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evil_yoda

Longer focal length scope suggestions

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Hi all,

I've currently got a Skywatcher ED80, HEQ5 and a ASI1600mm, set up in an 'obshedvatory'.  I like doing the bigger nebula etc with the ED80 and the focal reducer, but I'd also like to fill the frame a bit more with some of the smaller galaxies. The ED80 will be staying, so I'd like a companion for it.

Only having an HEQ5 limits me a bit to the weight of a new scope, so I've been looking at the CF RC8 as they seem quite light for the aperture.
I appreciate that it can be a bit tricky to collimate these scopes (and I've never colimated a reflector before). Also I think I'd need to bin 2x2 with my ASI1600 with the pixel size. Otherwise I keep coming back to the RC8.

Does anyone have any other suggestions of a scope that would fit the bill without spending a fortune?

Matt
 

 

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Depends a little bit on how good your guiding is I think - do you have any details on the ability of the HEQ5 to guide successfully? I say this, as it gets increasingly hard to guide at longer focal length, though a catadioptric is probably easier than a longer tube scope like a newt (less moment of inertia).  

If the mount is up to it, then a Newtonian would be possibly the cheapest way forward I think in terms of price vs aperture - you will need to learn to collimate though (it's not too hard once you've done it a few times - it takes me longer to take out the camera, and reconnect it afterwards than the actual collimation these days). Having a perm setup also helps enormously. 

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Hi Graeme,

From memory, my Tot RMS as reported by PHD2 varies between 0.8 and 1.5 depending on the night. 
To be honest, I've not spent much time fine tuning the the mount and polar alignment to improve the guiding as it's always been perfectly adequate for the ED80, but I appreciate a longer focal length would need much better guiding.

I did look around at some of the newts, but they all looked to be quite heavy and probably more difficult to balance the shorter tube RC or cassegrain designs

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+1 for RC 8" - I have such setup and very pleased with it. I also bin x2 in software for around 1"/pixel. It is on edge of HEQ5 capability - in terms of guide performance, you need nicely tuned mount to be able to guide at around 0.5" RMS total.

Scope hold collimation very well - I setup each session and I've collimated scope only twice (well, one could say that it was single collimation, because I did not get it spot on in first round, so I had to repeat the next day).

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Thanks vlaiv,

I've seen some of the superb images you've produced with the RC8 and the HEQ5 which is one of the reasons I thought it might be the way forward.
The belt mod might be on the todo list if I do get an RC8!

Cheers, Matt

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19 hours ago, vlaiv said:

+1 for RC 8" - I have such setup and very pleased with it. I also bin x2 in software for around 1"/pixel. It is on edge of HEQ5 capability - in terms of guide performance, you need nicely tuned mount to be able to guide at around 0.5" RMS total.

Scope hold collimation very well - I setup each session and I've collimated scope only twice (well, one could say that it was single collimation, because I did not get it spot on in first round, so I had to repeat the next day).

Hi vlaiv,

I've managed to find myself a used carbon fibre RC8! Cloudy nights for a while longer I suppose.

Not got it yet, but it's totally standard with the factory focuser etc.
I assume you upgraded the focuser on  yours, and was wondering what you upgraded it to? Doesn't look like the standard focuser has a threaded attachment for the imaging train.

Might be a while before I can upgrade the focuser, but it's always fun to make an upgrade list :)

Matt

 

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59 minutes ago, evil_yoda said:

Hi vlaiv,

I've managed to find myself a used carbon fibre RC8! Cloudy nights for a while longer I suppose.

Not got it yet, but it's totally standard with the factory focuser etc.
I assume you upgraded the focuser on  yours, and was wondering what you upgraded it to? Doesn't look like the standard focuser has a threaded attachment for the imaging train.

Might be a while before I can upgrade the focuser, but it's always fun to make an upgrade list :)

Matt

 

Hi, yes, I ended up upgrading it precisely because of lack of threaded connection. Stock focuser was otherwise quite usable - never had slippage issues with it, but I try to keep my imaging train light. I plan to use that monorail focuser on my SW Evostar 100 F/10, just need to get suitable adapter to it (OTA tube to M90).

This is the one I upgraded to:

https://www.teleskop-express.de/shop/product_info.php/info/p6970_TS-Optics-2-5--Rack-and-Pinion-Focuser---holds-Acc--up-to-6kg---travel-53mm.html

I also have this extension before focuser, screwed into OTA:

https://www.teleskop-express.de/shop/product_info.php/info/p2773_TS-Optics-50-mm-Extension-Adapter-for-M90x1-thread.html

At the time of my purchase it was not included into delivery (I have TS version - regular one, not carbon fiber), but from what I see, TS includes it now with their RC units.

I also added rotator and suitable thread adapter (because need to mount 2" filters and reducers):

https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p6400_TS-Optics-Adapter-from-M54x0-75-to-M48---T2-Focal-Adapter-for-M54x0-75.html

 

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Why are you keen to add resolution via focal length and then reduce it via binning? It really doesn't matter whether you fill the frame or not. What matters is how many pixels (single or 4 pixel superpixels) you put under the object's image as projected by the telescope. It is this which determines the object's image's final size at full size (1 camera pixel = 1 screen pixel.) *

Earlier in the year I did a comparison in Astronomy Now between a 14 inch scope (2.4M FL) with large pixels and a 150mm scope (1M FL) with small pixels on galaxies. I found that the level of resolution and the size at which the images could be presented on screen was effectively the same. It seems that the considerably greater optical resolution of the larger scope was not translating into more final details. In both cases the mount was running with an RMS of less than half the image scale.

Personally I'd look for a scope with a FL which will give you about an arcsecond per pixel or a tiny bit less and make it the kind of nice simple design that you know you will find productive. I've struggled with one of those RCs with a guest and got nowhere. The theory is one thing but this example did not behave according to the theory.

Olly

*Not the best sentence I ever wrote! :BangHead:

Edited by ollypenrice
Typo

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19 minutes ago, ollypenrice said:

Why are you keen to add resolution via focal length and then reduce it via binning? It really doesn't matter whether you fill the frame or not. What matters is how many pixels (single or 4 pixel superpixels) you put under the object's image as projected by the telescope. It is this which determines the object's image's final size at full size (1 camera pixel = 1 screen pixel.) *

Earlier in the year I did a comparison in Astronomy Now between a 14 inch scope (2.4M FL) with large pixels and a 150mm scope (1M FL) with small pixels on galaxies. I found that the level of resolution and the size at which the images could be presented on screen was effectively the same. It seems that the considerably greater optical resolution of the larger scope was not translating into more final details. In both cases the mount was running with an RMS of less than half the image scale.

Personally I'd look for a scope with a FL which will give you about an arcsecond per pixel or a tiny bit less and make it the kind of nice simple design that you know you will find productive. I've struggled with one of those RCs with a guest and got nowhere. The theory is one thing but this example did not behave according to the theory.

Olly

*Not the best sentence I ever wrote! :BangHead:

I agree with you on that one - aim for final resolution, but this is where I found RC to be the best match.

Let's say that we want ASI1600 and 1"/pixel resolution with 8" aperture on HEQ5. That means focal length of about 800mm. There are couple of options to go here:

1. F/4 Newtonian with native pixels, or F/5 with reducer CC

2. F/8 RC with binned pixels.

For option 1, we need good CC. I'm not sure that I've found CC that will correct coma to good degree but not introduce SA or something else, over large enough field. Best CC that I've seen in terms of correction would be ASA x0.73, but it has 17mm corrected circle (or there about). It also means F/5 scope - and that will add its own complexity in terms of bulk being put on HEQ5. Somehow I can't seem to find good 8" Newtonian candidate for 1"/pixel - maybe I just have not searched enough.

With option two we have nice compact design that sits well on Heq5. It is flexible in terms of resolution / focal length as there are reducers for it - x0.67 or x0.75 (I would say that any reduction past x0.72 will not work well on this RC and asi1600 due to corrected field - x0.67 reducer can be "spaced" to x0.72 - x0.75, but there is also very good reducer - Riccardi FF/FR - x0.75).

Binning ASI1600 in software is exactly the same as using camera with larger pixels and having larger read noise - one of 3.4e read noise - still better than most CMOS cameras out there in terms of read noise. Only drawback for RC is FOV, and for galaxies it is of course less important.

So if we go by aperture at resolution (around 1"/pixel), I think RC is very viable if not the best option in 8" class to be mounted on Heq5 (with given camera).

Being true mirror system it has other advantages - broader use as astronomical instrument - for spectroscopy, photometry and astrometry - I mean it is most widely used by professionals in scientific role because of its characteristics.

Edited by vlaiv
forgot to mention target aperture

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5 hours ago, ollypenrice said:

Why are you keen to add resolution via focal length and then reduce it via binning? It really doesn't matter whether you fill the frame or not. What matters is how many pixels (single or 4 pixel superpixels) you put under the object's image as projected by the telescope. It is this which determines the object's image's final size at full size (1 camera pixel = 1 screen pixel.) *

Earlier in the year I did a comparison in Astronomy Now between a 14 inch scope (2.4M FL) with large pixels and a 150mm scope (1M FL) with small pixels on galaxies. I found that the level of resolution and the size at which the images could be presented on screen was effectively the same. It seems that the considerably greater optical resolution of the larger scope was not translating into more final details. In both cases the mount was running with an RMS of less than half the image scale.

Personally I'd look for a scope with a FL which will give you about an arcsecond per pixel or a tiny bit less and make it the kind of nice simple design that you know you will find productive. I've struggled with one of those RCs with a guest and got nowhere. The theory is one thing but this example did not behave according to the theory.

Olly

*Not the best sentence I ever wrote! :BangHead:

Hi Olly,

I'd been looking around at different scopes to fit somewhere in the focal length I was aiming for, that would be a 'reasonable' match for the HEQ5 in terms of weight. I know I'll need to tune the mount somewhat to get the best from it for a scope of this focal length. 
I'm happy with the camera I have, and also the mount (for the time being at least!)

I had considered a newt, but these generally seem to a bit more difficult to balance with the imaging train hanging off to one side. And quite heavy.
Also looked at something like a C8 edge. Longer focal length than the RC8, perhaps a similar weight though.
Another refractor would be nice. Skywatcher 120 Esprit with a focal length of 840mm gives me an arcsecond per pixel, but starting to get expensive. And the OTA is almost 10kg.

The RC8 is a longer focal length and means I'll need to bin 2x2. 

But...

It's lighter and more compact, and should be easier to balance than a newt perhaps.
Cheaper and lighter than a refractor.

I got this one second hand for a good price, so if after a year or so it isn't a good fit for me, I'm sure I'll probably get most of my money back if I sell it on.

Either way it'll be fun to try something a bit different than the little ED80 (which will still be getting lot's of use).

Cheers, Matt
 

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The SW ED120 with matched reducer gives 765mm FL and pretty close to 1arcsec per pixel. 

Not the ultimate apo, but has refractor simplicity, and the weight shouldn’t tax the mount too much

Edited by catburglar
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In my view these conversations spend too much time on the weight issue and not enough on the accuracy which, at these pixel scales, can be the dominant factor.

Olly

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200P-DS with 0.9 coma corrector would give you very close to 1" per pixel with the ASI1600.

Edited by Stub Mandrel

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After reading posts like this one:

http://www.astrofotoblog.eu/?p=856

Not really convinced in perfromance of coma correctors and fast newtonian systems.

Granted 200PDS is not that fast at F/5 - but slower the newtonian - longer the tube and harder on the mount and guiding it is.

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32 minutes ago, vlaiv said:

After reading posts like this one:

http://www.astrofotoblog.eu/?p=856

Not really convinced in perfromance of coma correctors and fast newtonian systems.

Granted 200PDS is not that fast at F/5 - but slower the newtonian - longer the tube and harder on the mount and guiding it is.

Interesting link.

With the SW CC and 130P-DS (f4.7) my curvature map is almost identical to the ES one in that link, except my curvature is only 12% rather than 26% and the corner stars are as round as a round thing. Strange that many people prefer the Baader?

image.png

Could it be that the 'economy' Skywatcher CC is the best of the bunch? Does suggest it might do a good job with the 200P-DS?

Not sure how big the weight difference is between 200P-DAS and 150PL, but the 1200mm 150PL guides well on a HEQ5.

 

 

 

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4 hours ago, Stub Mandrel said:

Interesting link.

With the SW CC and 130P-DS (f4.7) my curvature map is almost identical to the ES one in that link, except my curvature is only 12% rather than 26% and the corner stars are as round as a round thing. Strange that many people prefer the Baader?

image.png

Could it be that the 'economy' Skywatcher CC is the best of the bunch? Does suggest it might do a good job with the 200P-DS?

Not sure how big the weight difference is between 200P-DAS and 150PL, but the 1200mm 150PL guides well on a HEQ5.

 

 

 

F/ratio plays significant part in performance of CC - SW CC is good for F/5 scopes, although I'm not sure how much SA there is with that F/ratio.

Scope used in comparison of coma correctors is F/4. 8" F/4 has focal length of 800mm - and that puts it in right spot for 1"/pixel with ASI1600. F/5 8" with x0.9 reduction will provide 900mm focal length - that is 0.87"/pixel with ASI1600 and I would personally rather be on north side of 1"/pixel than below it - that would mean ASA x0.73 CC - and that one will not provide fully corrected field for ASI1600. It will for ASI183, and I think that would be really good combination, but then again I would bin ASI183 as well for effective resolution of 1.36"/pixel rather than leaving it at 0.68"/pixel.

There is characteristic signature of SA on star shapes - most SCTs have such stars (SA depends on wavelength with SCTs, and also with primary/secondary distance that changes when you focus) - almost "button" like rather than point like - smooth vs sharp so to speak.

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On ‎08‎/‎12‎/‎2018 at 12:31, ollypenrice said:

Why are you keen to add resolution via focal length and then reduce it via binning? It really doesn't matter whether you fill the frame or not. What matters is how many pixels (single or 4 pixel superpixels) you put under the object's image as projected by the telescope. It is this which determines the object's image's final size at full size (1 camera pixel = 1 screen pixel.) *

Earlier in the year I did a comparison in Astronomy Now between a 14 inch scope (2.4M FL) with large pixels and a 150mm scope (1M FL) with small pixels on galaxies. I found that the level of resolution and the size at which the images could be presented on screen was effectively the same. It seems that the considerably greater optical resolution of the larger scope was not translating into more final details. In both cases the mount was running with an RMS of less than half the image scale.

 

I didn't read the article myself but this in itself should not be a surprise.  I'm assuming the resolution of the test between the 14 inch and 6 inch were similar overall?  

The benefit of a larger scope comes from that at the same arcsec/pixel you can get more flux per pixel for the same amount of time.  So if you 0.8" per pixel on the 6" then on a 14" at the same resolution (and assuming same sensitivity) you'll get more flux on each pixel.  As such fainter details should show up earlier for less time observed. Usually though longer focal length instruments have "/pixel that are much smaller and spread the light out further so you don't see this gain in aperture.

I think sometimes this is where getting more resolution issue comes from.  There is likely a limit in any sky as to how well resolved you can make an object (without fancy tech anyway and 8000ft high telescopes). However you should get to the fainter stuff quicker *at the same "/pixel* which might make it seem like you are getting a higher resolution.

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46 minutes ago, Whirlwind said:

I didn't read the article myself but this in itself should not be a surprise.  I'm assuming the resolution of the test between the 14 inch and 6 inch were similar overall?  

The benefit of a larger scope comes from that at the same arcsec/pixel you can get more flux per pixel for the same amount of time.  So if you 0.8" per pixel on the 6" then on a 14" at the same resolution (and assuming same sensitivity) you'll get more flux on each pixel.  As such fainter details should show up earlier for less time observed. Usually though longer focal length instruments have "/pixel that are much smaller and spread the light out further so you don't see this gain in aperture.

I think sometimes this is where getting more resolution issue comes from.  There is likely a limit in any sky as to how well resolved you can make an object (without fancy tech anyway and 8000ft high telescopes). However you should get to the fainter stuff quicker *at the same "/pixel* which might make it seem like you are getting a higher resolution.

The resolutions were 0.66 for the big scope and 0.9 for the small one. Realized detail was honestly about the same.

The test was complicated by the greater sensitivity of the small pixel camera. In fact the smaller telescope with smaller but more sensitive pixels was probably faster but I didn't have the original raw data to explore this side of the comparison.

In priniciple I agree with you, though.

Olly

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I suppose you could put the larger CCD (I'm assuming the Atik?) on the 6 inch and then bin the 460 (I assume) until you get roughly the same "/pixel.  Then image the same object in 3nm narrowband.  Then you should have the calibration for the CCD sensitivity, then you can run the same exercise with the two telescopes making the "/pixel roughly equivalent.  Then compare a bright but non-saturated object flux (star is likely best) and divide out the response issue from the CCDs.  

Of course that means multiple striping of setups and retesting, and imaging time is valuable.

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