Sky travel 102 f5 vs 120 f5

[Smithfire]

Is it generally accepted that the 102 has better optics than its bigger brother and is better suited to astrophotography?....or is that a product of my fevered imagination?

Many thanks

Clive

[michael.h.f.wilkinson]

Less unsuitable for AP is the better choice of words. For a given F-ratio, chromatic errors go up rapidly as a function of aperture. For AP, it is better to go for a smaller ED or triplet than a larger achro. Alternatively, have a look at mak-newts. There is an Intes one on ABS-UK at the moment for a very reasonable price. That particular one is most suitable for smaller CCD chips, I gather,

[Stargazer_00]

Are either/any of the StarTravel F5 refractors achromatic doublets?

[Smithfire]

Thanks for the answer Micheal. Does that imply that even the 102 would be unsuitable for AP (using a web cam).

Clive

[Stargazer_00]

I had a ST120 and it was fine for widefield use.  Pretty poor at higher magnifications though.

I wouldn't recommend it as the scope in any serious AP set up though, better off saving a bit more for an APO instead of an achro.  ED80s work well as entry level AP scopes.

[Smithfire]

Thanks for that Stargazer.

Back to the drawing board I guess.

Clive

[Stargazer_00]

They would work, they just wouldn't be great. The fact that its an achromatic do count against it for serious AP.  That said it would produce images if you bolted a camera on the back and put it on a tracking mount, you just might not be very pleased with the results.

You get what you pay for really and this is especially true in AP.

[Stargazer_00]

This might help:

http://stargazerslounge.com/topic/145038-skywatcher-st102-achromatic-first-astro-pics/

Note the bright stars are purple.  That's the achromatic in full swing.

[ollypenrice]

Are either/any of the StarTravel F5 refractors achromatic doublets?

They all are, though they are not terribly achromatic! Normally achromats are not pulled down anywhere near F5. Remarkably they are not bad, though, in visual use at low powers.

The slower versions are still far from ideal for imaging but would be less unsuitable, to borrow a good phrase from Michael.

Olly

[oldfruit]

I had a ST 102. It was great for low to medeum power visual work but i tried it with my camera once and all the bright stars were purple and a bit bloated. Whilst i beleive this can be processed out to an extent you would be better aiming for an ED 80 or something similar for astrophotography. 

[michael.h.f.wilkinson]

Thanks for the answer Micheal. Does that imply that even the 102 would be unsuitable for AP (using a web cam).

Clive

Note that webcams are most suitable for planetary imaging, or very narrow field DSO IF they have a long exposure feature. For planetary imaging, I would have a look at a (preferably slow) Newtonian (6" F/8 would be great, but F/5 still works fine) or a Maksutov/SCT. This kind of imaging is much easier than deep sky.

[chrisbk]

If you did go for one of the ST's I would get the 120mm. 

By making a couple of cardboard aperture stops you could have a 120mm F5 when you want the extra aperture, a 100mm F6 or even an 80mm F7.5.

Each of the latter will still suffer from CA but not as badly.

[Stargazer_00]

It's worht pointing out that these wont operate as well as dedicated 100mm F6 or 80mm F7.5 scopes though.  Things like knife edge baffles and so on really help to cut down on CA as well.

[Smithfire]

Ok. I hear all the above and very interesting it is too. I was going to get a 127 mak with a goto mount and share that with a 102 f5. Sounds like a 120 for observation and a 80 apo for pics would be a better long term solution. My kids don't really need Christmas presents anway.

Clive

[Stargazer_00]

In fairness an 80 apo is probably as good as a 120 achro. APOs can handle much more magnification per inch.

[Smithfire]

Yes, but surely an 80mm apochromatic wont get near a 120mm achromatic for observational astronomy?

[Stargazer_00]

An ed80 can operate comfortably at 200x whereas an st120 struggles. APIs are just better, is one of those odditites. The resolution well be lower in the ed80 but the colour quality will be better, image sharpness and contrast too. If take an ed80 over an st120 for visual all day long.

[Smithfire]

....interesting.

Many thanks.

Clive

[chrisbk]

Like most things instrumental, it's swings and roundabouts.

I agree with Stargazer_00 that an APO refractor will give you the best image quality possible with a telescope but, inch for inch, they are the most costly scopes to buy. 

For photography the short tube acromats would be a poor choice due to excessive CA.

Visually though, however good an 80mm telescope might be, it cannot defy the laws of optics and will have the LGP  commensurate with that aperture.

Resolution wise it's a bit more complicated. Whilst a 120mm should have greater resolution than an 80mm it's not that simple.

Resolution is a function of aperture AND magnification. To reach the theoritical resolution of a scope you need a power of at least 40D in inches - preferably 50D.

Obviously it depends partly on the visual accuity of the observer but those above are average figures.

As Stargazer points out an ED80 will be able to reach these powers on a decent night whilst the ST120 would struggle.