Ritchey Chretien Questions

[Clarkey]

2 hours ago, Sarek said:

I'd be interested to hear what people think about this collimation guide for an RC. It seems to me the most straightforward and simple solution I've come across so far

I don't see anything wrong with this method. This is similar to the Reego collimator method - but unfortunately it does not work with the 8" version as you cannot see the edge of the primary mirror.

I use this:

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

However, I have modded the procedure slightly for my scope. As my primary seems to align very well with the focuser (which I believe is not always the case), I get the secondary perfectly aligned as this is relatively easy. Then I use the star test version in the DSI guide to get it spot on. It is a bit tricky, but once dialled in, it holds collimation very well. I typically do mine once per season. I think you need to develop the method that suits your scope. What I would say though is only do very small adjustments. It is very easy to overdo the changes and it is a pig to get back (trust me, I know). With any method, I would always recommend a star test. 

With regards the FL, the comments above are pertinent. With the RC8 I normally bin2 which gives a pixel scale of around 1.4" / pixel. This is about as good as my seeing will allow and also is within the tolerance of my mount which typically guides with an RMS of around 0.6 - 0.7". I also image with an 800mm refractor and in terms of resolution there is very little between them.

[Sarek]

5 hours ago, Clarkey said:

I don't see anything wrong with this method. This is similar to the Reego collimator method - but unfortunately it does not work with the 8" version as you cannot see the edge of the primary mirror.

I use this:

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

However, I have modded the procedure slightly for my scope. As my primary seems to align very well with the focuser (which I believe is not always the case), I get the secondary perfectly aligned as this is relatively easy. Then I use the star test version in the DSI guide to get it spot on. It is a bit tricky, but once dialled in, it holds collimation very well. I typically do mine once per season. I think you need to develop the method that suits your scope. What I would say though is only do very small adjustments. It is very easy to overdo the changes and it is a pig to get back (trust me, I know). With any method, I would always recommend a star test. 

With regards the FL, the comments above are pertinent. With the RC8 I normally bin2 which gives a pixel scale of around 1.4" / pixel. This is about as good as my seeing will allow and also is within the tolerance of my mount which typically guides with an RMS of around 0.6 - 0.7". I also image with an 800mm refractor and in terms of resolution there is very little between them.

Thank you. I'm learning a lot here. I didn't know about binning - although I'd noticed it in the ASi Air app.  Interesting to hear how well your collimation holds. 

[WolfieGlos]

Hi @Sarek , did you order this scope in the end, and if so, how have you been getting on with it?

I also have a HEQ5 and 72ED, also a DSLR…and I was considering a Stellamira 110 or Skywatcher 150pds https://stargazerslounge.com/topic/408459-stellamira-110ed-or-sw-150pds/ until it was suggested to look at an SCT. However I have come to the same conclusion as you; the Stellalyra 6” RCT! 

I’m still considering the others, but the longer focal length for a similar price as the 150pds is really tempting. Collimation is holding me back, I’ve had a nightmare with the 72ED corners and tilt, despite being a refractor i must have the worst example going.

Another question I would add to the topic, is how do people take flats with a scope this size? Is a flat panel necessary? I currently use the tshirt + white iPad method with the 72ED, but it’s probably not big enough for the RCT. 

 

[Sarek]

On 06/04/2023 at 23:10, WolfieGlos said:

Hi @Sarek , did you order this scope in the end, and if so, how have you been getting on with it?

I also have a HEQ5 and 72ED, also a DSLR…and I was considering a Stellamira 110 or Skywatcher 150pds https://stargazerslounge.com/topic/408459-stellamira-110ed-or-sw-150pds/ until it was suggested to look at an SCT. However I have come to the same conclusion as you; the Stellalyra 6” RCT! 

I’m still considering the others, but the longer focal length for a similar price as the 150pds is really tempting. Collimation is holding me back, I’ve had a nightmare with the 72ED corners and tilt, despite being a refractor i must have the worst example going.

Another question I would add to the topic, is how do people take flats with a scope this size? Is a flat panel necessary? I currently use the tshirt + white iPad method with the 72ED, but it’s probably not big enough for the RCT. 

 

Hi, Apologies for late response - been away. I have not yet made my mind up about the RCT. Collimation still worries me as I've had no experience of it all let alone with an RC which seems the most difficult by far! I've also looked at longer focal length refractors (perhaps second hand if one comes up)  as well as the SW 150PDS.  I am beginning to favour the latter.

Sounds like you have been unlucky with the 72ED.

[900SL]

On 03/03/2023 at 01:04, pipnina said:

That definitely busts the myth I heard then!

Maybe the myth applies when these scopes are used with the 0.67x reducer/flattener...

 

This thread is making me want one of these now ha.

The issue was to do with the baffling on the earlier version of this scope, which is sold under a number of rebrands. It was subsequently corrected and I think the current models don't have this issue. 

There is a prominent YouTube video that is the cause of concern over this, which is outdated...

[WolfieGlos]

22 hours ago, Sarek said:

Hi, Apologies for late response - been away. I have not yet made my mind up about the RCT. Collimation still worries me as I've had no experience of it all let alone with an RC which seems the most difficult by far! I've also looked at longer focal length refractors (perhaps second hand if one comes up)  as well as the SW 150PDS.  I am beginning to favour the latter.

Sounds like you have been unlucky with the 72ED.

Thanks, and no worries, it is Easter with good weather 😃 That’s exactly what I read about RCTs, but it’s still required for a reflector too, I guess they must be easier 🤷‍♂️ Need to do some research on it. The longer refractors are generally coming in at more cost for less focal length, so I’m leaning away from them.

Yeah, very unlucky with the 72ed in the corners and it’s put me off SW refractors entirely. Some processing fixes I’ve recently found assist but that’s not the point really.