4inch Refractor musing

[spudlet]

Hi all,

I mentioned in my welcome thread I was looking to get a 4inch refractor to take over from my old SV 80mm achro mainly for grab and go purposes - mainly visual use, but with the very occasional foray into imaging - and by occasional I guess you could say I am 95% visual, 5% imaging.

With that in mind, I don't need top of the line APO performance. I can't afford to pay top dollar for a Tak or TEC for example, but I'd like to try to minimise chromatic aberation and other impeffections within reason. I've suffered the past decade with scopes that give off oodles of violet fringing for example and I'd like to move away from that if possible ! I don't like the yellow cast my M-V filter gives me in my 6inch or 80mm scoeps!

I plan to mount it on the EQ4 mount I have spare for quick grab and go.

My main quesiton is with these so-called APO scopes (definition of apo seems to have been warped in recent years I think) - Altair 102mm F6.5 Super ED Triplet for example vs the Altair 115mm F7 triplet - is the light gain introuced by the extra 13mm of aperture significant? The 115 is heavier, and longer, and in those terms less grab and go friendly, but if the view of, say, Saturn on a night of good seeing was noticably more detailed, I'd happily take the small hit in convenience.

Secondly I'm wondering about the ED doublets SV110, AT111, Altair StarWave 110, etc . They're cheaper than the triplet described above, significantly so to be worth considering I think, but my goal is to minimise CA on bright objects, is the false colour through the Ed doublets significantly worse than that of the 102 or 115 triplet above.

I don't really care about the glass involved as long as it provides a better chroma corrected scope than what I have now, and be in a reasonably convenient short(ish) package.

Any insight? My budget tops out around £1200 mark.

[nightfisher]

Just a suggestion, a lot of people rate the skywatcher Evostar 100 Ed as a very good telescope, at £625 new including a focal reducer, its good value, though having never looked through one, i cant say how much CA it will show, and reading your post it seems you are looking or zero CA

[spudlet]

HI thanks for the suggestion, I admit I had dismissed the 100ED on the grounds it has a longer focal length, but perhaps 100mm longer than, for example the 115 isn't significant. But I also want to retain wide field views if possible, so I think I am still looking toward a short focal ratio.

Re: zero CA, I'm not expecting zero CA as such, I think for that you need to spend much much more than my head and my wife would ever allow I do want to minimise it and other aberration in the scope as much as my budget may allow though.

it is a shame there are no shops near me to go and view some scopes, closest I think is London, but that is a day trip to get there and back again! Astrofest isn't far away, if I can hold out until then..

[DirkSteele]

A member of my astronomy club owns the Altair 115 and it performs very well and is certainly worthy of being classed as an APO refractor (many telescopes really are not but labelled as such). It is a clone of the TMB 115 (which I own and is still produced by APM Telescopes in Germany, but will cost in excess of £3000 for the OTA). While there is a small performance drop compared to my TMB, I am not sure I could put my hand on my heart and swear that my TMB is 3x the scope, which is about the price differential, especially as that performance differential only really manifests itself on nights of the most tranquil seeing.

The Altair 115 is quite a bit longer though than my TMB 115 (despite both being F/7) as my scope actually "telescopes" at the back. However, both are considerably longer (and heavier) than my APM TMB 105 F/6.2 and this is the scope I use as my truly portable system (including as carry on luggage on a plane).

Unfortunately I do not have any experience with the Altair 102 F/6.5, but I am sure there will be an SGL member who owns one and will be happy to share their thoughts.

There is not getting away from the fact that well designed (with well mated lens elements) triplet will have better colour correction than a doublet, but I was fortunate enough recently to use a new design APM doublet (152mm at F/8 I believe), and the performance on false colour was extraordinary (i.e. I could not see any). I think this new lens arrangement was designed by Massimo Riccardi. So it may be worth exploring the available doublets on the market, especially as you state that you are not married to a completely colour free image.

You are certainly right not to care about the glass type of the ED element. Whether it be FPL-53, OK-4, or pure fluorite crystal, any optical designer will tell you it matters less than the design of each of the lens elements, the accuracy of their manufacture, and how well the elements are mated together to control chromatic aberration.

[spudlet]

Thanks for the reply. Sounds like the Altair 115 may be the scope I'm looking for, but equally something a little smaller with the obvious portabiity advantages may suit too. It is a hard to deiscion to make - it was a lot more straight forward to pick a scope ten years ago I think!

[Mr Fiery Jack]

Just a suggestion, a lot of people rate the skywatcher Evostar 100 Ed as a very good telescope, at £625 new including a focal reducer, its good value, though having never looked through one, i cant say how much CA it will show, and reading your post it seems you are looking or zero CA

I have an ED100, a very nice indeed, I don't care what anyone says, these ED scopes are Aporomatic scopes. Easily up-graded too should you wish to change the focuser, a moonlight is a nice up-grade later on.

[Bizibilder]

I have both the 120mm Evostar achromat and ED APO and there is certainly a difference in the ca between them. The achromat does show purple fringing when viewing very bright objects like Venus but is, to me anyway, acceptable on the other planets and stars. The ED APO is an entirely different animal and shows no discernable colour fringing with any object that I have looked at. It is also my normal scope for photography and again has shown itself to be false colour free - within the limits of my photographic abilities!

If you are a visual observer then i would suggest the ED APO range of scopes are well worth considering - the slightly longer focal length of the ED 100 may well be useful if you want to observe Solar system objects. (usual disclaimer applies).

[John]

I have 102mm and 120mm ED doublets. The 102mm is made by Vixen and has a fast focal ratio of F/6.5. The 120mm is a Skywatcher and is F/7.5. Both show very small "splashes" of chromatic aberration around the brightest stars but I can't see any along the lunar limb or around Saturn and Jupiter. Venus shows around the same amount as a bright star but it does not get in the way of observations. The Vixen weighs around 3.5kg and the Skywatcher around 6.5kg.

I've owned the Skywatcher ED100 and ED80 in the past and they, to my eyes, were effectively CA free for visual observing. I've also owned a William Optics Megrez 90 (another ED doublet) and that showed a similar amount of CA to my current 102mm and 120mm's.

Personally I've not felt the need to consider triplet refractors as I'm purely a visual astronomer. I feel my 102mm and 120mm ED refractors deliver everything you could ask for from scopes of their aperture, when the conditions allow, so I'm very happy with them

I've owned decent achromatic refractors from 80mm to 152mm in the past by Vixen, TAL, Skywatcher and Meade but the more compact format that the ED doublet lenses allow suits me very well now.

CA is just one aberration of course and arguably not the most detrimental if the objective lens is otherwise well figured. Spherical aberration is not uncommon with lower cost refractors and can really reduce sharpness and contrast at high magnifications.

[spudlet]

I've always wondered if my evostar 150 has a poor lens, but I've never really bothered to check it, I know it has delivered me some breath taking views of DSOs in years past that I'll never forget, and that is all that really matters I suppose.

I'd rather avoid aftermarket upgrades if possible. Moonlite focussers look lovely, but I'd only screw it up when fitting I'm sure. I got a 2inch diagonal stuck fast in my scope once, couldn't tell you how but I had to resort to something like WD40 to get it out in the end!

[John]

I have a Moonlite on my ED120 and it's a great focuser and really easy to fit - just 3 screws. One thing it does really well is to ensure that the optical axis of the focuser matches precisely that of the objective lens. This is often not the case with the standard focusers supplied I've found.

Testing and adjusting the collimation of the Evostar 150's is straightforward enough if you have a simple cheshire collimation eyepiece and the scope has the objective lens cell that is adjustable for tilt, as many do. I use this approach:

http://www.spacealberta.com/equipment/refractor/collimate.htm

[ollypenrice]

I have an ED100, a very nice indeed, I don't care what anyone says, these ED scopes are Aporomatic scopes. Easily up-graded too should you wish to change the focuser, a moonlight is a nice up-grade later on.

I think that's fair enough in visual use. Where the doublets (and indeed some of the triplets like the Meade 127) show their limitations is in imaging, where the blue can bloat somewhat. In fact even the TEC140 produces larger stars in blue than in the other channels. The Sky 90 and FS60 doublets from Takahashi are not as good as they should be in this respect either. The strength of the ED100 in visual use is its slower than average F ratio, hence its famously good colour correction. It does make an attractive visual scope, for sure. The one question mark over all the SW EDs is the way the focuser pushes into the tube and is retained by radial screws. This sometimes leads to non orthogonal focusers. I believe that focusers should always thread onto tubes. But hey-ho, they are bargain scopes with proven abilities.

The 4 inch refractor didn't become a classic tool for nothing.

Olly

[spudlet]

Testing and adjusting the collimation of the Evostar 150's is straightforward enough if you have a simple cheshire collimation eyepiece and the scope has the objective lens cell that is adjustable for tilt, as many do. I use this approach:

http://www.spacealbe...r/collimate.htm

That does make it look easy, but I'm pretty sure my unit is the original non-adjustable version of the lens cell. My curiosity has been piqued though, so I may well get a cheshire just to see. It can come in handy for other scopes I have or may have in the future.

[nightfisher]

If you have an Evostar 150 f8, and a decent mount to hold it.......just puta new focuser on it and get a new lease of life from it, the original syntra focuser leaves a lot to be desired, especially when it has a few years on it, i had a 6" that the focuser was so bad it made stars look like exclamation marks

[andrew63]

A 4" or there about refractor is such a lovely scope in use - very 'easy' to use little cool down and enough aperture to cover a few bases. One thing i would say, those 110 to 115 triplets are more weighty than the doublets and i would be concerned if you want to put it on a EQ4 on aluminium tripod as you indicate for a grab and go. When you add diagonal, finder, eyepiece rings your probably getting on for 10kg. A 4" doublet will be much less of a handful.

As for CA my 102ED is F7 - and while not the highest level of glass correction - there is no hint of colour along the lunar rim. Good luck with your choice.

andrew

[spudlet]

Thank you for all the input, guys. The choice is agonising

[nightfisher]

Thank you for all the input, guys. The choice is agonising

looks like this post went wrong

[spudlet]

looks like this post went wrong

Just a wee bit ! No idea what happened there. I suspect my machine was "having a moment"

[rwilkey]

My findings on the ED Doublet is that as far as visual astronomy is concerned, there is no noticeable CA, I believe it is only in AP that this becomes apparent, when a triplet would be ideal.

[estwing]

there's a 4" vixen on ebay at £350...second-hand in the rother-valley ebay shop with tripod!

[spudlet]

I do have some AP urges, so to speak, and from time to time I try to take basic images of moon, and the brighter planets, especially when in conjuction. A doublet would probably suffice for that I think, but I'd like to leave the door open for more advanced imaging later on, so I'm putting in the extra pennies for the triplet now.

I took the plunge on a 115 triplet from Altair after much thought. I wasn't entirely prepared for the size of it, huge! It came on Saturday, I am to blame for the cloud cover - desperate to give it a test!

All the contributions to this thread gave me plenty of food for thought, thanks to all - wishing you a merry Christmas.

[Mr Fiery Jack]

Well enjoy the scope and a very Merry Christmas to you and yours.

[Marki]

Cool! I hope you enjoy the scope and get a chance to use it over the holiday! Look forward to hearing how you get on with this! Merry Christmas and clear skies!

[spudlet]

Cool! I hope you enjoy the scope and get a chance to use it over the holiday! Look forward to hearing how you get on with this! Merry Christmas and clear skies!

Had it out briefly this evening, mounted on the CG5-GT, seemed to handle the load well. Didn't bother with the goto, the weather looked like it'd take a turn for the worse at any moment to - probably before completing a 3 star alignment ! so i swung it immediately to jupiter. The view was great, liight clouds passing through the fov more or less consntantly didn't spoil it - I would go so far as to say it is the best view of the planet i've had in ten years - so far so very happy.

i need to buy a new diagonal though, I was forced to use a really old 1,25inch diagonal, the freebie that came with my 150. The 2inch WO diagonal I have simply won't fit into the focuser of the new scope. It's a very tight fit on the other scope - so much so i've had it stuck before - and so i assume it to be a defect of that diagonal.

[Mr Onions]

Great to hear you're pleased with it.

You do know that photos of a new telescope are mandatory,don't you?

[Mr Onions]

I once bought a 2" errecting diagonal from WO and that wouldn't fit my TV Oracle 3, either.