Looking for a F8 or longer GoTo scope

[blusky]

Hi all, being a newbie looking for a planetary scope, hence an F/8 or better - constrained by a budget .

I think a Newtonian 130 would be acceptable, 150 would be happy with. One thing, as a newbie, looking for a GoTo mount, and with this cannot find any F/8 or better scopes in the market.

Sky-watcher seem to do 130P GoTo's only F/5, and the nearest match is Dobsonian 200P (which, with a GoTo mount is twice my budget, and without, well, is a Dobsonian

Any advice would be much appreciated...

[Chris H]

What is your budget? Does the scope have to be a Dob? There is the Skywatcher Skymax 127 Goto, it's F11.8 and delivers excellent planetary views. Costs £365 from FLO.

[blusky]

I'd say about £300. I understand those Mak scopes eat a lot of light power because of the additional reflection and the central mirror, so the 127 Mak would be equivalen to less than 114 Newtonian.

Not keen on Dobsons at all, it's just their motorised and GoTo version would be suitable, but is clearly over budget.

one more thing - is stargazing practical at all from a balcony in the middle of a large city?

[Chris H]

Dobs have a secondary mirror in the light path just like a Mak. Mirror coatings are pretty damn good these days so you won't be loosing much light at all. If using in a city your targets lunar and planetary will be pretty bright anyway so I would not discount a Mak on that basis.

What direction does the balcony face?

[blusky]

A reasonable aspect to the East, a bit of North and South. No balcony above, which helps a lot. Does it make much difference?

What disadvantages of a Mak and of a Goto do you see for city viewing (in Britain )

[blusky]

Ok, could I have a show of hands for a manual Dob 200P against an AZ Goto Mak 127 please

[umadog]

The only reason for going long focal ratio is if you really, really, want to get rid of all eyepiece astigmatism. You don't need to be f/8 or longer to have a "planetary scope." Why? Because if the central obstruction is <20% by diameter the scope is indistinguishable from an unobstructed instrument. Barring the spider diffraction spikes, of course. In other words, an off-axis Newt with no central obstruction gives the same planetary contrast as a Newt of the same size with a <20% central obstruction. The rigorous mathematical proof is all in [removed word] Suiter's book.

Optimising secondary size is not the correct thing to do. Why? Contrast is provided by the size of the telescope's point spread function (PSF; google it), which is basically how much the light is diffracted as a consequence of passing through a finite aperture. In the case of a mirror, it's how much it's diffracted bouncing off a finite mirror. Ultimately, it's related to the length of mirror's edge compared to the surface area. The result being, of course, that larger mirrors yield sharper images because their point spread function is smaller. This is why larger instruments are needed to split close doubles.

So a 12" f/4 with a 25% obstruction will probably produce better planetary images than a 5" f/8 with a 15% obstruction. The reason is that increasing aperture provides a resolution boost. The 12" may not be as good as, say, 12" f/6 with a 20% obstruction but it will be the 5". No question. Don't neglect the resolution boost gained by going larger. Of course, a larger scope may need a significantly longer cool down time. But that's a different story.

Speaking personally, I bought my 18" f/4 (20% obstruction) to look at DSOs. The first time I turned it at Jupiter I wasn't really expecting so much. Even though I knew better, I was conditioned by the "f/4 won't make a good planetary scope" crowd. All I can say is "wow." That thing has consistently produced the best planetary views I have ever had (seeing willing, of course). It's not a subtle difference. On a still night the polar caps turn into a series of fine bands and you see purple streaks along the equator. Yet the scope is f/4... The reason it works so well is because the optics are good, the scope is cooled down, and it's properly collimated. Even SCTs with >25% obstructions can produce great views if well cooled.

So take another think about long focal ratio "planetary Newts"....

[great_bear]

Ok, could I have a show of hands for a manual Dob 200P against an AZ Goto Mak 127 please

You may struggle on a balcony with a 200P - these are big scopes - but then I don't know how big your balcony is. Are you on a high floor? I wouldn't like to carry a 200P down many flights of stairs if I wanted to take it somewhere in the car.

[umadog]

On a small balcony you will have trouble getting views of lower elevations with a Dob.

[DoctorD]

Hi I have the SW102 MAK and it gives great views of Saturn and Jupiter and is great on double stars.

Brighter DSOs are OK and Globulars in particular.

So the 127 MAK would be even better!!

Hope this helps

Paul