Skywatcher 130/900 - parabolic?

[Alfian]

Hi all,

I have been contemplating a "summer" project of mounting something  a little larger than the Tal1's 110mm OTA on the sturdy pedistal EQ mount. (another thread pending)  and thinking that this would probably be a 150mm F5 tube. However I'm a bit unsure of going "F5" preferring something F6 and slower. (Altair?) 

I have just come across this:

 http://www.teleskop-express.de/shop/product_info.php/info/p4778_Skywatcher-Explorer-130-f7-9-Optical-Tube---130-900mm-Newtonian.html

I have always assumed that this OTA had a spherical mirror, but TS seem to be making a point that it has a "sharp parabolic mirror".  It is also being sold (off?) at half price for 85 euros which currently equates to about  £64.

Given that TS are showing it "F7.9" which it obviously isn't, 900/130 = 6.9, are they also mistaken about the "parabolic" primary? perhaps I'll drop them an email.

Anyone any thoughts?

[nightfisher]

Is it the same as this ota on astroboot 900mm focal length

[ronin]

Jules, seems you have missed or lost a link.

As it say's Skywatcher I would err towards saying spherical, Celestron used to advertise that all their mirrors were parabolic so had it said Celestron it may have been parabolic, but even then a little dubious. Seem to recall FLO saying not all were.

Unfortunately it falls into a size and length where either parabolic or spherical could be correct.

[Alfian]

Yes Jules, I think it is the same one as the OTA on Astroboot (£70) and its like most ads for this scope in that it does not specify what type of primary it  has. Interestingly in the TS ad  for the scope mounted on the EQ2 it again talks of the "crisp" image from the "parabolic" mirror. Anyway I've emailed them so we'll see what they say. TS are usually pretty good at responding. 

[Cornelius Varley]

The 130/900 has a spherical mirror. The Orion Spaceprobe 130 (Oroin badged Synta130/900) has a spherical mirror http://www.telescope.com/Telescopes/Reflector-Telescopes/Reflector-Telescopes-for-Beginners/Orion-SpaceProbe-130-EQ-Reflector-Telescope/pc/1/c/11/sc/339/p/9851.uts?refineByCategoryId=339 . A spherical mirror at the diameter shouldn't be a problem though.

[Peter Drew]

At that diameter/focal length a sphere should be adequate for decent performance. I would rather have a good sphere on that spec telescope than a poor paraboloid. 

[John]

I wonder why Skywatcher opted for a spherical rather than parabolic mirror on those ?

I agree with Peters point but Skywatcher can produce very decent parabolic F/5 mirrors in the 130mm and 150mm apertures so you would have thought a 130mm F/6.9 would be a piece of cake for them ?

[Alfian]

Thanks for the Orion link CV. Orion specify that it is a spherical mirror on the Spaceprobe 130. That seems to be a bit unusual. From what I've seen, if a 'scope has a parabolic mirror the ad' tends to mention it. If no mention then  the assumption tends to be that it will be spherical. That's what surprised me with the wording on the TS website being very specific. It will be interesting to see what TS says. Peter/John - your comments are noted!

 I had not planned on spending anything yet but for the money, with the exchange rate on the euro as it is,  it seems like quite a good deal.

[Alkaid]

If I'm not mistaken, the TAL-1 also has a spherical mirror.  At F7, it works just fine.  So I'm fairly sure that the 130/900 SW will be fine too.

[Peter Drew]

Good spherical mirrors can be produced in multiples on one machine, parabolic mirrors are a one at a time process. Probably all down to economics. 

[Alfian]

Yes the Tal1 does have a spherical and at F7.3 it works very nicely indeed. Your comments, Peter,  re the production of sphericals  compared to that of parabolics is interesting. Interestingly, I have had a reply from Edgar at TS Optics who concedes that the "ff7.9" figure is incorrect and will be changed to F6.9, but  also says "yes it has a parabolic mirror!". Still not certain whether I'll bother with it, and still not absolutely certain whether it has a parabolic mirror either. From what you say Peter, given its likely production cost, it will be better for having a spherical anyway. 

[Cornelius Varley]

Here's a review of the SW130/900 http://www.stargazing.org.uk/reviews/Explorer%20130M%20EQ2.html .

Extract from the review

" Its performance is perfectly adequate for general observation, and while the appearance of Airy discs was not perfect, with rather bright diffraction rings, this was not so bad as to result in poor images. The mirror is spherical rather than parabolic, but at f/7 this is not a big problem. You can also get a parabolic version, the 130P "

[Alfian]

Thanks for that link, Peter, an interesting read. The 130P I'm sure is a 650 f5.