nights they call the 130pds the 'apo killer'.  But comparing scopes of different types is a bit like comparing tools.  Yes, you can use an adjustable spanner to drive in nails, but a hammer is better.

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I've made a similar transition from a 130mm reflector to a 102mm f7 ED refractor, albeit via several other flavours in-between and do not regret the purchase.

The FPL-51 f7 doublet I own is in-focus, pretty much devoid of the dreaded chromatic aberration mentioned here and good for 200x. I've only ever experienced obvious (yet very slight) fringing when zoomed in on an LED street lamp and Sirius is a challenge. 

I considered the telescope a bargain, but the new price has recently increased to £600, moving it closer to the FPL-53 so if choosing now, I'd be more inclined to save an extra £300 for the Starfield or similar. After all, the experts must be right? I have thought about "upgrading" but do wonder if I'll kick myself if there isn't a huge improvement over my £495 scope... 🙄

Do not expect to see more. What you will see is greatly improved contrast, a cleaner view so to speak and obviously no diffraction spikes. These refractors do have other benefits for instance the focuser is a joy to use and they're less prone to dewing than a reflector.

 

Bear in mind, I call my AZ5 'Wobbly Bob' for good reason. When using medium-high eyepieces and the refractor, there is annoying focusing vibration even with the 1.75" steel tripod (essential) upgrade. My most enjoyable longer sessions have been when the refractor is held incredibly solidly on my Celestron AVX.

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To be honest, and especially if you have reasonably dark skies you may be better off with something like a Celestron Starsense dob for the money you're willing to fork out. That's for you to decide what your priorities are.

Also if dobs aren't for you, and if all you have is the AZ5, I'd seriously think about keeping the 130mm and save for a sturdier mount. A good mount really increases your options.

Edited by ScouseSpaceCadet
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 Ever since I started out in this hobby, a long lime ago now, I've tried to make sketches of the views I see through the eyepiece.  One thing that always frightened me was the Moon as it was so complex. Where to start and where to finish a lunar sketch is difficult, as is how much detail to include in the sketch. To help myself overcome these problems, I decided to make simple sketches of crater floors and the floors of walled plane's.  While sketching the floor of a crater named Werner using my 100mm refractor, I noticed what looked like a fine rille, so I drew it. The next time I came to Werner, I noticed another, and another, which I drew, but found that in conversation with more experienced selenographers, these rilles did not exist. They were not recorded on the maps I had, and images also failed to hint at them. Observing Werner through my 200mm Newtonian and also through a 10 Dob, revealed no hint of the rilles, yet each time I aimed my 100mm refractor at Werner, there they were, as fine as the lightest razor cut and visible even in full Sun. Was I imagining these rilles, or was I simply nuts?  I can hear you now shouting "Yes"!   Eventually I recieved an e-mail from a BAA member who had seen my sketches and decided to capture Werner. He sent me his images, and low and behold there were my fine little rilles.  Just recently I've acquired a new Atlas, The Duplex Moon Atlas, which is highly detailed, and on map 18 there is Werner and with careful study there also are my little rilles.  Seeing these rilles in the atlas will be a challenge in itself. They look like collapsed lava tubes and are far from obvious.  So why are these rilles easier to see in a 4" refractor than in the Atlas, and why do the larger aperture reflectors not even hint at them? Then if a small refractor reveals detail not seen in scopes with greater theoretical resolution and light crasp, how can it be said that a large Dobsonian will always show more than a good refractor?  My personal feeling is that with greater light grasp comes a reduction in fine definition with regard to already bright objects such as the Moon, and so the resolution gain which should make it easier to detect fine linear detail is hindered by the increase in light gathering. I've noticed a similar effect with regard the the rings of Saturn, where a superb view of the planet was presented through an 8" Dob, yet a 120ED standing alongside the 8" gave a much more intricate view of the ring detail.  So the power of a refractor lies in its sharp, high contrast, high definition performance and not in its resolution and light gathering, and in this they can deliver beyond their presumed capabilities; and so Bigger Is Not Always Better.

 

Edited by mikeDnight
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Great story Mike. I can’t compare views between a 4-5” refractor and a larger Newtonian, but I can compare them with those of 6” and 8” SCTs. In my experience I would choose the refractor every time - whether FPL-51 or 53/equivalent. But then I’m city based, and most interested in the planets, Sun and Moon. If I had access to dark skies, I would certainly have a bigger reflector to complement my refractors. 

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Thanks everyone for the input, suspect there are slightly too many responses now to do post by post replies, but thought I'd chime in before we descend into an astronomical version of West Side Story and things get really ugly between the Fracs and the Newts 😀

I'll add a couple of clarifications to my original post, which I didn't mention first time round as I had no idea the discussion would go quite so deep.

- A 4 inch refractor was always the plan. My first telescope was/is a 70mm Celestron Traveller, which I'd never used as much as I should, mainly because the lack of fine adjustment on a photo tripod killed the fun of. So, after a load of research, the end point was always going to be one of the new 4" refractors on a half-decent mount, but rather than blow the cash in a one'r I set out to get a mount first, just to check that fine adjustment was as useful as I'd hope with the traveller before splashing out on the better scope. I was about to pull the trigger on a mount and tripod when the 130 P-DS with mount came up on FB marketplace, and I bit their hand off to get it. So owning a Newtonian was a complete accident, but it seemed decent enough to act as a stop gap for a few months while I got into the hobby. And, yes, fine adjustment proved transformational to my viewing pleasure.

- A couple of other reasons that I want a Refractor (as well as the portability, convenience, etc that others have suggested) - can be used for terrestrial viewing, and doesn't flip things on both axis (just getting into star atlases, and the double reversal of Newtonians is a PITA for me). 

- I wear glasses and have astigmatisms in both eyes, so require LER eyepieces with any telescope. When you factor this in  with the quality/design of eyepieces required to perform well in a fast Newtonian, the range is really limited, and getting pricey. A fast Newtonian can never really be considered to be a remotely cheap scope for someone who wears glasses, assuming they want an eyepiece or two.

-The 130 P-DS is an astrograph first and a visual scope second (it seems that most of the sub f6 Newtonians on the market are also designed to this brief also). I did toy with buying a coma corrector to see if it helped, but my recessive common-sense gene kicked in at this point and said "it wasn't what you wanted, it doesn't perform like you need it to, so move it on". Likewise this also applies to wasting time flocking the interior or any other mini-projects which wouldn't change the fact it was a cheap fast Newtonian, and which would also probably knock money off the resale value (it's a really clean scope, and I want to move it on in original condition).

- I remain unshakeable in my belief that a f7 refractor will be easier on eyepieces than a f5 Newtonian. Even with my "best" eyepiece ( 30mm APM UFF which I got to see what the best the 130P-DS could do would be) I've had the sensation that its focus is on a knife-edge and if my eye strays 1 micron too far forward then it will become flaky. I've done some reading and found others who confirm this effect from fast scopes, especially Newtonians, where the eyepieces are "nervous" or "unhappy" despite making a fair image. Agree that the consensus is that f8 is the point of true "eyepiece unfussiness", but I humbly assert that f7 is a lot closer to f8 than f5 is.

- Yes, appreciate the AZ5 is the weakest of the mid-range alt-az mounts. Recently moved it onto the 1.75 steel tripod after I picked up on on ebay, transformed the stability, It'll do for a year or two, then I can get a Skytee or similar for the tripod and add a slow Newtonian to the setup for DSOs, should I feel the urge (though, on this topic, I'm pretty disappointed there's only one 6" f8 readily available these days, and seemingly no 8"ers above f6. Seems like the manufacturers no longer car about visual observers).

As for the "dangerous ideas" being floated (sheesh...) I'm aware I'm not buying Hubble. But I do like visual quality, and appreciate quality optics in photography e.g. Nikkor vs Hoya/Sigma. Guessing this is the same sort of ballpark as FPL51 vs FPL53

Thanks all, I'll let you know what I go for

cheers,

Dave

 

 

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In my post,  the gist was nothing to do with  'Fracts v Newts', only that a 4inch ED/Apo is not the best telescope scope for all objects on all occassions. Many people have learnt this and have more than one  scope anyway.  I've owned and used just about all types of scopes of all types and sizes over the years and enjoyed using  them all.  I currently have an  8inch Dob, 80mm ED doublet and 5inch SC.

Horses for courses, they each do at least one thing in which they excel over the others.

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23 hours ago, paulastro said:

In my post,  the gist was nothing to do with  'Fracts v Newts', only that a 4inch ED/Apo is not the best telescope scope for all objects on all occassions.

As an example, globular clusters in particular simply will not resolve at all in my 90mm APO FPL-53 triplet but resolve nicely in my 8" Dob.

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On 11/02/2023 at 13:12, paulastro said:

On some occasions more 'objective' advice would be more helpful.

 

 

 I found this fairly objective advice from an old Unitron advert shown in Sky & Telescope back in the 1950's that might help. :icon_cyclops:

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Reflectors did have a lot of problems back then.  Long focal ratio mirrors led to long, unwieldy OTAs and massive EQ mounts.  The ideas for Dobsonian mounts and EQ platforms hadn't yet dawned on anyone.  I'm not sure why short f-ratio mirrors took so long to be adopted even after the Dob mount and EQ platforms became popular.  Perhaps the eyepieces of the day looked terrible in them?  I don't think any new mirror grinding or testing methodologies have been invented for a long time.

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  • 3 weeks later...

Quick update to this - whilst I was weighing up the pro's and con's of FPL51 Vs FPL53 ,fate intervened and I stumbled across the attached on Astrosellbuy (or is it buysell...).

An Altair 102 F7 EDT APO triplet, picked up for slightly less than a new FPL53 doublet. A credit to it's former keeper. Just rigged up this afternoon, expecting clear skies late April or so...

 

PXL_20230305_155516905.thumb.jpg.88504f129d6d0edbf7ff5cdd4605f111.jpgPXL_20230305_154550661.PORTRAIT.thumb.jpg.24d7c32c121e264890d8421f0df446fc.jpg

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2 minutes ago, Dave-P said:

Quick update to this - whilst I was weighing up the pro's and con's of FPL51 Vs FPL53 ,fate intervened and I stumbled across the attached on Astrosellbuy (or is it buysell...).

An Altair 102 F7 EDT APO triplet, picked up for slightly less than a new FPL53 doublet. A credit to it's former keeper. Just rigged up this afternoon, expecting clear skies late April or so...

 

PXL_20230305_155516905.thumb.jpg.88504f129d6d0edbf7ff5cdd4605f111.jpgPXL_20230305_154550661.PORTRAIT.thumb.jpg.24d7c32c121e264890d8421f0df446fc.jpg

That is looking very swish!

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