Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Are they as good as they say?


Earl
 Share

Recommended Posts

It weighs less than 6kg wouldn't a HEQ5 be good enough?

Yep, just about, ideally an NEQ6 would be better suited but we can't have everything on our wish list at any one time, unless you've got lucky at Friday night bingo! :)

Link to comment
Share on other sites

For visual use, yes. But for use as a 150mm f9 (fl1350mm) astrograph you need some serious tracking hence the need for expensive mount. 1350mm is far too long for photographic use on an EQ-5 or EQ-6.

Link to comment
Share on other sites

For visual use, yes. But for use as a 150mm f9 (fl1350mm) astrograph you need some serious tracking hence the need for expensive mount. 1350mm is far too long for photographic use on an EQ-5 or EQ-6.

I don't agree with this. It is outside the 'really easy comfort zone' which I'd put at a metre but lots of people go out to around 1.5 metres with care. Take Mike (Yfronto) for example, and he has a whacking great OTA as well as this kind of fl.

The RC is certainly intended as an astrograph only, because the central obstruction is far too big to give decent visual contrast. But yes, f9 is slow. However, RCs are always slow unless glass is introduced. I think the design is theoretically optimal at f8.

What a scope like this does do is put a longer focal length within the reach of folks with more budget mounts.

I've heard grumblings about the collimation and would say that F9 would work better with CCD than DSLR where exposure time is limited.

Sometime soon I'll be getting the 8 inch version to test, something that should be intersting. I want to try it on the EQ6 as well as the Tak mount to see how the guiding holds up.

Olly

Link to comment
Share on other sites

A Tal 200k is a much more realistic option to the RC TAL 200K OTA Review Nenad Filipovi?'s Homepage and can be picked up much cheaper second hand. I sold my £1000 200K in mint condition and the best offer I had was a dismal £300 :) The 200K is a good all round performer on planets, dso's, visual and astography but the S/H prices often reflect the paranoid myth that there is a dark art to collimation that few would take on. I often regret selling my TAL but for me I found the brighter views from shorter focal length scopes easier on the eye.

SPACEBOY

Link to comment
Share on other sites

I owned one (briefly, unfortunately) and while I only used it twice so I didn't really get to grips with it, build quality-wise it seemed alright. Collimation isn't hugely fiddly but it is essential you get it right otherwise it shows up clearly in your images.

What the issue here is that GSO aren't making reducers for them at the moment and the majority of the available reducers that would work on these won't illuminate a big sensor (I think there's an Astro Physics reducer that would do this but it's not cheap or easily available). That's fine for someone like me who uses a smaller sensor but DSLR users are pretty much stuck at the native focal length for now.

Tony..

Link to comment
Share on other sites

A Tal 200k is a much more realistic option to the RC TAL 200K OTA Review Nenad Filipovi?'s Homepage and can be picked up much cheaper second hand. I sold my £1000 200K in mint condition and the best offer I had was a dismal £300 :) The 200K is a good all round performer on planets, dso's, visual and astography but the S/H prices often reflect the paranoid myth that there is a dark art to collimation that few would take on. I often regret selling my TAL but for me I found the brighter views from shorter focal length scopes easier on the eye.

SPACEBOY

If you were to use that TAL for astrophotography would you end up getting curved defraction spikes on long exposures due to the shape of the spider vanes. I understand the design is to reduce defraction spikes and an example of an image is given down the page (i assume its a fairly short exposure, i could be wrong though)

:p

Link to comment
Share on other sites

Sorry to go off topic slightly with a some what newbie question but something has come to mind while looking at the RC in question. 6" f/9 RC - 152mm Aperture / 1370mm Focal Length. For future reference would I be correct in saying that the f/9 is taking in to consideration the large secondary obstruction actually reducing the overall aperture and so in effect lengthening the focal length ???

Link to comment
Share on other sites

A Tal 200k is a much more realistic option to the RC TAL 200K OTA Review Nenad Filipovi?'s Homepage and can be picked up much cheaper second hand. I sold my £1000 200K in mint condition and the best offer I had was a dismal £300 :) The 200K is a good all round performer on planets, dso's, visual and astography but the S/H prices often reflect the paranoid myth that there is a dark art to collimation that few would take on. I often regret selling my TAL but for me I found the brighter views from shorter focal length scopes easier on the eye.

I'm not so sure about the 200K for imaging. Off the top of my head, there's a few issues with them that doesn't make them a logical choice. Visual though, they're a good scope :p.

Sorry to go off topic slightly with a some what newbie question but something has come to mind while looking at the RC in question. 6" f/9 RC - 152mm Aperture / 1370mm Focal Length. For future reference would I be correct in saying that the f/9 is taking in to consideration the large secondary obstruction actually reducing the overall aperture and so in effect lengthening the focal length ???

The size of the secondary has nothing to do with the focal length or aperture, that is set by the primary. It's not that big anyway!

Tony..

Link to comment
Share on other sites

The size of the secondary has nothing to do with the focal length or aperture, that is set by the primary. It's not that big anyway!

Tony..

Forgive me! Just trying to get my head around this. Would a large secondary not cast a shadow (so to speak) over the primary and in doing so reduce its light gathering surface ? I know the OTA to be slightly larger that the mirror but not so much so that the OTA aperture compensates for the secondary obstruction. IE: My 200P has a 200mm mirror with a 32mm Central obstruction but the OTA is only 10mm larger than the primary. Would this not mean that I am actually loosing 20mm of my mirror aperture and so changing the focal ratio of the scope :) IE: 180mm primary and so 1000mm / 180mm = f/5.5.

Am I just talking rubbish or am I making some kind of sense :p

Link to comment
Share on other sites

To my mind, you're not making a great deal of sense unfortunately!

Ignore the size of the OTA, the tube could be 5 feet wide but if your primary mirror is 8", then that's your aperture as it's the mirror that gathers the light.

IIRC, CO's are measured as a percentage of the diameter of the primary so if your mirror has a diameter of 200mm and your secondary is 20mm, it's 10%.

Tony..

Link to comment
Share on other sites

Confusing myself here :)

Forgive me! Just trying to get my head around this. Would a large secondary not cast a shadow (so to speak) over the primary and in doing so reduce its light gathering surface ? I know the OTA to be slightly larger that the mirror but not so much so that the OTA aperture compensates for the secondary obstruction. IE: My 200P has a 200mm mirror with a 32mm Central obstruction but the OTA is only 10mm larger than the primary. Would this not mean that I am actually loosing 32mm of my mirror aperture due to obstruction and so changing the focal ratio of the scope icon_confused.gif IE: 180mm primary and so 1000mm / 180mm = f/5.5 not f/5

A poor example: If I take the small 2" dust cap of my 8" objective cover and use the scope with the objective cover in place do I not change the focal length of my scope despite the primary still being 8" ? Or a 100mm unobstructed refractor be equal to a 130mm reflector ?

Edited by spaceboy
Link to comment
Share on other sites

SORRY FOR THE HIJACK EARL :)

I will open a separate thread as I am too simple to pick this up in a couple of posts :p

no worries, all valid points.

Im tempted to get one of these as i need another scope ill never use HAHA.

Link to comment
Share on other sites

No, f6 would take you to I.2 metres and most people would be happy to give that a go on an EQ6. But two metres is another matter entirely. I've only gone to 980mm on an EQ6 and it was pretty reliable but not totally so. I would have difficult nights when I'd lose some subs.

Expectation varies on this, it seems. Some people expect to lose a percentage of subs and inevitably one does, but I try not to lose any and three nights out of four that's possible with my setups. For longer FL I use the Takahashi mount. It isn't as much better than the EQ6 as it should be, in my view, but it is very consistent.

Olly

Link to comment
Share on other sites

No, f6 would take you to I.2 metres and most people would be happy to give that a go on an EQ6. But two metres is another matter entirely. I've only gone to 980mm on an EQ6 and it was pretty reliable but not totally so. I would have difficult nights when I'd lose some subs.

Expectation varies on this, it seems. Some people expect to lose a percentage of subs and inevitably one does, but I try not to lose any and three nights out of four that's possible with my setups. For longer FL I use the Takahashi mount. It isn't as much better than the EQ6 as it should be, in my view, but it is very consistent.

Olly

Thanks :)

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.