RC, CDK or Vixen Cassegrain?

Topic starter

Dec 10, 2021 · Edited: Dec 10, 2021

Hello everybody!

Looking for advise, opinions and experience sharing.

I have two scopes, bot wide field. Totally happy with them. But I think I want to add a third one with more “reach” in terms of focal length, to shoot planetary nebulas and more distant galaxies.

After having done some reading and searching, I think I have narrowed down my choice to three possible options:

Ritchey Cretienne (GSO RC8 )
Dahl-Kirkham Cassegrain (Mewlon 180)
Vixen Cassegrain (VC200L VISAC).

I know for sure that I do not want EdgeHD. These are very fine scopes, but a moving primary mirror design seems a bit too complicated to me.

Also I can’t go beyond 8” aperture, as I do not have much more storage space in the house 😊

I know, many astrophotographers use 8” RC scopes. But Vixen somehow appears to be not so popular. I was wandering if there’s any reason for this in terms of AP suitability. I’ve also seen absolutely beautiful planetary images, done with CDK. But these do not seem to be used too much for DSO…

TIA and clear skies!

Respectful Engaging

Andy Wray

Dec 10, 2021

or just go for a good newtonian with a coma corrector?

David Nozadze

Topic starter

Dec 10, 2021

Andy Wray:
or just go for a good newtonian with a coma corrector?

Already have an Epsilon160. Takahashi says they will release a 1.5x extender for it some time in the future. But it will only increase my F/L to 750mm and cost as much as 8" RC. A newtonian with a "native" 1500`mm-ish F/L will be just too large.

John Hayes

Frequent contributor

Exemplary contributor

Dec 11, 2021 · Edited: Dec 11, 2021

A RC is corrected for both spherical aberration and coma but it is particularly challenging to align correctly and they suffer from field curvature. A straight Cassegrain is only corrected for spherical aberration. A CDK is considerably easier to align and it can be corrected for SA, Coma, and field curvature. Of those three choices, the CDK is the best option–by far.

John

Helpful Concise

Yuxuan

Dec 11, 2021 · Edited: Dec 11, 2021

Voted for CDK. Keep in mind that the Mewlon 180 is a DK, but not a CDK. It is wonderful for visual, but has very large off-axis coma and small field of view, and thus is not suited for photography.

Well written Helpful Concise

David Nozadze

Topic starter

Dec 11, 2021

Yuxuan:
Voted for CDK. Keep in mind that the Mewlon 180 is a DK, but not a CDK. It is wonderful for visual, but has very large off-axis coma and small field of view, and thus is not suited for photography.

Thank you very much! I did not realize that. Well, Mewlon is definitely excluded then.

Well written Respectful

Reg Pratt

Dec 11, 2021 · Edited: Dec 11, 2021

I vote RC. They're excellent scopes and not nearly as difficult to collimate as people say they are. Use the DSI method, fine tune your corrector back focus, and you get a very good starfield. I have 2 8" myself.

Rouz Astro

Dec 29, 2021

Ive had all 3 and hands down the CDK gave me the best results.

The "premium" RC was very difficult the get perfectly collimated.

The VC200L was ok but my images didn't really turn out very good, the spider was ridiculously thick!

My CDK12 collimated in 15 minutes. I liked it so much ordered the CDK14 a few months ago.

Here is my review on the CDK12:

https://astrogeartoday.com/review-of-the-planewave-cdk-12-5/

Rouz
https://linktr.ee/Rouz_Astro

Concise

Rouz Astro

Dec 29, 2021

Reg Pratt:
I vote RC. They're excellent scopes and not nearly as difficult to collimate as people say they are. Use the DSI method, fine tune your corrector back focus, and you get a very good starfield. I have 2 8" myself.

I found the stars in the corners were never perfectly round.
Once you move to full frame will be even worse.
I suspect most users will notice that.

David Nozadze

Topic starter

Dec 29, 2021

Ive had all 3 and hands down the CDK gave me the best results.

The "premium" RC was very difficult the get perfectly collimated.

The VC200L was ok but my images didn't really turn out very good, the spider was ridiculously thick!

My CDK12 collimated in 15 minutes. I liked it so much ordered the CDK14 a few months ago.

Here is my review on the CDK12:

https://astrogeartoday.com/review-of-the-planewave-cdk-12-5/

Rouz
https://linktr.ee/Rouz_Astro

Thank you very much for your comment!

Indeed, CDK does offer the best performance than the other two. But, to my deep regret, the smallest CDK, available on the market, is 10". This is just too big for my current circumstances. Therefore, I am "forced" to consider only RC8 or VC200L.

I am not too afraid of RC collimation. I know it is tricky, but still doable. I am more concerned about the focuser. The stock GSO focuser does not seem to be of a very high quality. Also it is not compatible with ZWO EAF (which I am going to use). William Optics offers rack & pinion upgrade for GSO RC at around $800. Also I will need to add focuser tilt plate and Howie Glatter laser. So, from the budget point of view, RC8 will cost me more than Vixen. But, it is also one full stop faster and you made a very important point about the spider thickness. So, for the moment, I am more inclined towards GSO.

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Björn Arnold

Perfect post

Dec 29, 2021

David Nozadze:
I am not too afraid of RC collimation. I know it is tricky, but still doable. I am more concerned about the focuser. The stock GSO focuser does not seem to be of a very high quality. Also it is not compatible with ZWO EAF (which I am going to use). William Optics offers rack & pinion upgrade for GSO RC at around $800. Also I will need to add focuser tilt plate and Howie Glatter laser. So, from the budget point of view, RC8 will cost me more than Vixen. But, it is also one full stop faster and you made a very important point about the spider thickness. So, for the moment, I am more inclined towards GSO.

The tilt adapter is a MUST since the GSO mechanical design wouldn't otherwise allow a semi-decoupled adjustment of focuser and primary mirror.
IMHO, the original focuser is a bad design. You cannot rotate it without losing optical adjustment. There's the posibility to attach a Baader SteelTrack focuser. This would also be compatible with your ZWO EAF.

To adjust the optics, you have different options and you would need to find the one that fits you. You can use the Howie Glatter laser OR the Takahashi collimating eyepiece OR (my way) a standard collimating laser plus a standard Cheshire eyepiece.

A helpful tool you might want to purchase is a Ronchi eyepiece which will help you to confirm the mirror spacing. My GSO 8" RC came with incorrect mirror spacing.

Another note on my experience: I'm not using the RC with any field corrector or reducer. With my 4/3" sensor, the aberrations are negligible into the corners. If you have a larger sensor, you probably need a corrector.

CS,
Björn

Helpful Insightful Engaging

David Nozadze

Topic starter

Dec 29, 2021

Björn Arnold:
David Nozadze:
I am not too afraid of RC collimation. I know it is tricky, but still doable. I am more concerned about the focuser. The stock GSO focuser does not seem to be of a very high quality. Also it is not compatible with ZWO EAF (which I am going to use). William Optics offers rack & pinion upgrade for GSO RC at around $800. Also I will need to add focuser tilt plate and Howie Glatter laser. So, from the budget point of view, RC8 will cost me more than Vixen. But, it is also one full stop faster and you made a very important point about the spider thickness. So, for the moment, I am more inclined towards GSO.

The tilt adapter is a MUST since the GSO mechanical design wouldn't otherwise allow a semi-decoupled adjustment of focuser and primary mirror.
IMHO, the original focuser is a bad design. You cannot rotate it without losing optical adjustment. There's the posibility to attach a Baader SteelTrack focuser. This would also be compatible with your ZWO EAF.

To adjust the optics, you have different options and you would need to find the one that fits you. You can use the Howie Glatter laser OR the Takahashi collimating eyepiece OR (my way) a standard collimating laser plus a standard Cheshire eyepiece.

A helpful tool you might want to purchase is a Ronchi eyepiece which will help you to confirm the mirror spacing. My GSO 8" RC came with incorrect mirror spacing.

Another note on my experience: I'm not using the RC with any field corrector or reducer. With my 4/3" sensor, the aberrations are negligible into the corners. If you have a larger sensor, you probably need a corrector.

CS,
Björn

Dear Björn!

Thank you very much! This information is most useful. I got all my questions answered!

Luckily, I already have a Takahashi collimating eyepiece, as well as a standard laser and a Cheshire. Happy to know that these are sufficient. But I did not know anything about the Ronchi. I guess you saved me a lot of time and trouble with this advise

My largest camera is APS-C size. I hope that it is just small enough to be used without a corrector.

Still think to go with the William Optics focuser. First of all because it has a screw mount on the camera side. But, secondly, I am sooo dissapointed with Baader, that I don't feel like buying anything from them any time soon (they sent me a brand new, sealed, narrow band filter with scratches and were very late to follow up on the inquiry).

CS

David

Respectful Supportive

Björn Arnold

Perfect post

Dec 29, 2021

Dear David,

The Ronchi isn’t a necessity for proper adjustment. I was using it to confirm the back spacing of the scope and the presence of optical aberrations.

For the APS-C, you will see some astigmatism setting in in the image corners. If it’s to an acceptable level, you either leave it, crop it or go for a corrector.

CS,
Björn

Well written Helpful Insightful Respectful Concise

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