Saturn with ASI174MM...

[JamesF]

I do accept that this level of detail is a little obsessive, but it's often good to push the boundaries of one's understanding.  I'm the kind of person who prefers not to worry about things from a position of knowledge rather than from one of ignorance

James

[Nick Smith]

Guys, I have the following data from the Celestron white paper on SCT's

Design back focus from primary mirror baffle tube lock ring

C5 - 5"

C6 - 5"

C8 - 5"

C925 - 5.475"

C11 - 5.475" from the 3"/2" reducer plate

- 5.975 from the lock ring

C14 - 5.475" from the 3"/2" reducer plate

- 5.975 from the lock ring

Hope this helps

Do you have any way of attaching this white paper to your post?  I have never in my years of using SCTs considered this to be an issue so would be really interested to see the source material.

Thanks

Nick

[Kokatha man]

...somewhat similar James   - that quick measurement was an approximation of the total length with just the Moonlite focuser on...I store the scope upright in my studio without the EFW etc train attached...that's packed away atm!

[Nick Smith]

Okay guys.  Not sure if I am any the wiser than I was 30 minutes ago, but must go to bed.

Cheers

Nick

[JamesF]

Do you have any way of attaching this white paper to your post?  I have never in my years of using SCTs considered this to be an issue so would be really interested to see the source material.

Thanks

Nick

I think we might still allow for the possibility that whilst there is SA present at non-optimal mirror spacings that it might not be sufficiently significant to impact visual use or imaging or both.  I'd assume that it will have an impact on imaging before visual, so perhaps two interesting questions to answer would be:

1) How much SA does there need to be before it has an impact on image quality?

2) How does one calculate the amount of SA due to sub-optimal mirror spacing (perhaps working from the actual amount of backfocus required)?

I'm quite a long way out of my depth at this point

James

[Kokatha man]

...I think we are all beyond our depth  a bit James...but like you I want to be able to comprehend as much as possible, within limits - beyond whatever my limits might be at any one time I try to cover my bets by being as fastidious as earthly possible with everything I can control or that which I know/suspect is controllable - even things I'm suspicious of..!        

ps: got the email etc, thanks!  

[Kokatha man]

Do you have any way of attaching this white paper to your post?  I have never in my years of using SCTs considered this to be an issue so would be really interested to see the source material.

Thanks

Nick

Yes. I'd like to see this paper also.

A quick update - I measured the imaging train I'm now using with the ASI174MM before we packed up yesterday morning after being lucky enough to get a good night's imaging with the new camera on Jupiter & Saturn where we made a few comparisons: my "train" is still around 230mm so I don't want to think how much more it was before fwiw...

[Merlin66]

The celestron white paper actually discusses the HD Edge design compared with other SCT's. Unfortunately it is an 8 Mb PDF file.

Drop me an email address for a copy.

(The backfocus spacings actually came from the celestron blog where the question as asked)

The change of back focus affects all the aberrations, not only SA.....you really need a Zemax analysis and a series of spot diagrams to determine the outcome.

I'll have a look a Suiter, see if he discusses this issue.

Ken

[Nick Smith]

I think we might still allow for the possibility that whilst there is SA present at non-optimal mirror spacings that it might not be sufficiently significant to impact visual use or imaging or both.  I'd assume that it will have an impact on imaging before visual, so perhaps two interesting questions to answer would be:

1) How much SA does there need to be before it has an impact on image quality?

According to Damian Peach about 1/4 wave.  Here is the link:  http://www.damianpeach.com/simulation.htm

Can't find any other references on the subject.

Cheers

Nick

[MarsG76]

Fantastic Result.... I keep looking at your image... cant get enough of it, if you got something this good at bad-average seeing, imagine what you will image when your seeing is perfect...

Well done

[Kokatha man]

Thanks, Saturn is starting to wind down for us...we'll keep imaging in iR for the +63N rift/spot but we'll start to concentrate more on Neptune & Uranus from now on.

Incidentally, with all the commentary on optimal imaging train lengths for SCT's (about 6" for the C14, all those figures posted are on the Celestron site) it was pointed out to me that this distance should be measured only to the prime focal plane for imaging applications, not to the camera sensor...a barlow (if used) would be positioned just inside said prime focal plane...this can be calculated, but for situations like our own means that we are working pretty close to said distance.