8" Ritchey-Chrétien for Astrophotography?

[AndyThilo]

Anyone have one of these, various brands available from what I've seen, Orion/TS-Optics/i-Optron. How well do they perform and how problematic are they for collimation? I'm guessing the fixed primary means there's no worry of differential flex so a guide scope is fine?

I know that TS-Optics sell a 0.67 reducer which makes it a 1000mm f5.4 which is quite nice. 

Opinions, recommendations etc.. all welcome :). 

[johninderby]

Not to forget FLO.

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-8-f8-m-lrs-ritchey-chrtien-telescope-ota.html

[inFINNity Deck]

The collimation of a RC8 was recently discussed in this forum:

As you can read it is not overly complicated. Main importance is to only use two of the three collimation screws to avoid alteration of the mirror distance.

Nicolàs

[Seelive]

Is far as I'm aware, despite the apparent lack of aberrations, they still suffer from a strong field curvature so a field flattener will still be required for imaging.

[lukebl]

17 hours ago, inFINNity Deck said:

The collimation of a RC8 was recently discussed in this forum:

As you can read it is not overly complicated. Main importance is to only use two of the three collimation screws to avoid alteration of the mirror distance.

Nicolàs

I was the person who originally started that thread about collimating a Ritchey-Chretien, and have only just realised that it ran to 126 replies!

On the subject of the collimation, I did finally get to grips with it and now know that collimating an RC is nowhere near as difficult as it seems at first. You just have to be very methodical. I am pretty confident that I could now collimate an RC in 5 minutes, even without a laser collimator, although it shouldn't be necessary as they are very robust and hold collimation perfectly, especially if permanently pier-mounted.

On the original query of their suitability for astro-imaging, well that is what they're primarily intended for, so I guess they should perform well.

I got mine because I wanted something with a longer focal length so that I could capture smaller objects like galaxies and planetary nebulae. Unfortunately, due to persistent bad weather and personal commitments, I've barely used it this year although I intend getting out a lot more this winter. I currently use an Atik428ex, which has a small sensor and small pixels, meaning that it's oversampling and a the field of view is narrow, but I'm generally happy with the results. I've been limited by my NEQ6 mount which has not been tracking properly, probably due to my shoddy belt mod. Anyway, here's a couple of captures with the RC.

[vlaiv]

19 hours ago, AndyThilo said:

I know that TS-Optics sell a 0.67 reducer which makes it a 1000mm f5.4 which is quite nice. 

I'm not sure that works well. I have one, I tried it once and I did not like what I saw. I still need to try it some more to be 100% sure of how it performs as the issues I saw might have been related to tilt.

Main thing to realize is that 8" is corrected for up to 26-27mm diagonal. This means that you should be able to use APS-C sensor size without reduction, however, once you reduce the focal length - you effectively "increase" sensor size. Said 27mm diameter gets squeezed to x0.67 of its original size and that is around 18mm.

Have sensor that has more than 18mm diagonal? It is likely that you'll start seeing stars that are not as nice on the edges. For this reason, you'll often hear that people use this reducer more at x0.72-x0.75 "setting" (you can change magnification of reducer by changing distance to sensor).

This of course makes more sense as with x0.75 reduction - it gives about 20mm of corrected reduced field and that fits better with most sensors up to 4/3 size.

This reducer will be useful if you have smaller sensor, otherwise maybe look into dedicated reducer/flattener or perhaps Ricarrdi x0.75 reducer/flattener - I've read some reports that it works very good with RC scopes.