C11 EdgeHD Imaging and Collimation Woes

[DeepSkyMan]

Hi there

I am currently trying to get to grips with my imaging setup (relatively new to imaging).  Setup is:

C11 EdgeHD

Esatto Focuser

Extension tubes

Atik Horizon

The scope has had Bobs Knobs added to it, so I was expecting a need for collimation, and hoping to do this using the imaging rig and software rather than the traditional approach.  However the scope seems so far out of whack that I’m wondering if I should just remove the imaging rig and use the visual rig from my 8”, this would be a Baader 2” click-lock diagonal and collimate the traditional way.  Towards this end does anyone know if it is possible to use the click-lock with the Esatto focuser?  How would this relate to the back-focus requirements for the 11” Edge?

I did a little experimentation with the imaging rig this evening on the basis that I could get the scope into something nearing focus (collimation notwithstanding), I managed to get a very grainy image of Capella which more resembled a fanciful image of an angel than a star.  My expectations were that I would see a distorted image on a black sky background, not an angel on grainy horizontally striped grey background.  I was using MaximDL for this exercise.  Thought I’d managed to save an instance of the image, however this failed to pass.  This learning curve is getting so steep, I think I’m slipping off😏
 

Kind Regards

Paul

[Elp]

How does a defocused star look through an eyepiece?

I'd also keep it simple, forget the auto stuff and just connect the camera and spacers to the visual back with the correct backfocus distance to the sensor. You haven't mentioned a reducer, are you aiming for DSO or planetary imaging?

Edited March 13 by Elp

[DeepSkyMan]

Thanks for the feedback Elp.  Subsequent to my starting this thread yesterday evening, popped out to the obsy for a last try.  I believed I had followed the recommendations for backfocus on the rig to the letter, using a sequence of extensions calculated and recommended to me by a reputable vendor to give me the desired backfocus, however I suspect this might be my issue.  My last effort was to rack the focuser on the SCT out as far as it would go, and this resulted in me at least getting to a recognisable black background, with a very out of focus moon in the top right of the sensor.  So it looks like I need to extend the image train out further to hit the backfocus.  Looks like I will need to order another extension tube.

 

Kind Regards

Paul

[bosun21]

The back focus is the distance between the reducer/flattener and the actual camera sensor. It has nothing to do with the focus travel by turning the focuser knob. If you haven't got a reducer in the imaging train then you don't set any back focus. Just attach the camera to the telescope and rack the focus knob in and out until you see the moon in focus or the star becoming as small as possible. To collimate on a star it has to be perfectly centered on the screen/eyepiece and tracked. Polaris is a good choice of star.

[dan_adi]

On 14/03/2024 at 00:49, DeepSkyMan said:

This learning curve is getting so steep, I think I’m slipping off

SCTs do one thing well, driving people nuts 😆

[Elp]

As @bosun21 has said if you're not using a reducer you don't really need spacer rings. Without a reducer I've done it visual back > diagonal > t ring nosepiece > camera.

With the F6.3 Celestron reducer I've done it back of scope (no VB) > reducer > spacer rings > camera. For my C6 the BF spacer range is around 105-120mm (it's quite forgiving for an SCT as there's so much focus travel), your scope would be different spacing I'd assume.

At present, you haven't confirmed how you've connected it all up.

Also have you checked a defocused star visually as you'll see collimation issues straight away?

[DeepSkyMan]

Thanks all, appreciate the feedback.

Ok, so now I’m seriously confused.  Trying again this evening, and I get no joy at all for the entire range of the stock focuser, with the Esatto set at the zero step position.  I took this image with the Atik Infinity software, this is what I’m getting for the entire range of the focuser.  Also attached is the image train in question.

Kind Regards

Paul

 

[Oddsocks]

Hi Paul.

On published documentation (that I can find):

The 11" HD has a coma corrector built in to the baffle tubes and Celestron's white paper for the Edge HD series shows that the distance from the top surface of the visual back thread to the sensor should be 146.05 +/- 0.5mm.

https://celestron-site-support-files.s3.amazonaws.com/support_files/91030-XLT_91040-XLT_91050-XLT_91060-XLT_EdgeHD_OpticalTube_Manual_English_web.pdf

Here is an extract from the paper linked above showing the BF distance for the Edge HD series with the 11" HD highlighted:

 

A "rough" back focus (BF) calculation for your current set up gives:

2" Essato BF= 67mm 

PL3600212 large SC adaptor BF = 2mm

PL3600218 M56 (Essato) to T2 (camera) with stop-ring BF (minimum) = 4mm.

Atik Horizon sensor to T2 distance BF = 13mm

Total BF used = 86mm

Additional BF required with Essato at minimum (all way in) position 146.05mm - 86 = 60.05mm

But...

You would not have the Essato set to the minimum (all way in) position when calculating the BF distance, it should be approx half way, plus, the PL3600218 M56 (Essato) to T2 (camera) with stop-ring adaptor would also not be at the minimum position otherwise you could not use it to rotate/orientate the camera.

PrimaLuceLab don't say the maximum length that the PL3600218 M56/T2 with stop-ring adaptor will adjust to and from memory, I think it is about 10mm, but the thread pitch for T2 is 0.75mm, so, recalculating the distances with the Essato at 50% extension and the M56/T2 stop-ring adaptor at 1.5 turns out to give you one full turn of the camera plus a little spare gives:

2" Essato at 50% of full range (67mm minimum BF +  1/2 range extension of 7.5mm) = 74.5mm.

PL3600212 large SC adaptor BF = 2mm.

PL3600218 M56 (Essato) to T2 (camera) with stop-ring, 4mm minimum BF + 1.5 turns out (1.5 turns x 0.75mm = 1.125 mm) BF = 5.125 mm.

Atik Horizon sensor to T2 distance BF = 13mm

Total BF used = 94.625mm

Additional BF extension required = 146.05mm - 94.625 = 51.425mm (see diagram below). 

Going by the pictures you posted it appears you have too much additional BF distance added.

If you adjust the Essato position to 50% extension and the PL3600218 M56/T2 adapter to 1.5 turns out then the nominal T2 distance spacer required between the camera and the PL3600218 M56/T2 with stop ring adaptor is 51.425mm.

Given that the range of focus travel for the 2" Essato is 15mm and the nominal position of the focuser for calculating the BF is at 50% extension then you have a small tolerance on the length of extension tube you can use and still be able to adjust focus using the Essato and while still being within the Celestron specified BF for the built in coma corrector of 146.05 +/0.5mm.

This allows you a reasonable distance of 51.425mm +/- 6mm for your additional T2 spacer (allowing 1.5mm tolerance either side of fully out or fully in of the Essato focuser).

A T2 spacer of 45.425mm min length to 57.425mm max length between the PL3600218 M56/T2 adapter and the ATIK Horizon is required according to my rough calculation.

You do have a little more tolerance on the minimum length of T2 BF spacer that you use by extending the PL3600218 M56/T2 stop ring adaptor by a few turns, but there is no safety stop on that M56/T2 adapter and if not careful you can unscrew that fully and drop the camera.

The above calculations are just quickly done and will require you to double-check by referring back to the manufacturers data sheets and manuals.

IMO the Edge HD is a difficult OTA to fit an external focuser to because of the BF requirements dictated by the internal coma corrector fitted inside the baffle tube. To achieve the best performance the sensor plane must be at the Celestron specified distance of 146.05mm +/-0.5mm, from the rear surface of the visual back and that means that using an external focuser to move the camera you are moving away from the specified BF distance and image quality will be degraded.

I have no idea how much image performance is degraded once you move away from that 146.05mm distance for the sensor,  perhaps somebody who has an Edge HD 11" will add to the discussion but for practical purposes I think you will need to set up the camera sensor to be as close as possible to that 146.05mm distance with the Essato at 50% extension, then use the OTA main mirror focuser to bring the image to focus, then, only adjust the Essato in/out focus by a fraction of a mm either way to fine-tune the focus bearing in mind that doing so moves the camera sensor away from the ideal BF distance of 146.05mm as stipulated in the Celestron documents.

HTH

William.

 

Edited March 17 by Oddsocks
Corrected BF distance for 2" Essato

[michael8554]

I like William's explanation.

Though your post heading talks of Collimation too.

If that is badly out, not just the focus, I would start with a Collimation with a high-power eyepiece.

Michael

[DeepSkyMan]

Not really had a chance to sort out the collimation since I put the Bobs Knobs on, was hoping as I said to collimate with software, will get to it though when I resolve my backfocus issue.

William, I did start off with the Esatto at midpoint, which seemed logical, however after the initial results I ended up assessing the situation with it at the zero position.  I’m going to take the train of the scope this evening and carry out some experimentation. 
 

Thanks Everyone for your valued input

Paul.

[Starflyer]

If an SCT is miles out of collimation take it off the mount, balance it securely on a table and look into the front of it from about 5 - 10m away.  Get an assistant to tweak the collimation until everything is showing as concentric circles.

In my limited experience with SCTs this is the easiest way to get them close, it'll still need careful collimation on a star though.

[Elp]

It helps to use a tripod mounted camera to do this as it won't move around like your head or eyes will.