RC vs Dall khirkham

[Nova2000]

Hi. 

When I was browsing the net I found about a type of reflector named Ritchey chretien and dall khirkham. These are specifically designed for astrophotography. What exactly is their speciality and why are they used so much? 

[MARS1960]

16 minutes ago, Nova2000 said:

Hi. 

When I was browsing the net I found about a type of reflector named Ritchey chretien and dall khirkham. These are specifically designed for astrophotography. What exactly is their speciality and why are they used so much? 

An RCT is not a reflector. it's basically an SCT variant. 

I had a 6" RCT and tbh found it a pain to use and collimate, it was also fairly slow at f9, a frac is so much easier to use for imaging IMO.

[MARS1960]

PS, there is a great thread on the RCT.

 

[mikeyj1]

11 minutes ago, MARS1960 said:

An RCT is not a reflector. it's basically an SCT variant. 

sorry but i have to take issue with that.  it is a pure reflector, no lenses, which is not true for the Dall K.

here is a good link: http://www.alluna-optics.com/Alluna-Blog/111-blog-Ritchey-Chretien-or-DK.html

i don't have either, but the common argument for a RC would be to compare to a Apo refractor, in which case its aberration free image is cheaper for the aperture, lighter to mount, and also faster cool down compared to an SCT, and coma free compared to a reflector (debateable?).  For the downsides like collimation, i would ask to anyone that has one

cheers Mike

[Nova2000]

Do rc require any flattner? Or just a reducer?  

[Peter Drew]

The Ritchey-Cretien telescope is a variant of the Cassegrain, not a SCT, it has no Schmidt corrector. Both mirrors are hyperbolic to remove off axis aberrations, the secondary is typically larger than for the classic Cassegrain. A Dall-Kirkham employs a 70% corrected parabolic primary and a spherical secondary. This system is "easier" to produce but has a narrower FOV than the R-C.  

[MARS1960]

6 minutes ago, mikeyj1 said:

sorry but i have to take issue with that.  it is a pure reflector, no lenses, which is not true for the Dall K.

here is a good link: http://www.alluna-optics.com/Alluna-Blog/111-blog-Ritchey-Chretien-or-DK.html

i don't have either, but the common argument for a RC would be to compare to a Apo refractor, in which case its aberration free image is cheaper for the aperture, lighter to mount, and also faster cool down compared to an SCT, and coma free compared to a reflector (debateable?).  For the downsides like collimation, i would ask to anyone that has one

cheers Mike

Fair point, i should have said the optical design is more like an SCT.

[MARS1960]

4 minutes ago, Peter Drew said:

The Ritchey-Cretien telescope is a variant of the Cassegrain, not a SCT, it has no Schmidt corrector. Both mirrors are hyperbolic to remove off axis aberrations, the secondary is typically larger than for the classic Cassegrain. A Dall-Kirkham employs a 70% corrected parabolic primary and a spherical secondary. This system is "easier" to produce but has a narrower FOV than the R-C.  

Fair point to i should have omitted the C from my SCT.

[fireballxl5]

Just now, MARS1960 said:

Fair point to i should have omitted the C from my SCT.

maybe the S lol

[MARS1960]

1 minute ago, fireballxl5 said:

maybe the S lol

Lol, that would make even more sense.

I think the point i was really trying to make is that for a newbie to all this an RCT @ F9 is not the best choice, i would start with an easy to use and easier to image frac like an 80ED.

[Nova2000]

Yup f9seems very slow. I'm thinking of a new scope for advance astrophotography. Till then I will be using  a ts70ed. I also need to get a mount for my self. And a good autoguider. 

[Louis D]

2 hours ago, Nova2000 said:

Do rc require any flattner? Or just a reducer?  

An RC has a curved field and requires a field flattener for imaging unless the imager uses a curved focal plane.  An RC also has some off-axis astigmatism.  Read up here for more basics on RCs.

[Nova2000]

7 minutes ago, Louis D said:

An RC has a curved field and requires a field flattener for imaging unless the imager uses a curved focal plane.  An RC also has some off-axis astigmatism.  Read up here for more basics on RCs.

Then how do I use a focal reducer? If I use a Flattner? 

[Louis D]

While I'm not familiar with RC imaging, most SCT and refractor field flatteners are also focal reducers.  Hopefully someone with RC imaging experience can chime in here.  I did a quick google search and turned up quite a few discussions on this topic out there on the various astro imaging forums.