Back Focus Issue?

[vlaiv]

I think it is due to spherochromatism - SCT scopes are corrected for spherical aberration in only one wavelength - blue part of spectrum as well as IR is very susceptible to this.

Another thing with SCTs is that spherical correction depends on primary/secondary distance and depending where your imaging gear sits (and how long the train is) - there will be some "shift" in corrected wavelength and hence blue or IR "parts" can be less or more affected (it will be either blue or IR that is worse than in optimum configuration) - IR is not of particular concern if using IR cut, but blue is.

This is why external focuser helps - if one locks mirror in proper position this spherical aberration is at minimum, and of course it helps with mirror related tilt.

[geoflewis]

I've just come across this excellent thread. I have an old C14XLT (non Edge) which shows poor shape stars in the corners (to be expected) even when using an Optec x0.63 Telecompressor (FF/FR) - probably still to be expected with the inherent coma of these old C14 optics, but I have wondered about trying some spacers, but didn't now what, where and how they should be inserted in the optical train. I image with a QSI583wsg-5 using Astronomic Type 2 (LRGB)+Ha filters. The Optec lens was provided with a bespoke spacer for the QSI camera, so I've presumed that allowed for filter glass correction, but I'm just guessing..... I'm certainly not looking for perfection imaging DSOs with the C14, but might be worth some experimentation, so where does one get these very short spacers (+/- 1mm), so that I can have a play with them?

Cheers, Geof

[knobby]

1 hour ago, geoflewis said:

I've just come across this excellent thread. I have an old C14XLT (non Edge) which shows poor shape stars in the corners (to be expected) even when using an Optec x0.63 Telecompressor (FF/FR) - probably still to be expected with the inherent coma of these old C14 optics, but I have wondered about trying some spacers, but didn't now what, where and how they should be inserted in the optical train. I image with a QSI583wsg-5 using Astronomic Type 2 (LRGB)+Ha filters. The Optec lens was provided with a bespoke spacer for the QSI camera, so I've presumed that allowed for filter glass correction, but I'm just guessing..... I'm certainly not looking for perfection imaging DSOs with the C14, but might be worth some experimentation, so where does one get these very short spacers (+/- 1mm), so that I can have a play with them?

Cheers, Geof

I use these, not brilliant but does the job well enough

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

[vlaiv]

I use couple of these:

https://www.teleskop-express.de/shop/product_info.php/info/p7893_TS-Optics-Aluminium-Fine-Tuning-Ring-for-T2-thread---thickness-0-5-mm.html

(there are other thicknesses available):

https://www.teleskop-express.de/shop/index.php/cat/c124_Extension-Rings.html

There are more expensive, but very robust and do the job. I'm using them to shorten T2 thread because filter drawer that I'm using has very short thread and if you thread in something with longer thread it will protrude inside and block filter drawer operation.

There are actually cheaper "non astro" alternatives that you can maybe fit to purpose. Circlip / Seeger ring - or simply retaining ring of suitable diameter can be used - there are couple of variants, and you want "shaft" one - that has "smooth" inside like this one:

This was solution that I used for above problem and 1.25" filters that had too long thread (again it would stick out of drawer so it was impossible to insert drawer), since I was not able to find any 1.25" "astro" spacers.

[geoflewis]

42 minutes ago, knobby said:

I use these, not brilliant but does the job well enough

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

Thanks knobby, at that price they're worth checking out. If I get good results then I can always consider the more expensive option suggested by vlaiv as a permanent solution....

31 minutes ago, vlaiv said:

I use couple of these:

https://www.teleskop-express.de/shop/product_info.php/info/p7893_TS-Optics-Aluminium-Fine-Tuning-Ring-for-T2-thread---thickness-0-5-mm.html

(there are other thicknesses available):

https://www.teleskop-express.de/shop/index.php/cat/c124_Extension-Rings.html

There are more expensive, but very robust and do the job. I'm using them to shorten T2 thread because filter drawer that I'm using has very short thread and if you thread in something with longer thread it will protrude inside and block filter drawer operation.

There are actually cheaper "non astro" alternatives that you can maybe fit to purpose. Circlip / Seeger ring - or simply retaining ring of suitable diameter can be used - there are couple of variants, and you want "shaft" one - that has "smooth" inside like this one:

This was solution that I used for above problem and 1.25" filters that had too long thread (again it would stick out of drawer so it was impossible to insert drawer), since I was not able to find any 1.25" "astro" spacers.

Thanks both,

Geof

[PhotoGav]

The FLO pack of spacers looks good - lots of different sizes to play with. I have two metal T2 spacers that I tried out. No idea where they came from, they were what I found in my bottom drawer in the obsy when trying to fiddle with spacing! I’m interested to hear how you get on and if they make any difference @geoflewis.

While out walking the dogs I was pondering the SCT collimation / spacing issue and just wondered whether it makes a difference which side of focus I carry out the collimation. I noticed that it is different depending on which side I go out of focus. Should I collimate going ‘In’ - that is the direction of the final move to bring the system to focus during autofocus, so to me it would make sense to do it there. Thoughts? Unfortunately the current collimation was done after a focus move ‘Out’. I think I will try this before imaging tonight, if the forecast is correct and the cloud clears...

[steppenwolf]

4 hours ago, knobby said:

Just ordering new book for the Mrs...

Making every cross Stitch count ?

As those who know me will already be aware, I have an in-house expert on this subject!

13 minutes ago, PhotoGav said:

Should I collimate going ‘In’ - that is the direction of the final move to bring the system to focus during autofocus, so to me it would make sense to do it there.

Yes, I would be sure to do this - every little nuance helps.

[PhotoGav]

Thanks Steve, I will do that and see if the difference is noticeable. Now, come on clear skies...

[geoflewis]

4 minutes ago, PhotoGav said:

The FLO pack of spacers looks good - lots of different sizes to play with. I have two metal T2 spacers that I tried out. No idea where they came from, they were what I found in my bottom drawer in the obsy when trying to fiddle with spacing! I’m interested to hear how you get on and if they make any difference @geoflewis.

While out walking the dogs I was pondering the SCT collimation / spacing issue and just wondered whether it makes a difference which side of focus I carry out the collimation. I noticed that it is different depending on which side I go out of focus. Should I collimate going ‘In’ - that is the direction of the final move to bring the system to focus during autofocus, so to me it would make sense to do it there. Thoughts? Unfortunately the current collimation was done after a focus move ‘Out’. I think I will try this before imaging tonight, if the forecast is correct and the cloud clears...

Hi Gav, I will post back here in due course, but I'm currently set up for DSO imaging with my 4" APO, so won't be testing the C14 for a while yet, though will probably get back to some galaxy close ups as galaxy season comes around again.

With regards to focus and collimation with my C14, I always focus inward as last move, then lock the focus down with the flop stoppers that were custom fitted to my scope. I then only make subsequent focus adjustments with my electronic Moonlite focuser. I do all my collimation using a bright star viewed on my laptop via my ASI120MM camera, as that way I can see the star image on screen whilst standing on steps to access the collimation screws, also using the mount's HC to keep the star centred as I make small adjustments.

Cheers, Geof

[vlaiv]

Out of curiosity, I know that regular SCTs have spherical primary, is that the case with EdgeHD as well (since it is better corrected, maybe primary has changed curve)

Just trying to wrap my head around difference between primary tilt of other conical shapes vs spherical - because when you tilt sphere you get another sphere .

I think I get it though - it is position of "lowest" point on the sphere that counts? Right - if it's on optical axis (in relation to secondary / corrector) then it's collimated, but if its at slight offset (I think that happens when you "tilt" primary) - then you get out of alignment. But surely tilting secondary and corrector can fix this - at expense of optical axis not being aligned with main physical axis of the scope (causing apparent sensor tilt?)

Not sure that I fully understand what could be going on with that.

 

[PhotoGav]

Vlav - I think you are probably right. I am on the edge (ha ha!) of my abilities with understanding optical systems here, but I think you are explaining the problems encountered with the EdgeHD. As for fixes... the best solution I have is: get a refractor! In the meantime, I will accept the best that I can get from the Edge, which isn't too bad to be fair.

[steppenwolf]

9 minutes ago, vlaiv said:

Out of curiosity, I know that regular SCTs have spherical primary, is that the case with EdgeHD as well (since it is better corrected, maybe primary has changed curve)

I believe that the corrections are done purely by optics just behind the visual back within the OTA?

[vlaiv]

7 minutes ago, PhotoGav said:

Vlav - I think you are probably right. I am on the edge (ha ha!) of my abilities with understanding optical systems here, but I think you are explaining the problems encountered with the EdgeHD. As for fixes... the best solution I have is: get a refractor! In the meantime, I will accept the best that I can get from the Edge, which isn't too bad to be fair.

I'm sorry if I'm adding any sort of confusion or anything, I'm just pursuing academical details rather than implying what you should do or change. After all, no need to have perfect optical system (and there is none that can be described that way) to enjoy beautiful images - and yours do look like they are going to be excellent.

10 minutes ago, steppenwolf said:

I believe that the corrections are done purely by optics just behind the visual back within the OTA?

Have no idea, I did not think that there is rear optical element with EdgeHD, but it's quite possible. I was under the impression that secondary and corrector (possibly even primary) were changed in shape to produce flatter, less aberrated field (harder to produce - hence price increase). Here is what I found on Celestron website in relation to this:

"EdgeHD is an aplanatic flat-field Schmidt optical system designed to produce aberration-free images with pinpoint stars all the way to the edge of the field of view of today’s largest imaging sensors and wide field eyepieces"

aplanatic - implies not conical (as far as I know, so not sphere, parabola, hyperbola or ellipse).

However, down the page:

"Guarantee ideal performance every time. Edge HD’s precision-machined mechanics and a spherical secondary mirror make collimation simple and accurate. "

So it turns out that secondary is spherical? This would mean that primary might not be?

[geoflewis]

24 minutes ago, vlaiv said:

Out of curiosity, I know that regular SCTs have spherical primary, is that the case with EdgeHD as well (since it is better corrected, maybe primary has changed curve)

Just trying to wrap my head around difference between primary tilt of other conical shapes vs spherical - because when you tilt sphere you get another sphere .

I think I get it though - it is position of "lowest" point on the sphere that counts? Right - if it's on optical axis (in relation to secondary / corrector) then it's collimated, but if its at slight offset (I think that happens when you "tilt" primary) - then you get out of alignment. But surely tilting secondary and corrector can fix this - at expense of optical axis not being aligned with main physical axis of the scope (causing apparent sensor tilt?)

Not sure that I fully understand what could be going on with that.

 

Hi vlaiv, I don't have the Edge HD version (I wish...), but this article by Celeston might answer your questions. Download the white paper pdf if you want a fuller read.....

https://www.celestron.com/pages/edgehd-optics

Cheers, Geof

[PhotoGav]

I think you’re right Steve, I was under the impression that there is some extra optical wizardry between the secondary and the visual back that makes it all ‘perfect’.

Vlad - it is very interesting and useful to pursue the academical details and I thank you for your great input to this thread. As for ‘beautiful images’, that is what I’m in this for and with any luck your optimism with regards to what I produce will be well founded!

The good news is that the sky is clearing......

[steppenwolf]

6 hours ago, vlaiv said:

Have no idea, I did not think that there is rear optical element with EdgeHD, but it's quite possible. I was under the impression that secondary and corrector (possibly even primary) were changed in shape to produce flatter, less aberrated field (harder to produce - hence price increase). Here is what I found on Celestron website in relation to this:

Page 3 of this document confirms what I thought - lens elements inside the OTA. The Edge HD 9.25 is what I was basing my belief on but the 1400 is apparently the same, however, I have no idea why the others have the lenses in markedly different places!

6 hours ago, PhotoGav said:

The good news is that the sky is clearing......

I can confirm that, I am actually imaging right now!?

[PhotoGav]

48 minutes ago, steppenwolf said:

I can confirm that, I am actually imaging right now!?

Hoorah! And snap... clear skies over England, at last!

I fiddled with collimation at the start of the session and I have a feeling that it has had a positive impact on quality again. Though I also think that different wavelengths are affected differently. I started with the green filter and the corners seemed almost perfect. The same cannot be said for the blue filter - not bad, but not as good. I will go through the data in the morning and post some subs. At least we are imaging though! Conditions are coming and going, I think there must be some high hazy moisture as guiding has good periods and then not so good. What exposure length do you use for guiding Steve?

[Davey-T]

15 minutes ago, PhotoGav said:

Hoorah! And snap... clear skies over England, at last!

Not this bit 

Dave

[PhotoGav]

2 minutes ago, Davey-T said:

Not this bit 

Dave

That’s not fair, Dave. Anyway, get ready for snow in the next few days....

[geoflewis]

Nor this bit.... 30+ mph winds and driving rain here right now. Bird feeder has been blown over, even though I have extra concrete weight on the base. No way would I roll back the obs roof in this ?

[Davey-T]

40 minutes ago, PhotoGav said:

That’s not fair, Dave. Anyway, get ready for snow in the next few days....

Just looked out and still cloudy so I'm turning in now, CO says clear in the morning so may get up early to catch some planets and a bit of solar after that, how's that song go about looking on the bright side ? 

Dave

[steppenwolf]

1 hour ago, PhotoGav said:

What exposure length do you use for guiding Steve?

I use 3 second exposures with an original Lodestar bin 2x2/OAG/1070 FL/F7 - currently 0.19 arcseconds RMS guiding tonight but I have seen it as high as 0.27 but I ain't complaining at that!

The cloud will probably upset things but, hey ho ........

[PhotoGav]

9 hours ago, steppenwolf said:

I use 3 second exposures with an original Lodestar bin 2x2/OAG/1070 FL/F7 - currently 0.19 arcseconds RMS guiding tonight but I have seen it as high as 0.27 but I ain't complaining at that!

The cloud will probably upset things but, hey ho ........

Interesting, thank you. I have used 2s in the past, but changed to 3s exposures last night in an attempt to settle things down and increase SNR on the faint star that PHD2 selected. You are just showing off with your RMS figures...! I have been getting around 0.5” +/- 0.1”, but certainly not down to the 0.2” area. I am at 1422mm FL, but still, I’m just jealous of your stats! I wonder what is the cause? Our setups are very different in the software department. Makes me wonder! I still have the pointing model rattling around in my brain as a potential point of improvement? Despite ruling it out once already!

[vlaiv]

4 minutes ago, PhotoGav said:

Interesting, thank you. I have used 2s in the past, but changed to 3s exposures last night in an attempt to settle things down and increase SNR on the faint star that PHD2 selected. You are just showing off with your RMS figures...! I have been getting around 0.5” +/- 0.1”, but certainly not down to the 0.2” area. I am at 1422mm FL, but still, I’m just jealous of your stats! I wonder what is the cause? Our setups are very different in the software department. Makes me wonder! I still have the pointing model rattling around in my brain as a potential point of improvement? Despite ruling it out once already!

I don't think focal length has anything to do with RMS figures. I've got the same stats (0.5" +/- 0.1") on HEQ5 with 1625mm FL (RC 8").

There are couple of things that can cause "substandard" performance out of premium mount (Mesu 200, from what I've seen people report should be in 0.2" or less range easily) - not related to mount tracking:

- Do you have guide FL and guide camera pixel size properly set? It can be the case of wrong "units" for RMS.

- What are you using for guiding (I'm guessing 683 WSG8 should have OAG on camera, right?), or what is your guide resolution. Some setups don't have enough precision in guide star position calculations and can give wrong RMS value (guiding could be better or worse than RMS figures would indicate).

- How stable is your mount? Is it pier or tripod. Is it susceptible to vibration? Is there a source of vibration nearby that can cause this - like motorway with heavy trucks passing by or construction site? (this latter might be the case if you have periods of poor performance interspersed with periods of good performance - passing truck or operating machinery).

- How about wind? Are you shielded from it? Was it windy on nights of poor performance? Wind can impact setup in multiple ways - poor tripod, OTA / mount connection that is not firm enough, if using OAG - connection between camera and scope (camera can be quite distant to OTA - larger arm momentum with wind, and if connection is susceptible to flex - this can also be problem).

[PhotoGav]

Thanks Vlad. Everything should be in favour of the Mesu operating perfectly...

To answer your points:

Just checked pixel size settings in PHD2 and I have entered 8.30. The spec for my guide camera, a Lodestar X2, says pixel size is 8.2 x 8.4?m. I obviously hedged my bets and went for 8.3 when entering the details in PHD2! Which should I enter and will it make a difference anyway?! I also see that I am not binning the guide camera. Would that be a good idea and improve the situation I wonder? It looks as though it is going to be clear to begin with again tonight, so will give binning a try.

Yes, I'm using the OAG on the QSI 683.

The mount is on an isolated pier and should be vibration free, no obvious sources of vibration nearby.

It was fairly windy last night, but I use a Pulsar dome, so the system is reasonably protected from wind.

I have seen the guiding error RMS down in the 0.2" region in the past, but that was on a night of particularly good sky quality and with the Esprit 100 refractor on the mount. I live on the edge of a small market town, so there is some light pollution and my sky quality is good, but not brilliant. I have a feeling that this makes a difference to the general ability of the system, including guiding accuracy. I will post some subs in a while for scrutiny... of both back focus / collimation and guiding.