RC's and Newts and Theory

[Outbackmanyep]

It seems to have a HUGE secondary! Is this normal for a RC?

[netwolf]

The larger secondary provides a faltter field. Good for DSO not so good for Planet imaging.

[Gama]

Originally Posted by netwolf

The larger secondary provides a faltter field. Good for DSO not so good for Planet imaging.

Provides a Wider illuminated field.



Theo.

[toryglen-boy (Duncan)]

i dont know much about the RC design, or the pros and cons to it. Would this better than an F5 200mm reflector for DSO imaging?

thanks

[Octane (Humayun)]

Duncan,

Depends on what focal length you want, really.

I'm sure there's more to it, but, essentially, for a lot of people, that's what it'd come down to. Approximately 2 metres focal length compared to 1 metre.

Regards,
Humayun

[pmrid (Peter)]

Originally Posted by toryglen-boy

i dont know much about the RC design, or the pros and cons to it. Would this better than an F5 200mm reflector for DSO imaging?

thanks

Unequivocally yes.
Peter

[gbeal]

Unequivocally yes, but only if your mount, your guiding etc all works OK at the longer focal length.

[toryglen-boy (Duncan)]

Originally Posted by pmrid

Unequivocally yes.
Peter

thats quite a resounding answer Peter, with no explanation !!

care to elaborate?

[AlexN]

True... Despite it being lighter than a 8" F/6 newt, weight isn't the only thing that dictates whether or not a mount will accurately track for photography... I know with my current rig I can achieve 30 minute guided exposures no worries with the 480mm focal length of my scope. however I dare say the 8" RC on the HEQ5 would be difficult to achieve 10 minute subs, and being an F/8 system, I'd say 10 minute subs would be about the minimum you'd want to run in the 8" RC to achieve very nice images...

I would love to get another one one day.. We'll just have to wait and see how things go..

[toryglen-boy (Duncan)]

Originally Posted by AlexN

True... Despite it being lighter than a 8" F/6 newt, weight isn't the only thing that dictates whether or not a mount will accurately track for photography... I know with my current rig I can achieve 30 minute guided exposures no worries with the 480mm focal length of my scope. however I dare say the 8" RC on the HEQ5 would be difficult to achieve 10 minute subs, and being an F/8 system, I'd say 10 minute subs would be about the minimum you'd want to run in the 8" RC to achieve very nice images...

I would love to get another one one day.. We'll just have to wait and see how things go..

i hear you, i will stick to my 200mm F5 reflector i think, when i use the MPCC i get a pretty flat field anyway, and its more forgiving on guiding.

[AlexN]

Yep... Providing your optics are sharp, and the field is flat, there is not much reason to go to the RC unless you want much longer focal length... The RC will better illuminate a DSLR sized sensor, that said, an 8" F/5 newt will effectively illuminate the sensor enough that flats will take care of any vignetting...

[toryglen-boy (Duncan)]

Originally Posted by AlexN

Yep... Providing your optics are sharp, and the field is flat, there is not much reason to go to the RC unless you want much longer focal length... The RC will better illuminate a DSLR sized sensor, that said, an 8" F/5 newt will effectively illuminate the sensor enough that flats will take care of any vignetting...

Thanks for clearing that up Alex, its just as i thought.

[gregbradley]

It depends on what type of image you want to produce.

F8 200mm = 1600mm focal length = more closeup more highly magnified than F5 200mm = 1000mm focal length.

A common and very useful focal length for imaging is 1200mm.

F5 is a preferable focal ratio than F8. Not that there is anything wrong with F8 but it is more demanding on accuracy of tracking and will give more closeup views of objects. Maybe you prefer widerfield images. F5 will also get a bright image faster - more than twice as fast as F8 for the same aperture. So that may be good for your imaging requirements - perhaps you live in area where clouds often interrupt imaging. Well F5 is more productive than F8.

Collimation requirements for both will be important but a Newt is easier to collimate than an RC.

Then again F5 200mm will not capture too many galaxies whereas F8 at 200 will get quite a few.

So different strokes for different folks. No right or wrong about it but what type of image would you like to produce is the primary question coupled with what environment are you imaging from. Also what size camera chip as the bigger the chip the more you need a large corrected field which only some configurations of scopes can provide.

For a DSLR I imagine you are in safe territory with either scopes.

Also longer focal length often means a longer tube which is more wind affected.

If you travel to a dark site to image then F5 200mm would be far more productive and less likely to result in a nonproductive trip is clouds are hanging around. F8 200mm would require clearer less windy conditions to be productive but you could image a nice galaxy shot with it or a closeup of some DSO.

For less experienced imagers shorter focal length is always the way to go as the longer the focal length the more accurate everything has to be and the longer the exposure time to get a decent image.

Personally I think you would find most people would prefer an image taken from an F5 200 Newt well setup to a F8 200 RC. You'd get more positive feedback on most images (not all).

That is my experience anyway.

Greg

Originally Posted by toryglen-boy

i dont know much about the RC design, or the pros and cons to it. Would this better than an F5 200mm reflector for DSO imaging?

thanks

[toryglen-boy (Duncan)]

Thanks for that Greg




Yeah, i am gonna use the F5 Reflector, it serves me very well, and if i wanna view planets, then i can use my 12" dob.

infact, i have just put the 200mm SCT in the IIS classifieds off the back of these replies !!

[Peter Ward]

Greg, your thoughts are no doubt appreciated by many and while it may seem I'm on your case, mate, I'm not, this was simply not right.

Originally Posted by gregbradley

F5 will also get a bright image faster - more than twice as fast as F8 for the same aperture.

This old chestnut is false. With CCD imaging the limiting stellar magnitude and extended object S/N ratio is determined by aperture alone. I think it was Russ Croman... who did a pretty comprehensive analysis on SBIG users as to the how and why.

Originally Posted by gregbradley

Collimation requirements for both will be important but a Newt is easier to collimate than an RC.

Again false. The GSO RC's have adjustable secondaries only. (the rest it locked in at the factory) . Newtonians have both primary and secondary adjustments.

Originally Posted by gregbradley

Also longer focal length often means a longer tube which is more wind affected.

Again false. In all Cassegrains (ie RC's), the light path is folded back through the primary..OTA length = FL divided by 3 or about 21 inches. Newtonians don't, so the OTA's are similar in length to the FL , 8" F5 so about 40 inches.

[Satchmo]

Hi Peter

I would have to support Greg on all three counts, but happy to stand corrected with convincing facts.

1. He didn't specify particularly limiting magnitude of stellar objects so we can assume that he had speed of imaging extended objects like nebulae and galaxies in mind . An F5 optics has an image intensity 2.5X greater than an F8 for the same aperture, so recording speed of extended objects will have to be similarly quicker. This applies to nebulous extended objects only.

2. Collimation of the GSO RC's depends on the optical center of the primary coinciding with the optical center of the squared on focusser. If there is any decenter error there from the factory , no amount of secondary angle tweaking will help. Are there any user adjustments for either of these mechanical adjustments ( SCT primaries are spherical so not as sensitive to decentering). There is nothing to stop manuafcturers making fixed primary mirrors in Newtonians so this potential 'feature' is not a privelage of RC scopes.

3. The short tube will help but an F8 scope is going to be more sensitive to vibration han an F5 whichever way you look at it as it is operating at high prime focus magnification.

Originally Posted by Peter Ward

Greg, your thoughts are no doubt appreciated by many and while it may seem I'm on your case, mate, I'm not, this was simply not right.



This old chestnut is false. With CCD imaging the limiting stellar magnitude and extended object S/N ratio is determined by aperture alone. I think it was Russ Croman... who did a pretty comprehensive analysis on SBIG users as to the how and why.




Again false. The GSO RC's have adjustable secondaries only. (the rest it locked in at the factory) . Newtonians have both primary and secondary adjustments.




Again false. In all Cassegrains (ie RC's), the light path is folded back through the primary..OTA length = FL divided by 3 or about 21 inches. Newtonians don't, so the OTA's are similar in length to the FL , 8" F5 so about 40 inches.

[Peter Ward]

Mark,

1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Do you disagree with Stan's analysis?

2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes and not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8

3) The point Greg made was the *tube was longer*.

It is patently not,

and will suffer less from wind loading as a result....though I do take the point the longer focal length could be more sensitive to seeing and tracking errors.

[multiweb (Marc)]

What an interesting thread. Got to agree with Peter on this one.

1)_ Recently attented an astro-photo meeting at the ASNSW and Mike BJ did a talk that reflects exactly what Stan Moore is talking about.

2)_ Don't know much about RCs, I trust Peter had enough in his hands to play with and knows what he's talking about. but I know my 5" f/5 newt is very fiddly to collimate from scratch.

3)_ Longer FL harder tracking. Longer tubes catch the breeze. That's a given

Originally Posted by Peter Ward

Mark,

1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Do you disagree with Stan's analysis?

2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes are not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8

3) The point Greg made was the *tube was longer*.

It is patently not,

and will suffer less from wind loading as a result....though I do take the point the longer focal length could be more sensitive to seeing and tracking errors.

[Satchmo]

Peter

I had a quick skim of the article. I was distracted by the fact that you mentioned specifically stellar limiting magnitude. So it seems it matters not to the CCD that the physical image at the focal plane will be 2.5X brighter in the F5. I am a bit of planetary nebulae nut and its been my closet dream to make a photographic catalogue of far southern planetaries. I guess this means with a CCD I can shoot at F16 with no loss of speed over an F4, which makes my project all the more attractive.

Regards collimation: I would have been very nervous about purchasing a budget telescope with highly aspheric surfaces where you couldn't collimate the primary element but your findings allay my fears.

I don't really see the difference in difficulty of collimating a small Newtonian against a SCT or GSO RC. The secondary you don't routinely touch after it has been aligned, and at F5 a common or garden laser collimator gets you close enough. At F4 to F4.5 a squizz at high power at a defocused star tells you which primary collimation knob to move just as you would do to tweak the secondary on an SCT or RC.


[QUOTE=Peter Ward;490652]Mark,

1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Do you disagree with Stan's analysis?

2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes are not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8

[Peter Ward]

Time for a group hug?