Saturn with ASI174MM...

[Pete Presland]

A very informative review of an excellent image. It will be interesting what you can achieve with some good conditions in the future.

[JamesF]

Thanks once again fellas - yes, a C14 James.

I have been wondering if this camera might work well with the C14 because it's perhaps easier to achieve what might be an optimal focal ratio compared with, say, some of the large dobs that people are using for planetary imaging.

I'm looking forward to giving it a go myself.  I used to enjoy the ease of imaging at f/20 to f/25 with my ASI120MC, but now I've gone mono I'm struggling to keep the focal ratio down with my C9.25 because of the extra kit in the optical path.  Perhaps returning to the seven to eight metre focal lengths that would suit the ASI174 on the same OTA might be the way forward.

James

[Kokatha man]

Thanks once again fellas!

Being honest I'm finding the larger pixel size is a bit of a relief...I had a "variable amplifier" I machined up using the lens element from a Televue 2X barlow & could get down to about 1.2X for the ASI120MM's.

But the problem is, these SCT's are put together with an optimum primary-secondary spacing (& thus image plane/focal point) which is also for visual...

Add your after-market focuser, EFW & barlow + camera - not forgetting the profile/length of all fittings/couplings - & you are really "throwing" the image way back outside the tube & all sorts of issues can arise therein...optically as well as mechanically - the optical degradation increases the closer you bring the primary & secondary towards each other & this is what you have to do to project the focal position further outside the scope tube.

Just with the quick, jerry-rigged assortment of what I had in hand I have brought the focus back towards the OTA a very significant amount for the ASI174MM & we honestly believe we can clearly notice the improvement in the Diffraction Rings symmetry (not concentricity, that's collimation) since we've done this - which must be a good thing.

[JamesF]

Sounds like you were pretty much in the same position as me Darryl, though to an even more extreme point.

My "guesstimate" of the optimal position for the focal plane on the C9,.25 is about 100mm behind the SCT thread.  I might expect the C14 to be a little more to work well with a 2" diagonal.  Is that consistent with your belief?  If you have a definitive source for this information I'd be very pleased if you'd share it.  For some time I've been posting that SCTs and Maks only achieve their stated focal length in one configuration, but I had no certainty of where that was.

Perhaps I should just stop faffing about and just decide where I'm going to have the focal plane of my camera (whichever one I happen to decide to use) and then make up/modify everything else in the optical train to achieve that.  I could perhaps look at using a low-profile newt focuser to allow the camera to be brought closer to the optimal position.  It might even be possible to mount a lower-powered barlow inside the drawtube.

James

[Kokatha man]

...actually I think it is probably considerably less that that figure James - they are designed as visual scopes & they're meant to be focused via the primary mirror adjuster!  

I've got a manual in pdf form somewhere...a fella who works for a well-known Oz scope seller sent it to me as a return favour a few years back - he worked in Cal. as a Celestron tech. for a few years & this was the manual they were given...only trouble is I have nfi where I've placed it as I have replaced my entire pc system in the meantime...but I know I wouldn't have deleted it!

It's a good 10-15 years old now but I think it's still relevant...will see if I can find it sometime - & btw the f/l does change as the mirror spacing is adjusted...

Depending upon whether I can make minor changes to the (new) setup I think I'll be ok but if not I will machine a new adapter etc up...one idea I had was like you suggestion of an "in-focuser" barlow but I don't think I'll need anything that radical now...

The Diffraction rings were definitely appearing "squished" at times & since the shorter train we are pretty certain that they are clearly more symmetrical...

[astroman001]

Darryl

Interesting discussion. I had wondered about this myself, but had put it to one side because of the great results Damian Peach gets with an electric focuser, diffraction corrector, barlow, filters and camera.

I'm pretty sure I am seeing limited rather than optics limited.

regards

Peter

[JamesF]

...actually I think it is probably considerably less that that figure James - they are designed as visual scopes & they're meant to be focused via the primary mirror adjuster!  

I could certainly believe that I've over-esimated.  I was thinking that the visual back would add about 40mm and the diagonal about 60mm (for a scope normally expected to be used with 1.25" fittings), but both of those figures might well be too high.  I know it won't necessarily be suitable for me, but I'd certainly be interested in hearing about whatever configuration you eventually end up with.

James

[JamesF]

I'm pretty sure I am seeing limited rather than optics limited.

I think that's a fair point.  My gut feeling however is that if you do the best you possibly can with the optics then you have the greatest chance of addressing issues with the seeing by stacking/deconvolution etc.

And it's good to have something to do on those cloudy nights

James

[U Cyg]

Excellent image, and a very informative thread.

Angie

[Kokatha man]

Thanks Angie...I'll put this red channel Jupiter in that surprised me when I calculated the elevation of Jove when we captured it - 31°!

Actually the rgb was quite reasonable but I didn't save it as an rgb at the time of processing but might go back & repro it as such.....really waiting for some half-chances to continue testing but it's clouds, clouds & more clouds here..!

[Nick Smith]

Thanks once again fellas!

Being honest I'm finding the larger pixel size is a bit of a relief...I had a "variable amplifier" I machined up using the lens element from a Televue 2X barlow & could get down to about 1.2X for the ASI120MM's.

But the problem is, these SCT's are put together with an optimum primary-secondary spacing (& thus image plane/focal point) which is also for visual...

Add your after-market focuser, EFW & barlow + camera - not forgetting the profile/length of all fittings/couplings - & you are really "throwing" the image way back outside the tube & all sorts of issues can arise therein...optically as well as mechanically - the optical degradation increases the closer you bring the primary & secondary towards each other & this is what you have to do to project the focal position further outside the scope tube.

Just with the quick, jerry-rigged assortment of what I had in hand I have brought the focus back towards the OTA a very significant amount for the ASI174MM & we honestly believe we can clearly notice the improvement in the Diffraction Rings symmetry (not concentricity, that's collimation) since we've done this - which must be a good thing.

Hi Kokatha Man!!

Thanks for the post.  I would really appreciate a bit more detail regarding "the optical degradation increases the closer you bring the primary & secondary towards each other" 

I am not an expert on telescope optics

Cheers

Nick

BTW reaaly nice pic!!!

[JamesF]

I did some hunting when Darryl mentioned this issue as I'd not heard of it before.  If I've understood what I read correctly, an SCT is only properly corrected for spherical aberration at  the stated design focal length, which is only achieved with the primary and secondary at some specific spacing (which can probably be calculated by working backwards from the focal length if desired).  Reducing that spacing or increasing it gives under- or over-correction of SA at the focal plane.

James

[Nick Smith]

Maybe true James, but have you ever noticed this when you are collimating an SCT with a Barlow/Powermate?

[JamesF]

I can't say that I have, but that may be due to the fact that I wasn't expecting to see it.  It might also be because SA doesn't show up particularly obviously when collimating?

James

[JamesF]

Interestingly I've just read through a paper relating to vignetting in SCTs in which it is suggested that the design backfocus for my C9.25 is about 115mm and for the C14 should be about 170mm.  That must be without additional optics though.  I'd guess that if you wanted to hit the right spot when using a barlow it would be necessary to get the focal plane in the right place without one purely by changing the position of the primary, then add the barlow and use the secondary focuser to find focus.

James

[Nick Smith]

It can do James

A good star test on one side of focus and poor on the other could be due to SA.

Cheers

Nick

[Nick Smith]

Interestingly I've just read through a paper relating to vignetting in SCTs in which it is suggested that the design backfocus for my C9.25 is about 115mm and for the C14 should be about 170mm.  That must be without additional optics though.  I'd guess that if you wanted to hit the right spot when using a barlow it would be necessary to get the focal plane in the right place without one purely by changing the position of the primary, then add the barlow and use the secondary focuser to find focus.

James

Hmmm

I would just be more inclined (clouds permitting!) to ignore this stuff and just give it a go with what you have

Nick

[JamesF]

Hmm.  I'm not sure I can see how it might show up.  I'd think that you'd still get a set of concentric rings on a star test, but they'd be a bit more diffuse and less clearly-defined when SA was present than when it wasn't.  That might be pretty hard to spot under less-than-ideal conditions or without having something to compare against, depending on the level of SA.

James

[Kokatha man]

...Spherical aberration was the main culprit of closer spacing as I understand - that "understanding" being limited btw, being honest

As i said above, we noticed on 4 occasions over these 2 nights that during collimation (for Jove earlier & Saturn in the morning) the Diffraction Rings appeared much more symmetrical than we have seen them for a very long time - I'm a bit compulsive/obsessive & I've worried about this for the last year or 2 & the best I could research (not a lot, but perhaps saying more about how/where I googled etc) I couldn't find any other (additional) reasonable explanations...

There has  been this slight distortion, perhaps symmetry isn't quite the right description - more an elongation or compression of the circular nature that we were used to for quite some time - I link it to when we installed the longer train that we employed when we switched to the ASI cams from the PGR...but I have to say this is all a bit obscure tbh! 

This of course is with the scope right at ambient etc & regardless of its orientation...I formerly employed locking bolts on the primary but have disengaged them recently - but this didn't seem to impact either way...

I guess I am linking the improved appearance (noted by both Pat & myself) to this shorter train...although I must admit that it can appear worse at times than others so seeing conditions must be at work also - or our ageing eyes..! 

Other than that the comments by Don & that manual were my only other references...haven't found it - although I haven't really searched yet... 

Also: James, I think you're probably right - I'd forgotten that the diagonal would be part of the visual train but this would still mean our new shorter train is considerably longer than this type of set-up...

[Nick Smith]

Agreed James

See my above post!

Nick

[Merlin66]

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

[Kokatha man]

...missed both of your posts while I was typing mine...this is all rather obsessive I appreciate & don't want anyone unduly fretting btw...that's my job!!! 

Sending you a PM James...

[JamesF]

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

That's very useful.  Thank you.

James

[Kokatha man]

...and thanks for that Merlin - puts a bit of perspective on my worries! 

...but, a quick ruler against the C14 reveals that I would be a good 7.5"+ (maybe 8"+) even with this new, shorter train so maybe I can go back to my compulsive/obsessive worrying again..! 

I don't think vignetting is of any consequence with planetary imaging James, but I'd like to look at that paper regardless...  

[Merlin66]

You'll find the focal length (check the plate scale) varies with back focus position.

The C11 (f10 base) is actually f11 when used with the spectroscope!!