What refractor to get

[johninderby]

Maybe? 🤔 but not sure.

Perhaps using a tall dovetail plate would move the OTA out enough to clear.

https://www.firstlightoptics.com/adm-vixen-type-v-series/adm-vixen-type-dovetail-with-wide-top.html

 

[Neil H]

I will have to try it and see i hope it wont as it is shorter and thinner 200mm by 1200mm is a big scope the bresser is 102 by 1000 so should not hit  

Buy it and see fingers crossed

[johninderby]

At least it won’t be hard to fix if it does either by fitting a pillar extension or a tall dovetail plate.👍🏻

[Neil H]

Yes John your spot on  i am trying every way not to fit extension  as it will be nice to swap telescopes  if i want without unbolting extension etc 

[johninderby]

And the extensions are very heavy. 🙂

[Louis D]

1 hour ago, Neil H said:

John what CA ?

 

1 hour ago, johninderby said:

CA (chromatic aberation) or false colur becomes more of a problem the shorter the focal length with a achromat which is more important for lunar / planetary observing as you will be using higher magnifications.  Not such a problem with DSOs at lower magnification.

CA (chromatic aberration) is a function of aperture and focal ratio.  Focal length alone won't give you any indication of a scope's CA.  The following table shows how CA behaves as a function of aperture and focal ratio:

You can cross reference individual telescope's specs with this table to get a feel for how good or bad the CA will be for it.  Additionally, moving to FPL-51 or FPL-53 doublets or even triplets allows the green and yellow areas to slide to the left.

Field curvature is strictly dependent on focal length.  It's roughly FL/3.

Edited September 8, 2020 by Louis D

[johninderby]

If the aperture remains the same it is the focal length that affects the amount of CA. If you change the aperture then that does effect the amount of CA as well.

Further reading on the subject.

http://www.maa.clell.de/Scholar/chromatic_aberration.html

[Louis D]

1 hour ago, johninderby said:

If the aperture remains the same it is the focal length that affects the amount of CA. If you change the aperture then that does effect the amount of CA as well.

Further reading on the subject.

http://www.maa.clell.de/Scholar/chromatic_aberration.html

But your original post on the subject simply stated that "false colur becomes more of a problem the shorter the focal length" without reference to holding the aperture constant.  This made it sound like any shorter focal length scope will have higher CA than any longer focal length scope as is the case for field curvature (shorter always has more FC for a simple doublet design regardless of aperture).  More accurately, the shorter the focal ratio (or commensurately, focal length) for a given aperture increases the amount of CA.

Edited September 8, 2020 by Louis D

[Alan64]

Longer refractors do benefit from pier-extensions, and for more comfortable eye-placements.

These issues would vanish into thin air with a Maksutov instead.  No false-colour, no diffraction-flares or -spikes, and with the smallest secondary-obstruction among mirrored telescopes per its length, and longer even.

A refractor like this one would be ideal... https://www.altairastro.com/starwave-ascent-102ed-f7-refractor-telescope-geared-focuser-468-p.asp

Apparently, it's quite popular.  I'd save up for that one.  By that time they may be back in stock.  If I'm not mistaken, it's the same one sold here...

https://www.teleskop-express.de/shop/product_info.php/info/p4964_TS-Optics-ED-102-mm-f-7-Refractor-Telescope-with-2-5--R-P-focuser.html

...and here... https://www.astronomics.com/astro-tech-at102ed-4-f-7-ed-refractor-ota.html?___SID=U

That would be all the refractor one would ever need.  It may also be the only refractor ever acquired.

[Deadlake]

This one gets reviews, f/11 to the one above

https://www.teleskop-express.de/shop/product_info.php/info/p11262_TS-Optics-102-mm-f-11-ED-Refractor-with-2-5--RAP-Focuser.html

hence less CA. I did consider it when going for a 100 mm refractor, but decided I'd go with better quality glass and a more compact length.

 

 

Edited September 9, 2020 by Deadlake

[johninderby]

Same as the Altair version. .

https://www.harrisontelescopes.co.uk/acatalog/altair-starwave-f11ed.html

[John]

All way over the original posters intended budget of £280, unfortunately.

 

[Neil H]

Nice refractors but cant do that price just paid out 6k so a bit short now 

I did message astroboot they said they only have whats on the site so none there 

[Neil H]

Hi guys I found bresser127t for sale for£270 shall I give it a miss and stick with the 102

 

 

[johninderby]

The 127L is a very well regarded scope but bigger and heavier of course. 8kg v 5kg. Think you would need the pillar extenstion with it.

If the size isn’t a problem then yes worthy of consideration.

 

Edited September 9, 2020 by johninderby

[Neil H]

Hi John that is what I was thinking not much of a saving £309 new and would have to get extension it's as long as my vx8L 

[Rob]

Love my 127L, But as John says be ready for length of the OTA. I do have an extension (not shown here) 16" one plus CG5 tripod with 2" steel legs

 

Edited September 9, 2020 by Rob

[Neil H]

Looks really nice how comes you don't need an extension I was hoping on my HEQ5 it would not need one

[Rob]

3 minutes ago, Neil H said:

Looks really nice how comes you don't need an extension I was hoping on my HEQ5 it would not need one

Thanks. Only because I like to not extend the tripod legs.. plus I'm a 6ft 3in high fella 😄

Edited September 9, 2020 by Rob

[johninderby]

Partly depends on if you want to point your scope at the zenith or not. 

Long scopes do need a bit pf planning to avoid the legs if not using an extension. BTW the 127L isn’t a long scope. This is 🙂

[Rob]

Yeah!.. big red is a long'n for sure

[Neil H]

God your knock the moon out of orbit