SCT and the moon

[Paxo]

Fair point Gaz and the one I guess I was trying to make, I could recall a few recent threads that's all. As I said I have been a critic myself regarding some aspects of their manufacturing but in general very happy, and in keeping with the thread, wouldn't use anything else for imaging the moon in fine close-up detail.

Steve

[ngc2403]

hello steve,

i would be surprised about the 1/6th wave optics because from what i have been told the more optical surfaces the better each one has to be. starting with thefirst one it has to be far better than 1/6th wave and even the last mirror (secondary) has to be better than 1/6th wave optics and from the problems that happen with making large accurate mirrors i would say the a 12'' 1/6th wave mirror would be alot however because it is not the last optical surface i has to be better than 1/6th wave as i said.

all in i would say that the meade that i used was very good at tracking however it was one of the older LX200's and unless major steps have been taken then i would have to doubt the idea that meades are 1/6th wave apon reaching the focal plane. reason they make alot of telescopes and they don't test them all for optical quaitity apon assembley. if RCOS are not 1/6th wave then i don't think meade are!

if you think that ur meade is 1/6th wave then it is well above averge for a mass produced telescope and although tracking and gears are things which improve with PEC etc SCT's will always suffer a narrow field of view and poorer contrast than almost all other telescope designs.

on a side note i look at things like the meade telescope and wonder why you say that they are affordable? look at the MAX mount personally i think that it is a waste of money because the Paramount is both better and far cheaper. the ACF RC like telescopes they make are very expensive to ok they are not the price of RCOS but then they are far well worse value for money.

Nobody seems to tell me why they wanted a long focal length telescope, to me all you get is more seeing in ur images?

ally

[GazOC]

Nobody seems to tell me why they wanted a long focal length telescope, to me all you get is more seeing in ur images?

Not everyone images DSOs. Long focal length scopes do very well for lunar/ planetary (visual and imaging), splitting doubles..

[narrowbandpaul]

if seeing at your site is 2" FWHM then all objects that you look at will have a maximum detail of 2", regardless of whether the object is the moon or planets or a quasar.

planetary imagers can use anything up to F40, which strikes me as odd, as the camera has a higher resolution than the telescope can resolve. This problem is made worse by using an Near IR filter, as resolution drops with increasing wavelengths.

the reason planetary guys use long F numbers is so that the object is made very big in the image. But regardless of size spanned in pixels, the etail is limited by the seeing, and for long F ratios...the telescope aperture.

So as ally says, zooming in to say 0.5"/pix for a planetary image, means all you will see is the atmosphere frozen in time.

Yet despite this, many good planetary images are produced.

And ally,

i would agree that most mass produced telescopes arent figured to 1/6th wave. And also, meade are expensive, for what is offered (say 12" aperture), they cost a lot more than a similar sized newt. The case in point is the max mount. It is like £20000...and its not as good as the paramount, and twice the price. So i dont agree with the prices meade think are OK. Ok it is more compact, and the F ratio can be reduced easily, but the contrast is poorer than a newt, and overall RMS error is worse than a newt (depends on manufacturer)

Having used a 16" LX200 at the observatory in Glasgow, I can honestly say it was one of the most unfortunate experiences of my life. The only thing it offers is aperture, making it OK for spectrometry/photometry

Just my opinion, others will vary

[GazOC]

I know for a fact that there are planetary/ lunar images taken at f40 - f60 that show details that the telescope cannot theoretically resolve. I assume the same applies to seeing as well? I'm sure these guys don't go to all the trouble of imaging at f60 for the fun of it when they could get the same images by doing it at f30 and just resizing the image x2?

[Keagle]

So if you had a 500mm(f/l) scope, that was f/10, the image would be bigger than a 1000mm scope that was f/5?

Thanks

Kurt

[GazOC]

No, size is just dependent on focal length, which is the same in both the scopes you mentioned.

[narrowbandpaul]

i remember an image of mars taken last year with a 9" TMB at F54. Quite remarkable. Contrast has a lot to do with minimum angular separation resolvable. And deconvolution can certainly help remove seeing. Thus you may be able to artificially improve the seeing at your site, but the resoltion of a telescope is set by diffraction (ie physics). Yet scopes seem to be punching above their weight.

Evidently there is a lot more than meets the eye.

A detailed calculation involving contrast brightnesses and obstructions for telescopes may yield an answer.

Just a thought...

consider an image of a planet...say jupiter

image a column of pixels passing through the planet. By subtracting pixel intensity at (x,y) in the array from pixel intensity below it (x,y+1)

ie I(x,y+1)-I(x,y)....this give a measure of contrast. How quickly the brightness changes across the image. Then from this you could work out how many pixels you can resolve, dependant on the brightness change.

Eg major bands in jupiter can easily be resolved cos of their big changes in brightness (contrast) however the encke gap in saturn is harder cos of the small brightness change.

Essentially we want the intensity gradient at each pixel in the image.

Equivalent to dI/dx and dI/dy.

Ally in matlab (sorry...you might punch me on friday) just use dI(x,y)/dy then x=x+1. is this the right idea?

Not asking you to do it. Its more a theoretical idea

Anyway, this analysis might be useful.

The contrast resolvable also depends on the thermal noise generated. With a noisy image, dI/dy would be very spiky, limiting detail. A smooth image with almost no noise allows separation of signal from noise, increasing resolvable contrast

just an idea

paul

[rusirius]

I have an 8in Meade L90 and my wife a Skywatcher Evostar 120 refractor. I find that the Meade is good at deep sky and excellent for the moon giving clear crisp views with my Ortho eyepieces. The scope is less impressive for planetary use and the Skywatcher refractor beats it hands down here giving much clearer, brighter and more detailed views. The Meade SCT gives much brighter images of DSO's though and I have been able to see many Messier and other cluster, galaxy and nebulae objects. Unfortunately although I live in the country, some objects are just too faint to be seen from my area due to moderate light pollution but I've enjoyed using the SCT for visual observation.

As you have observed, SCT's are fairly heavy and my LX90 is much heavier than the 5in Skywatcher refractor.

[Paxo]

hello steve,

i would be surprised about the 1/6th wave optics because from what i have been told the more optical surfaces the better each one has to be. starting with thefirst one it has to be far better than 1/6th wave and even the last mirror (secondary) has to be better than 1/6th wave optics and from the problems that happen with making large accurate mirrors i would say the a 12'' 1/6th wave mirror would be alot however because it is not the last optical surface i has to be better than 1/6th wave as i said.

all in i would say that the meade that i used was very good at tracking however it was one of the older LX200's and unless major steps have been taken then i would have to doubt the idea that meades are 1/6th wave apon reaching the focal plane. reason they make alot of telescopes and they don't test them all for optical quaitity apon assembley. if RCOS are not 1/6th wave then i don't think meade are!

if you think that ur meade is 1/6th wave then it is well above averge for a mass produced telescope and although tracking and gears are things which improve with PEC etc SCT's will always suffer a narrow field of view and poorer contrast than almost all other telescope designs.

on a side note i look at things like the meade telescope and wonder why you say that they are affordable? look at the MAX mount personally i think that it is a waste of money because the Paramount is both better and far cheaper. the ACF RC like telescopes they make are very expensive to ok they are not the price of RCOS but then they are far well worse value for money.

Nobody seems to tell me why they wanted a long focal length telescope, to me all you get is more seeing in ur images?

ally

Ally, I know it's better than 1/6th wave (just) as I've had the optical quality thoroughly checked, so, you think what you want, the proof of the pudding is in the eating so to speak. I don't particularly care that it has a narrow field of view, have you considered that maybe after careful consideration of the alternatives I, and many others, decided to buy an SCT on their relative merits for their given circumstances . You seem to be rejecting the personal choices of anyone but yourself, all of this is down to personal choice. As for affordable, that's all relative too, I am relatively privalaged to be able to afford such equipment, many can't. Many of the affordable comments relate to £/inch when compared to refractors.

So I guess you wont be buying Meade then. Anyway, that's the rant over.

Steve

[ngc2403]

hey paxo,

i understand the reasons some people buy SCT's but it's just that they are few in number and yet soon many people have them,

newts; cheap, big apature, fast focal ratio, with a BMPCC coma free?, excellent for DS imaging

refractors; high contrast, excellent optics, compact, good at everything really

SCT; good for long focal lenght stuff, compact for apature

i struggle for more reasons off the top of my head to be honest?

MAIN POINT

where did you get it tested? cause the interformeter they use at orion optics is hundreds of thousands of pounds. they have a 4'' collimation lens, it cost maybe ten thousands i can' remember (Narrowbandpaul knows). anyway they're are not many around that can test the accuracy of a mirror to high standards.

p.s. i will not be buying MEADE anytime soon

[Paxo]

Ally,

I had the benefit of being an academic at one of the UK's top research establishments and had access to equipment way beyond what Orion Optics or most optic manufacturers have, any more information I am not able or quite frankly prepared to divulge. I am happy with my purchase - end of story! Meade SCT's do benefit however, from significant tampering with to get them how you want it, Peterson Engineering in the US have made a business from these small but significant tune-ups. Why do some people use them (including many top quality imagers)- just take a look at Rob's recent bubble nebula. that will rest the case for the defense.

Steve

[ngc2403]

Hey Steve,

i agree that that is the end of the story but i think that you have hit the nail on the head.

People will always buy SCT's type telescopes but unlike other telescopes which work out of the box. The SCT's can be adapted to do anything. However that is my point they can't do it from the off, they need experience and the little upgrades to work properly/easily on certain things.

If you are referring to Rod Gendlers bubble nebula, then i feel that the comparison is a little off, he has spent loads of money on a setup he has never once seen. which works like robot under some of the best skies in the world where hours of continuous exposure are possible. though i think you maybe referring to someone on the site who's picture i have not seen.

thank you all for the very informed discusion

ally

[Paxo]

Hey Steve,

i agree that that is the end of the story but i think that you have hit the nail on the head.

People will always buy SCT's type telescopes but unlike other telescopes which work out of the box. The SCT's can be adapted to do anything. However that is my point they can't do it from the off, they need experience and the little upgrades to work properly/easily on certain things.

If you are referring to Rod Gendlers bubble nebula, then i feel that the comparison is a little off, he has spent loads of money on a setup he has never once seen. which works like robot under some of the best skies in the world where hours of continuous exposure are possible. though i think you maybe referring to someone on the site who's picture i have not seen.

thank you all for the very informed discusion

ally

Ally,

look a little closer to home to this weeks POW, that was the Rob I was referring to.

Steve..

[ngc2403]

hey

had a wee look there, nice image i have to say i would be more than a little happy to be able to say that it was mine.

ally

[batfire]

I enjoy the cpc1100 better than any scope I have owned and thats quite a few over the years. I do believe the 1100 is about as big a mobile unit I would go. I also have the xx12 and man it is a handfull also. Of the two I think the cpc1100 is best optical and all around use. I confess with the cpc1100 I use it mostly for ccd imaging work and I dont think for the price and size there is a better scope. I have the wedge and that is a pain when setting up due to having to lift the scope onto the wedge. I had neck pains a few times. I find that if you are mobile I use the 11inch much more than the xx12. It great as second scope at times when Im working and people stop by. I can keep working with cpc and let people view with the xx12. Optics quite a bit better in cpc oviously than the xx12. Both for viewing are great with the binoviewers. Just cant go back to mono once you own a set of the viewers. Hope that helps.

[steve_bham]

I have a 6" SCT and its a great little scope use it for both DSO and planets. Im a SCT fan small size and good views. But i only do visual so cant comment on astro-photography.

[Herakles]

Wow !

Wish I could comment on having used my C9 1/4 , sadly I have had like 30mins due to bad sky when not working and great skies when I am working ( I am cursed more than most , I work three nights on three off ! )

Recently sold my 200mm Newt ( F5) , yes visually the contrast was better but then again using the Newt was cumbersome for AP ( still learning this granted ). Being of typical height for my Scots generation ( 5' 6" ) the newt was awkward to set up compared to the SCT. The SCT has a lower ( IMHO ) vibration ratio than the 1000mm Newt I had . The SCT gets me 2350mm, with quicker damp down time . Lugging the SCT is a breeze compared to the Newt as well.

Add the FR to the SCT I get f6.3 , I can live with that . The SCT IMO gives ''me'' better options for AP , which is really where I want to be heading. It fits in the boot of my estate , Newt never did , to my satisfaction. Easier to collimate the SCT and not as often ( I'm told ! ) than the Newt .

So all in all I think it's horses for course with choiceof scopes that folk pick from. The C9 1/4 hopefully works better for me over the Newt I sold. That said , as I explained earlier , I really have yet to ''fully '' test the SCT . Those are 'my' reasons why I went the SCT route.

Had I more space at home I would have kept the Newt but TBH I think it would have been more an ornament than a tool. I also have a wee 80mm Fract, for wider DSO stuff.

[Merlin66]

I had and used various Newtonians from 10" to 18". Great optics, great views, but the cost of mounting them on an GEM or modified fork mounting, took away the ability to easily move and use them...

Sold them all and went over to the 12" SCT on HD tripod/ wedge.

Much more fun and easier to use. When I stopped imaging and started spectroscopy the SCT was MUCH easier to use ( back focus) and a joy!

Now I have the 12", 10" and 8" SCT with a combination of the Lx200 forks and the NEQ6pro. There's not much they can't do....