6" F9 Ritchey Chretien Astrograph Telescope

[Bigwings]

Ok guys and ladies

Anyone used one and if so how good are they. 

Please also comment on visual as well as photo mode.

Thanks

[Tim]

RC telescopes are generally  better for photography than for visual use, the large central obstructions reduce the contrast at the eyepiece.

Colliiating them can be the devil's own work, but once properly set, they can produce excellent images. There is a review of the Omegon RC telescope in a recent Sky at Night magazine.

[Bigwings]

52 minutes ago, Tim said:

RC telescopes are generally  better for photography than for visual use, the large central obstructions reduce the contrast at the eyepiece.

Colliiating them can be the devil's own work, but once properly set, they can produce excellent images. There is a review of the Omegon RC telescope in a recent Sky at Night magazine.

Thanks Tim. 

Sounds like I am better off with my Mak 127.

My interest is caused by an urge to try my hand at some photography and more light pollution is driving me to a goto mount anyway and a nice mount for sale is paired with an RC scope.

Mainly I am visual...

Perhaps an EQ5 goto is the way to go.

My main scope is a 10" DOB, not to difficult to collimate.

 

 

[vlaiv]

I'll present my view on this topic, it might be a bit controversial, and it is not based on very big experience (although I've got some experience with RC scopes - I own larger brother of mentioned scope - RC8" F/8).

I've done a sort of comparison of scopes in this class (6") and here is what I think:

Let's take 5 representatives of different scope designs and compare, Newtonian, Refractor (we will limit ourselves to achromats due to price difference, and let's be honest, good APO triplet in 6" class will simply outclass all other designs on most comparison criteria), SCT, MCT and RC.

RC vs Newtonian:

- Newtonian will have an edge on planets, especially F/8 variant with small secondary obstruction. Such scope will be harder to mount (due to momentum arm, except for Dobsonian mount), but will have smaller FOV than RC (due to 1.25" focuser -  because of small secondary). In faster ratios coma will be an issue. RC will also have larger light throughput - due to 99% dielectric coatings, regardless of larger CO - newtonians usually have 94% mirror coatings (enhanced versions) - you can go for special coatings like 97% hilux - then they will be better matched.

- RC has better corrected field for AP so you can use pure mirror system without need for corrector. Corrector can be used to lower F/speed and further flatten already pretty flat field. Due to large illuminated circle - you can use 2" eyepieces and combined with focal reducer you can have wider views.

- For planetary AP, due to processing and sharpening, central obstruction and loss of contrast have tiny impact, so these two will be pretty much matched (long focal length newtonian).

- Price wise they are very close, newtonian being slightly ahead (cheaper).

- Light baffling and stray light protection - win for RC.

- Collimation ease - newtonian wins here.

RC vs SCT

- Shorter focal length and larger FOV for visual (f/9 vs f/10, 2" focuser).

- Less prone to dew problems, and better thermal properties (open design)

- Better photographic field (no coma, less curvature, ...)

- Similar "format" for mounting, similar weight

- SCT will have very slight edge on planets due to somewhat smaller CO, but light through put will be on RC side (again depends on mirror coatings, but SCT has additional corrector plate).

- price +RC, -SCT

RC vs MCT

- just look at difference between SCT and MCT usually mentioned on internet (thermal stability, planetary performance, smaller FOV) - and apply to previous section (RC vs SCT) - all RC strengths will be emphasized, while planetary performance will lag.

- price +RC, -MCT

RC vs Refractor

- it all comes down to fact that refractor - especially faster like F/8 or below will have very big CA issues - this means less contrast on planets, less contrast on DSO in spite of better ligth throughput - no CO at all. Photographic usability of such scope is limited to narrow band (you can do LRGB or OSC, but CA will have huge impact on final result).

- Achromat will be heavier and harder to mount, and it will loose in price department.

All in all, for that target budget, I think that RC is very overlooked option for good all around scope - both AP and visual. If you are worried about contrast loss on planets, have a look at this:

This is simulated MTF of 8" RC vs 5" refractor (both ideal figure). MTF diagram is usually used to represent contrast loss - X axis represents spatial frequencies (or think in terms of large/small features, large features close to origin, small features to the right) and Y axis represents contrast loss (or attenutation in %, going from 0 at origin to 1 or 100% at top). What you don't usually see is such diagram comparing two different scopes - different aperture sizes and different CO characteristics. When you align spatial features axis (X axis) then you can see that smaller scope, although having "higher" contrast, actually looses on detail, and if you use scope with large aperture and CO with small magnifications (equivalent to what you would use with smaller unobstructed scope - you can actually have less contrast loss).

Bottom line, RC might not perform as good on planets as other 6" options, however, it will  probably be on par or even better than 4" apo if you keep your magnification the same (up to x200).

Just to mention, those RCs seem to be optically very good instruments - I tested mine to system Strehl 0.94

[Bigwings]

15 hours ago, vlaiv said:

I'll present my view on this topic, it might be a bit controversial, and it is not based on very big experience (although I've got some experience with RC scopes - I own larger brother of mentioned scope - RC8" F/8).

I've done a sort of comparison of scopes in this class (6") and here is what I think:

Let's take 5 representatives of different scope designs and compare, Newtonian, Refractor (we will limit ourselves to achromats due to price difference, and let's be honest, good APO triplet in 6" class will simply outclass all other designs on most comparison criteria), SCT, MCT and RC.

RC vs Newtonian:

- Newtonian will have an edge on planets, especially F/8 variant with small secondary obstruction. Such scope will be harder to mount (due to momentum arm, except for Dobsonian mount), but will have smaller FOV than RC (due to 1.25" focuser -  because of small secondary). In faster ratios coma will be an issue. RC will also have larger light throughput - due to 99% dielectric coatings, regardless of larger CO - newtonians usually have 94% mirror coatings (enhanced versions) - you can go for special coatings like 97% hilux - then they will be better matched.

- RC has better corrected field for AP so you can use pure mirror system without need for corrector. Corrector can be used to lower F/speed and further flatten already pretty flat field. Due to large illuminated circle - you can use 2" eyepieces and combined with focal reducer you can have wider views.

- For planetary AP, due to processing and sharpening, central obstruction and loss of contrast have tiny impact, so these two will be pretty much matched (long focal length newtonian).

- Price wise they are very close, newtonian being slightly ahead (cheaper).

- Light baffling and stray light protection - win for RC.

- Collimation ease - newtonian wins here.

RC vs SCT

- Shorter focal length and larger FOV for visual (f/9 vs f/10, 2" focuser).

- Less prone to dew problems, and better thermal properties (open design)

- Better photographic field (no coma, less curvature, ...)

- Similar "format" for mounting, similar weight

- SCT will have very slight edge on planets due to somewhat smaller CO, but light through put will be on RC side (again depends on mirror coatings, but SCT has additional corrector plate).

- price +RC, -SCT

RC vs MCT

- just look at difference between SCT and MCT usually mentioned on internet (thermal stability, planetary performance, smaller FOV) - and apply to previous section (RC vs SCT) - all RC strengths will be emphasized, while planetary performance will lag.

- price +RC, -MCT

RC vs Refractor

- it all comes down to fact that refractor - especially faster like F/8 or below will have very big CA issues - this means less contrast on planets, less contrast on DSO in spite of better ligth throughput - no CO at all. Photographic usability of such scope is limited to narrow band (you can do LRGB or OSC, but CA will have huge impact on final result).

- Achromat will be heavier and harder to mount, and it will loose in price department.

All in all, for that target budget, I think that RC is very overlooked option for good all around scope - both AP and visual. If you are worried about contrast loss on planets, have a look at this:

This is simulated MTF of 8" RC vs 5" refractor (both ideal figure). MTF diagram is usually used to represent contrast loss - X axis represents spatial frequencies (or think in terms of large/small features, large features close to origin, small features to the right) and Y axis represents contrast loss (or attenutation in %, going from 0 at origin to 1 or 100% at top). What you don't usually see is such diagram comparing two different scopes - different aperture sizes and different CO characteristics. When you align spatial features axis (X axis) then you can see that smaller scope, although having "higher" contrast, actually looses on detail, and if you use scope with large aperture and CO with small magnifications (equivalent to what you would use with smaller unobstructed scope - you can actually have less contrast loss).

Bottom line, RC might not perform as good on planets as other 6" options, however, it will  probably be on par or even better than 4" apo if you keep your magnification the same (up to x200).

Just to mention, those RCs seem to be optically very good instruments - I tested mine to system Strehl 0.94

Thanks for that informative contribution....food for thought.

Mike

[Tim]

Having used most variants of scopes for imaging, I would suggest an RC is perhaps not an ideal starter scope. There are so many other things to get right, you dont want to be mucking about with the scope too. Hard to beat a decent refractor for starting out with AP.

[kirkster501]

If you are new to imaging I feel very strongly you need to start with and ED80 atop the EQ5 and forget RC scopes.  Collimation worries need to be off the table completely whilst you learn the other nuances of imaging.

[ollypenrice]

I had an RC here belonging to a guest. We read the manuals, we watched the videos, we understood the theory, we had the tools (Howie Glatter) and we could not get it right. Random effects prevented the process from behaving as anticipated. One adjustment interacted with another unpredictably. 

For me, life is too short and rafractors are too good.

Olly

[Bigwings]

17 hours ago, ollypenrice said:

I had an RC here belonging to a guest. We read the manuals, we watched the videos, we understood the theory, we had the tools (Howie Glatter) and we could not get it right. Random effects prevented the process from behaving as anticipated. One adjustment interacted with another unpredictably. 

For me, life is too short and rafractors are too good.

Olly

Thanks Olly. Better get a Frac....but try a few with the Mak 127 firstly

On 11/10/2018 at 17:19, Tim said:

RC telescopes are generally  better for photography than for visual use, the large central obstructions reduce the contrast at the eyepiece.

Colliiating them can be the devil's own work, but once properly set, they can produce excellent images. There is a review of the Omegon RC telescope in a recent Sky at Night magazine.

Thanks Tim. 

Sounds like I am better off with my Mak 127.

My interest is caused by an urge to try my hand at some photography and more light pollution is driving me to a goto mount anyway and a nice mount for sale is paired with an RC scope.

Mainly I am visual...

Perhaps an EQ5 goto is the way to go.

My main scope is a 10" DOB, not to difficult to collimate.

 

 

[newbie alert]

Lots of very good experienced imagers haven't got to grips with a RC...collimation has been described as the B swear word..

[Jkulin]

14 hours ago, newbie alert said:

Lots of very good experienced imagers haven't got to grips with a RC...collimation has been described as the B swear word..

For me collimation is so easy and no different to any other collimation as long as you take your time.

I own a Howie Glatter and occasionally put it in and check and then I check on a star, here's mine that I checked in September, you just need to remember to slacken off the opposite collimation nuts before tightening on the opposite side and to do it with very tiny amounts like an 1/8th of a turn.

I don't use mine for visual, just AP and I find it brilliant, mines the 8".

There were as far as I can tell some inconsistencies with the quality of the GSO's some were exceptional and others not so, so make sure if you do buy one that you check it first and make sure it produces good quality stars.

Hope that helps?

[newbie alert]

10 hours ago, Jkulin said:

For me collimation is so easy and no different to any other collimation as long as you take your time.

I own a Howie Glatter and occasionally put it in and check and then I check on a star, here's mine that I checked in September, you just need to remember to slacken off the opposite collimation nuts before tightening on the opposite side and to do it with very tiny amounts like an 1/8th of a turn.

I don't use mine for visual, just AP and I find it brilliant, mines the 8".

There were as far as I can tell some inconsistencies with the quality of the GSO's some were exceptional and others not so, so make sure if you do buy one that you check it first and make sure it produces good quality stars.

Hope that helps?

They certainly have a stigma attached to them..stunning images but on the flipside a good few can't get on with them collimation wise..maybe one day once I've gained a good bit of experience!

[Jkulin]

3 hours ago, newbie alert said:

They certainly have a stigma attached to them..stunning images but on the flipside a good few can't get on with them collimation wise..maybe one day once I've gained a good bit of experience!

I'd never had to collimate a scope before I got this and it was so easy, I coudln't believe what all the fuss was about. It could probably still do with a tweak, but I'm not that fussed.

[kirkster501]

I put some of these videos up, I did them when my late wife was very poorly in 2013.  I sold the RC long ago but never really had an issue collimating mine once I got the hang of it.  There are three planes that need to be in sync.

1.  The focuser needs to be aligned with the primary (not possible when an adjustment ring to do this is not provided as default on the cheaper RC scopes)

2. Align the secondary with the primary. 

3. Then realign the primary with the secondary to produce concentric circles on the wall.

Having said all that, you can get 99% of the way there, much more conveniently, with a Cheshire - as I also describe on the videos.  Once you get them collimated they will stay that way for months.  I could deliberately wreck the collimation on mine and get it back within 20 mins or so, most of the time.....!  As I explained to people, no one is going to die if you get it wrong...  You won't break anything!  It's twisting a few screws! Remove the emotion and work the problem as we say in engineering....

However,  I agree, this is all too much for a beginner - a refractor is the way to go.  With a refractor, you know any issues with your images cannot be attributed to collimation.   

I think RC scopes are more trouble than they are worth for an inexperienced imager, despite the apparent attraction of a largish apperture for reasonable cost.  

[Demonperformer]

Do you happen to have a link to your videos. Just bought one of these nd it would be good to see it done by someone who "never had an issue".

Thanks.

[Ikonnikov]

For fine tuning collimation under the stars with imaging camera attached the procedure below works beautifully. You need to start with the collimation reasonably accurate though, e.g. set using a cheshire, collimating telescope etc.

https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf&ved=2ahUKEwjou_G9h43eAhXlJcAKHdUvDtoQFjAAegQIAhAB&usg=AOvVaw1TQ8YAhohC9_gr_IRXBo3x

 

 

[vlaiv]

I've found this useful, and modified it slightly for my use (not using Bahtinov mask, but rather fwhm measurement in SharpCap):

https://deepspaceplace.com/gso8rccollimate.php

 

[ollypenrice]

1 hour ago, kirkster501 said:

I put some of these videos up, I did them when my late wife was very poorly in 2013.  I sold the RC long ago but never really had an issue collimating mine once I got the hang of it.  There are three planes that need to be in sync.

1.  The focuser needs to be aligned with the primary (not possible when an adjustment ring to do this is not provided as default on the cheaper RC scopes)

2. Align the secondary with the primary. 

3. Then realign the primary with the secondary to produce concentric circles on the wall.

Having said all that, you can get 99% of the way there, much more conveniently, with a Cheshire - as I also describe on the videos.  Once you get them collimated they will stay that way for months.  I could deliberately wreck the collimation on mine and get it back within 20 mins or so, most of the time.....!  As I explained to people, no one is going to die if you get it wrong...  You won't break anything!  It's twisting a few screws! Remove the emotion and work the problem as we say in engineering....

However,  I agree, this is all too much for a beginner - a refractor is the way to go.  With a refractor, you know any issues with your images cannot be attributed to collimation.   

I think RC scopes are more trouble than they are worth for an inexperienced imager, despite the apparent attraction of a largish apperture for reasonable cost.  

We looked at your videos, Steve, but the scope we were working on didn't follow the pattern. There were interactions between adjustments which sent us round in circles. Perhaps there was a maechanical problem somewhere.

Olly

[kirkster501]

2 hours ago, ollypenrice said:

We looked at your videos, Steve, but the scope we were working on didn't follow the pattern. There were interactions between adjustments which sent us round in circles. Perhaps there was a maechanical problem somewhere.

Olly

It would almost certainly be because the focuser was not aligned with the primary Olly.  Without the additional focuser collimation ring - not included with the budget RC scopes - there is no way to adjust the focuser's alignment with respect to the mirrors and this is crucial in the RC hyperbolic design.  This is a glaring oversight in my opinion.

All too complex and too much bother?  Yes  (to me at any rate).  Plus other issues that I do not like about reflectors in the context of imaging.   That's why I flogged it and moved to my TEC refractor.  Each to their own.  Others love the RC design.

Edited October 17, 2018 by kirkster501

[Big Jim Slade]

23 minutes ago, kirkster501 said:

It would almost certainly be because the focuser was not aligned with the primary Olly.  Without the additional focuser collimation ring - not included with the budget RC scopes - there is no way to adjust the focuser's alignment with respect to the mirrors and this is crucial in the RC hyperbolic design.  This is a glaring oversight in my opinion.

All too complex and too much bother?  Yes  (to me at any rate).  Plus other issues that I do not like about reflectors in the context of imaging.   That's why I flogged it and moved to my TEC refractor.  Each to their own.  Others love the RC design.

Yes, hanging the focuser off the primary was always a massive design flaw with the GSO RCs. Worth noting that the current generation of truss GSOs have changed this. In the original form you can't hang anything substantial off it without the collimation slipping with every slew.

The 6" compounds the problem by having the collimation screws right against the focuser mounting ring, so you can't adjust them if you fit a tilt adjuster. It also makes it impossible to swap screws for some decent sized knobs.

I'm thinking about getting a ZWO OSC for mine, to go after smaller targets. It seems well suited for that.

[dasphoto2010]

Nice scope for astrophotography. I got mine used on ebay it was only 4 months old. optics were spot on out of the box.