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6" RC for imaging..?

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#1 betelgeuse91

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Posted 20 March 2019 - 09:55 PM

Hi! I've been imaging with a wide field set up of 80mm apo refractor + avx + dslr.

 

And I see a 6" RC sold just for $400 and I was thinking maybe this could be an economical add for my imaging choices (it'll only be for DSOs)

 

I'm here to ask for advises on my impulsive, dangerous thoughts...

 

I used to image with a 6" f/5 Newtonian, which weighs about the same as a 6" RC. My avx performed well with guiding error around 1.5 arcsec rms. So I don't worry too much about guiding. 

 

I also have a x0.8 flattener/reducer for my refractor, which I could try for the RC. I have a laser collimator I used for the Newtonian but will I need it for an RC?

 

I hear that the collimation is difficult with RCs... How hard is it and what kind of collimation tools do I need? 

 

In particular, I am looking at Astro-Tech's at6rc, just because I have a good experience with their at80edt. 

 

Any advises or thoughts? Should I just walk away from the temptation and continue living the easy life as a refractorer?

 

Thanks so much in advance...! 


Edited by betelgeuse91, 20 March 2019 - 10:05 PM.


#2 pfile

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Posted 20 March 2019 - 10:39 PM

if you have to collimate it, you will have to become an expert. i think it's a little easier to collimate the 6 since the field curvature is greater and the star shapes at the corner are very evident. you may not have to collimate it though. if the collimation is close to correct, then it can probably be done only under star test. no matter what you use to collimate it (cheshire, glatter laser, takahashi scope) you will always have to do the final tweak under star test anyway.

 

a lot of people replace the focuser with something sturdier (and motor-driven), which easily doubles the price of the OTA.

 

the astro-tech RC6 is made by GSO in china and is the same as every other RC6 out there.

 

i used a 0.8x reducer/flattener with mine (TRF-2008) and it worked great.

 

rob



#3 Ranger Tim

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Posted 20 March 2019 - 10:40 PM

I have had this particular scope since it first came out. It is universally loved and hated -- depends on your experience with it. It is a very inexpensive RC and can yield great images if tuned optimally. Getting it tuned can be extremely frustrating. Many have sold them after losing patience with their shortcomings. Here's a list of typical pros and cons, depending on the sample that you might obtain:

 

Pros

Small scope that is easy to mount (compact)

Tends to keep collimation if not roughly handled

Inexpensive

Somewhat dew resistant (better than SCT)

There is a lot of imaging potential for the 1380mm focal length

 

Cons

Focuser is not highest quality and won't tolerate really heavy loads

Focuser often not aligned with secondary

Collimation is almost voodoo hard

Diffraction spikes are fat and thick, but short

Curvature must have field flattener

f/9 is slow

Is a poor choice for viewing only

Requires a lot of counterweight on the front end to balance cameras and main mirror.

 

I like mine and have used it a lot. I have learned how to collimate it and have found what works for me. Try not to get too far from factory if you can help it. It is radically different from collimating a newtonian. I have the focuser alignment adapter/ring and the AT2FF and use them religiously. The AstroPhysics Flattener is also highly recommended. You need more than a simple laser to collimate it properly -- the process is more important than the tools, and there are several different methods.

 

This is an economical scope and is not meant to compete with the big name RC's, but it can be a useful tool if you have some patience and luck. If you are truly interested read the zillion threads involving it and make a choice. Sometimes it is a roll of the dice. I am not as detail oriented as others so maybe I settle for lesser quality, but I have to keep my budget in mind when purchasing scopes. You may not have the same restrictions...


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#4 bobzeq25

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Posted 20 March 2019 - 10:54 PM

The $400 price is a snare.  I have about $1200, total, in mine.  Great optics, mediocre mechanicals.

 

Reducer, electric focuser to make the stock focuser tolerable (people sometimes sub a Moonlite, which costs more than the scope), dovetail for the top to stiffen the tube and mount a guidescope (that seems to work OK), collimation tools.  It took hours to figure out how to collimate it, more hours to get it right.

 

RCs are particularly sensitive to mechanical issues, a cheap RC is something of a contradiction.

 

Why did you get rid of the Newt?



#5 betelgeuse91

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Posted 20 March 2019 - 11:12 PM

Thank you all for the insights. It seems like I will need to think about if I will be patient enough to handle the RC, and if I can keep it stock without upgrades without losing too much. 

 

 

 

Cons

Diffraction spikes are fat and thick, but short

Haha, I always envied its thick short spikes!

 

 

 

Why did you get rid of the Newt?

It was a Celestron 6", my first telescope for imaging which wasn't really made for imaging. It was like a test scope to see if I can get into imaging... 

I suppose buying a 6" imaging Newtonian would be an option too, but I thought an RC would give much different perspective in imaging. 



#6 bobzeq25

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Posted 21 March 2019 - 12:11 AM


Diffraction spikes are fat and thick, but short

Huh?  Do these look short and fat to you?

 

I think that's miscollimation.  As everyone says, it's really hard to collimate the scope.  I had to work many hours to get this.

 

Pleadies.jpg


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#7 betelgeuse91

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Posted 21 March 2019 - 02:57 PM

Huh?  Do these look short and fat to you?

 

I think that's miscollimation.  As everyone says, it's really hard to collimate the scope.  I had to work many hours to get this.

 

attachicon.gif Pleadies.jpg

I also get an impression that the RC spikes are shorter and fatter compared to Newtonian spikes. I think it's probably because RC has longer focal length and slower f number, and stars in the field are dimmer. I personally liked it that way. Also compared to Newtonians, RC spikes are so clean and nice where Newtonians have another diffraction from the mirror clips. 



#8 Pauls72

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Posted 21 March 2019 - 07:38 PM

I have one too. Mine is a TPO from OPT, but it's the same GSO scope.

Collimation will test your patients. You need at a minimum a Cheshire eyepiece and artificial star to perform collimation. On the good side once done, it doesn't move.

I added a second dovetail bar to the top to mount my guide scope. Took 13 months to get it, back ordered forever.

Astro Tech AT2FF field flattener.

I replaced the focuser with a GSO Liner Bearing focuser with a homemade focuser motor controller.

AT6RC-ST80-2s.jpg


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#9 bobzeq25

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Posted 21 March 2019 - 09:15 PM

If anyone is still reading this, and interested.

 

I got my top dovetail from ADM accessories, he may have them in stock.



#10 B. Hebert

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Posted 22 March 2019 - 12:16 AM

I will put in my vote for the 6" RC (mine is a TPO).  I have it on a Sirius (EQG) mount with my own guide scope (a "chin" mounted 500 mm mirror lens).

 

I have not had collimation problems, but mine came well collimated and has not moved much.  The focuser has not been a problem for me with a smaller DSLR (Canon Rebel) and I will put in a good word for the AT2FF.

 

I am probably pushing the limits of an EQG mount but it works for me.

 

If I won the lottery I might move up slightly; an 8" or 10" is just my daydream scope, but portability is also high on my list and my current system is comfortably portable.



#11 Complexmystery

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Posted 22 March 2019 - 01:13 AM

My first ever imaging scope was the AT6RC. I absolutely loved it. Hated the collimation though. I moved to an AT65EDQ. Sold it, went back to the RC6. I bought it againlol.gif . I found that I could get away with the KAF8300 size sensor and have little to no curvature around the corners. It's a fantastic scope for the money. Although, like others have stated, if I would ever buy it again I would get a new focuser. The stock one is okay for a DLSR but anything beyond that it is useless. Just my two cents-

 

 

 

 

Josh 



#12 rockstarbill

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Posted 22 March 2019 - 03:28 AM

Hi! I've been imaging with a wide field set up of 80mm apo refractor + avx + dslr.

 

And I see a 6" RC sold just for $400 and I was thinking maybe this could be an economical add for my imaging choices (it'll only be for DSOs)

 

I'm here to ask for advises on my impulsive, dangerous thoughts...

 

I used to image with a 6" f/5 Newtonian, which weighs about the same as a 6" RC. My avx performed well with guiding error around 1.5 arcsec rms. So I don't worry too much about guiding. 

 

I also have a x0.8 flattener/reducer for my refractor, which I could try for the RC. I have a laser collimator I used for the Newtonian but will I need it for an RC?

 

I hear that the collimation is difficult with RCs... How hard is it and what kind of collimation tools do I need? 

 

In particular, I am looking at Astro-Tech's at6rc, just because I have a good experience with their at80edt. 

 

Any advises or thoughts? Should I just walk away from the temptation and continue living the easy life as a refractorer?

 

Thanks so much in advance...! 

Lots to unpack here, but I think the most important thing to state here is that RC telescopes are not any more difficult to collimate than other optical systems are. The problem is not with the RC design itself, the problem is with the design of the inexpensive and poorly made RC telescopes. The Astro-Tech RC's are fairly inexpensive, but as others have noted here -- have design flaws in them that are difficult to overcome. There are some GSO mirrored designs that are more forgiving in terms of the design, but they will not fall at the same price point. I think the TS Optics Carbon RC and the Carbon RC sold by Agena Astro (whom will star-test collimate the RC for you) are likely going to yield people better results. So if that is in your budget range, I would look at those. 

 

I dont think the "easy life with a refractor" is a good way to go if you already have experiences with refractors. I was completely afraid of adding a Newt and an RC to the herd, but in the process I learned a lot about these systems that has squashed those fears. Sure, it takes more investment in terms of your time to get used to these systems and how to tame them, but there are really great advantages to using Newts and RC's for imaging purposes. On the RC side of the equation, they are REALLY cool if you want to get into spectroscopy. For pretty pictures, they can give a good balance of focal length and aperture for the price. 

 

With all of that said, the SCT telescopes are by and large much easier to use if focal length if the real driver you have. While an assumption on my part, that is usually why people start to look at RC's. Is that the case here? 



#13 bobzeq25

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Posted 22 March 2019 - 09:54 AM

Lots to unpack here, but I think the most important thing to state here is that RC telescopes are not any more difficult to collimate than other optical systems are.

Actually, they are harder to collimate.  The fundamental reason is that the two hyperbolic mirrors interact, so moving one affects the collimation of the other.  You can't just adjust one, then the other, you have to sneak up on collimation, going back and forth.  Toward the end you're making the smallest motion you possibly can, just breathing on the Allen wrench, so the last adjustment on whichever doesn't mess up the other one.  BTDTGTTS.

 

There are reasons why someone might think the collimation spikes are short and fat.  <smile>  Or why people just give up on the inexpensive RCs, the mediocre mechanicals do make it that much worse.

 

The primary adjustment is far more sensitive than the secondary, in the sense that small adjustments affect it much more.  You have to deal with that, too.

 

Then there's the fact that mechanical collimation and optical collimation are usually different things.  That, on the 6 and 8, collimating can introduce tilt, since the primary and focuser move together.

 

As I said, many hours.  One persons "best" method (after trying others) involved disassembling/reassembling the scope.  Seriously.

 

RC's are great, if you're building a professional observatory, maybe sending it into space.  <grin>  Or if you can afford to pay for a Knaebel.  Look at the pictures, that's what it takes to make/have a good RC.

 

http://www.knaeble-e...m/telescope-1-1

 

The manufacturer has bragged here about them being very "very affordable".

 

https://www.cloudyni...ull-carbon-rcs/

 

As I said, a cheap RC is something of a contradiction.


Edited by bobzeq25, 22 March 2019 - 10:16 AM.


#14 billdan

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Posted 22 March 2019 - 10:03 AM

@Betelgeuse91 -- I hope you realise that the RC6 has a 3 inch secondary or a 50% obstruction, which means its light gathering capabilities is about the same as your 80mm refractor, or maybe worse with the RC6 longer focal length.


Edited by billdan, 22 March 2019 - 10:06 AM.


#15 bobzeq25

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Posted 22 March 2019 - 10:11 AM

@Betelgeuse91 -- I hope you realise that the RC6 has a 3 inch secondary or a 50% obstruction, which means its light gathering capabilities is about the same as your 80mm refractor, or maybe worse with the RC6 longer focal length.

It's not quite that bad.  You need to square that 50% (it's area, not diameter, that counts), it's effectively 25%.  I've pretty much replaced mine with a 130mm F7.  That's about the same light gathering as my 150mm RC.  The square root of (150 squared minus 75 squared) is 130.


Edited by bobzeq25, 22 March 2019 - 10:14 AM.


#16 billdan

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Posted 22 March 2019 - 11:15 AM

waytogo.gif  Good on ya Bob, I jumped the gun there.


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#17 TelescopeGreg

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Posted 22 March 2019 - 12:14 PM

It's not quite that bad.  You need to square that 50% (it's area, not diameter, that counts), it's effectively 25%.  I've pretty much replaced mine with a 130mm F7.  That's about the same light gathering as my 150mm RC.  The square root of (150 squared minus 75 squared) is 130.

I think you also need to apply the reflectivity factor of the two mirrors, and the transmission of the glass.  Net, you lose some 10% from the theoretical reflector light gathering capability, right?.  Not a huge difference, I suppose, but just to be complete.

 

Harder to quantify, how does one compare the loss of contrast from the center obstruction?  I know (have observed) that refractors give a better visual experience, but how does that affect imaging?



#18 rockstarbill

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Posted 22 March 2019 - 02:10 PM

Actually, they are harder to collimate.  The fundamental reason is that the two hyperbolic mirrors interact, so moving one affects the collimation of the other.  You can't just adjust one, then the other, you have to sneak up on collimation, going back and forth.  Toward the end you're making the smallest motion you possibly can, just breathing on the Allen wrench, so the last adjustment on whichever doesn't mess up the other one.  BTDTGTTS.

 

There are reasons why someone might think the collimation spikes are short and fat.  <smile>  Or why people just give up on the inexpensive RCs, the mediocre mechanicals do make it that much worse.

 

The primary adjustment is far more sensitive than the secondary, in the sense that small adjustments affect it much more.  You have to deal with that, too.

 

Then there's the fact that mechanical collimation and optical collimation are usually different things.  That, on the 6 and 8, collimating can introduce tilt, since the primary and focuser move together.

 

As I said, many hours.  One persons "best" method (after trying others) involved disassembling/reassembling the scope.  Seriously.

 

RC's are great, if you're building a professional observatory, maybe sending it into space.  <grin>  Or if you can afford to pay for a Knaebel.  Look at the pictures, that's what it takes to make/have a good RC.

 

In full context, you and I are saying the same thing. The inexpensive ones introduce challenges above and beyond working with the optics themselves. That aside you are working with two mirrors and are aligning them. Using the methods like the DSI method, you should be able to get an RC into good collimation. I dont think they need to be limited to professional observatories or being launched into space. Getting a good quality one, will cost one some money though, no doubt. The used RCOS 10" I have works wonderfully, but was not cheap and are very difficult to find anymore.


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#19 pfile

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Posted 22 March 2019 - 04:35 PM

collimating reflecting telecopes with spherical secondaries is significantly easier than telescopes with non-spherical secondaries. i think all of the various Dall-Kirkhams on the market fall into this category.

 

rob


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#20 the Elf

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Posted 22 March 2019 - 05:30 PM

Nothing to add to Ranger Tim. Almost nothing: the weak rear cell problem is solved by off axis guiding.

 

Here are some samples I got on my first rig, the RC6 on an AVX with reducer and OAG:

(click image for full screen, full res in the table below the image)

 

http://www.elf-of-lo...unning2018.html

http://www.elf-of-lo...rlpool2018.html

http://www.elf-of-lo...flower2018.html

 

My collimation procedure was a) get all circles you seen when looking into the scope concentric, b) adjust the secondary on a star until the diffraction pattern looked ok.

That is not the recommended procedure but something a beginner can deal with. May now work with all copies.

Meanwhile I updated to an RC8 and an SW EQ6-R. Both are much better IMHO.

Spice lenght is a question of star intensity. Here is an extreme example taken with the RC8:

http://www.elf-of-lo...dFlame2019.html


Edited by the Elf, 22 March 2019 - 05:32 PM.

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#21 bobzeq25

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Posted 22 March 2019 - 06:08 PM

I think you also need to apply the reflectivity factor of the two mirrors, and the transmission of the glass.  Net, you lose some 10% from the theoretical reflector light gathering capability, right?.  Not a huge difference, I suppose, but just to be complete.

 

Harder to quantify, how does one compare the loss of contrast from the center obstruction?  I know (have observed) that refractors give a better visual experience, but how does that affect imaging?

Not much.  Increasing the contrast in processing compensates pretty well.  It's mostly a visual issue.


Edited by bobzeq25, 22 March 2019 - 06:08 PM.

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#22 Stephen Kennedy

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Posted 22 March 2019 - 07:45 PM

Not much.  Increasing the contrast in processing compensates pretty well.  It's mostly a visual issue.


Actually, what professional astronomers use them for, the larger central obstruction of the RC actually confers some benefits in resolution compared to other mirror telescopes like Newtonians.
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#23 zxx

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Posted 22 March 2019 - 11:09 PM

I have the GSO 6RC ,got mine on sale for $299 last year great scope for the price.

On my CGEM  DX my guiding had to be down to a total RMS of 1.00 for nice round stars

with my DSLR . At 1370mm ,or 1100mm with a 0.8x FR you may need to do better then a total RMS of 1.50 with your AVX.


Edited by zxx, 22 March 2019 - 11:14 PM.

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#24 rockstarbill

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Posted 23 March 2019 - 12:15 AM

Not much.  Increasing the contrast in processing compensates pretty well.  It's mostly a visual issue.

Perhaps.

 

Here is a JPG of a stack of OIII data of M97 I took with a 10" RC and a QSI6120 camera. The image scale was 0.27"/px.

 

Seem pretty impressive to me. I dont think I could do this with any of the other scopes I have.

 

1_RC_Downrez_OIII.jpg


Edited by rockstarbill, 23 March 2019 - 12:18 AM.

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#25 betelgeuse91

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Posted 23 March 2019 - 09:24 AM

I really appreciate the input from everyone here. Thank you so much. 


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