TheSkySearchers

flord.lord wrote: ↑Tue Jul 21, 2020 10:28 pm Hi,

The two scopes in the title are:

https://www.rothervalleyoptics.co.uk/sk ... drive.html

And

https://www.rothervalleyoptics.co.uk/sk ... scope.html

I was a bit confused by this:

- Is the only difference the multi-speed handset, and DC power/battery? How useful is the multi-speed handset?

-Do I need motorised tracking at all on this scope, let's say at 160x magnification (as without it is significantly cheaper)

Thanks!

Refractordude wrote: ↑Wed Jul 22, 2020 12:06 am At 166 magnification I would say a tracker is not needed. If I wanted a reflector around your price range I would get this https://www.firstlightoptics.com/beginn ... onian.html, and upgrade with a red dot finder if the stock finder is difficult to use. The 150mm aperture dob has a much better/very easier to use mount. Being a f/8 it will knock the planets and moon out the park.

Lady Fraktor wrote: ↑Wed Jul 22, 2020 5:57 pm An interesting looking new version of a EQ-2, it will be interesting to hear how stable it is.
The tripod can be stiffened up quite easily for less shake as well.

If the f/7 telescope has a corrector and spherical mirror they are not great for mid high powers and up without a lot of rework.
Spherical do work well when f/10 or slower.

Lady Fraktor wrote: ↑Thu Jul 23, 2020 5:17 pm No they do not install correctors on all models of spherical mirrored reflectors.

Normally a Newtonian with a spherical mirror will have a long focal length to deal with the larger light cone produced.
They also will have a larger secondary mirror because of this compared to a same sized telescope with a parabolic mirror.

The first two you listed are the correct tube length to be a natural f/6.92 and the Skywatcher website states they have a parabolic primary.

helicon wrote: ↑Fri Jul 24, 2020 3:13 pm I had an f/10 reflector on an EQ mount with a spherical mirror which worked just fine.
fas
However, as others have stated a parabolic is highly recommended in shorter focal lengths.

terrynak wrote: ↑Fri Jul 24, 2020 10:19 pm

helicon wrote: ↑Fri Jul 24, 2020 3:13 pm I had an f/10 reflector on an EQ mount with a spherical mirror which worked just fine.
fas
However, as others have stated a parabolic is highly recommended in shorter focal lengths.

Michael, I'm thinking you're referring to the Edmund Palomar Jr. - the 4.25", F/10 scope. Nice long focal length, given the aperture.

The famous Criterion RV-6 Dynascope (6") had a reputation for excellent performance despite its spherically shaped mirror at F/8.

notFritzArgelander wrote: ↑Fri Jul 24, 2020 10:45 pm
The technical detail is whether a spherical mirror deviates from a parabolic mirror in a way that satisfies the Rayleigh criterion that a defect in the shape of a mirror should produce no more than a 1/4 wave error in the wave front.

In Jean Texereau's How to Make a Telescope the application of this is discussed in detail. On pp. 16-17 there are formulas and a table:

Measuring f and D in cm: f^3 = 34.9D^3.
Measuring in inches one gets: f^3 = 88.6D^3.

The table has f/D = 7.0 for a 4" and f/D = 8.2 for a 6".

Both the examples you cite are "close enough" to parabolic so that within the Rayleigh criterion I wouldn't care about the difference between parabola and sphere. If the difference is smaller than what is required to have perfect optics, there is no functional difference. I wouldn't care whether the Edmund Palomar Jr or RV-6 Dynascope was advertised as a parabola or a sphere. Rayleigh has them as being equivalent.

I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.

notFritzArgelander wrote: ↑Fri Jul 24, 2020 10:45 pm
The technical detail is whether a spherical mirror deviates from a parabolic mirror in a way that satisfies the Rayleigh criterion that a defect in the shape of a mirror should produce no more than a 1/4 wave error in the wave front.

In Jean Texereau's How to Make a Telescope the application of this is discussed in detail. On pp. 16-17 there are formulas and a table:

Measuring f and D in cm: f^3 = 34.9D^3.
Measuring in inches one gets: f^3 = 88.6D^3.

The table has f/D = 7.0 for a 4" and f/D = 8.2 for a 6".

Both the examples you cite are "close enough" to parabolic so that within the Rayleigh criterion I wouldn't care about the difference between parabola and sphere. If the difference is smaller than what is required to have perfect optics, there is no functional difference. I wouldn't care whether the Edmund Palomar Jr or RV-6 Dynascope was advertised as a parabola or a sphere. Rayleigh has them as being equivalent.

I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.

terrynak wrote: ↑Sat Jul 25, 2020 1:19 am

notFritzArgelander wrote: ↑Fri Jul 24, 2020 10:45 pm
The technical detail is whether a spherical mirror deviates from a parabolic mirror in a way that satisfies the Rayleigh criterion that a defect in the shape of a mirror should produce no more than a 1/4 wave error in the wave front.

In Jean Texereau's How to Make a Telescope the application of this is discussed in detail. On pp. 16-17 there are formulas and a table:

Measuring f and D in cm: f^3 = 34.9D^3.
Measuring in inches one gets: f^3 = 88.6D^3.

The table has f/D = 7.0 for a 4" and f/D = 8.2 for a 6".

Both the examples you cite are "close enough" to parabolic so that within the Rayleigh criterion I wouldn't care about the difference between parabola and sphere. If the difference is smaller than what is required to have perfect optics, there is no functional difference. I wouldn't care whether the Edmund Palomar Jr or RV-6 Dynascope was advertised as a parabola or a sphere. Rayleigh has them as being equivalent.

I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.

An 8" F9 Newt means a scope 1803mm long or almost 6 ft. in focal length!

I was lucky enough to find a 5" F/8 Newt (127/1020mm) - the Meade 127NT, which was available only in '97/'98 before it was discontinued. Fits the Rayleigh criterion for a spherical, but someone mentioned that it has a parabolic mirror as well. Came with a LXD 500A mount, with a payload capacity of 15 lbs (according to manual). Very sturdy mount for an OTA this size.

Bresser currently markets a 130/1000mm or F/7.7 Newt in Europe (Messier NT-130/1000 EXOS-1) - like the Meade above, it may even have a parabolic mirror as well (may not make a noticeable difference with a spherical at high magnifications, but still nice to have); the EXOS-1 mount also has a payload of 15 lbs.

Flord.lord, if you still want a long focal length 5" Newtonian, the Bresser scope is the one to get. Here's the link:

https://www.bresser.de/en/Astronomy/Tel ... scope.html

notFritzArgelander wrote: ↑Sat Jul 25, 2020 1:55 am

I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.

An 8" F9 Newt means a scope 1803mm long or almost 6 ft. in focal length!

Yes, and the mount I built for it was a beast. :sigh:

https://www.bresser.de/en/Astronomy/Tel ... scope.html

Yes, that would be a good performer.

terrynak wrote: ↑Sat Jul 25, 2020 2:26 am

notFritzArgelander wrote: ↑Sat Jul 25, 2020 1:55 am

I built an 8" f9 with a mirror that was spherical with only slight parabolizing. The table gives f/D = 9.6 for a 10".... So a REALLY long tube.

An 8" F9 Newt means a scope 1803mm long or almost 6 ft. in focal length!

Yes, and the mount I built for it was a beast. :sigh:

https://www.bresser.de/en/Astronomy/Tel ... scope.html

Yes, that would be a good performer.

I was going to ask about the mount (and whether it was an EQ or Dob) on my previous post but decided to hold back...

I was able to pick up a Bresser 150/1200mm OTA new from a seller who managed to get a hold of a few of these, as well as a few 130/1000 OTAs. Should have snapped up a Bresser 130/1000 OTA as well, even though I already had the older Meade 127NT. Seriously thought about it, but never managed to pull the trigger.