Help with spacing please.

[Adreneline]

In an attempt to banish gremlins from my setup please can someone clarify a spacing dimension that is causing me some angst.

I (think?) know the spacing between the SW R/F and CCD sensor is 55mm but I'm not sure where exactly I am measuring that dimension.

Is it between 'A' and 'D' or 'B' and 'D'?

Some articles infer it is between the final lens element 'A' and the sensor line 'D' and some indicate it is between the end face of the F/R body 'B' and the sensor line 'D'.

If it is between 'A' and 'D' how do I know where 'A' is?

I have searched around on this and other forum on the web but I am still confused.

Thank you.

P.S. If 'B' - 'C' is 42mm I'll be a happy AP'er

 

[Marky1973]

Hi there - I am pretty sure it is from B to D to include the 13mm back focus on the Atik...I had the same queries as you in this thread which might help!

 

[Filroden]

My Skywatcher flattener comes in two pieces. The optical element which has a nice grip to it and an 11mm extension which is smooth. This takes the 62mm thread down to a 48mm thread. For me, I have to measure from the reducer element without the extension. 

Your image looks like it could be similar. If so, I would measure from A. If it is a single unit then you are in luck and it is from B

[Adreneline]

Thank you for your responses. I will check the F/R I have to see if it separates. I was under the impression it was 'B' but then I seemed to find a number of articles that suggested it was measured with respect to the centre line of the lens - but maybe that was a different model of R/F. I seem to have general image sharpness issues and I am just trying to eliminate possible causes. Thanks again.

 

[Waldemar]

Hi Adreneline,

 

It is not as hard as it looks:

The backfocal distance of your reducer/flattener is 55 mm
the backfocus of your camera is 13 mm, hence 55 - 13 =42 mm, So yes, you are correct ... you need a 42 mm spacer to get your camera in the right place.
There may be some little differences because of production flaws, things are never 100%, but you made a smart purchase by buying the Baader Varilock, so you are able to make small adjustments.

If you want to check if you are absolutely spot on, you could download a trial version of CCD Inspector. This will give you all details is anything is off even by the smallest bit.

[Adreneline]

Hi Waldemar,

Thank you for the advice re. CCD Inspector - I will give that a go.

I am just becoming a bit obsessed with driving out focus problems I seem to have been plagued with since starting to use a CCD. When I first started I used a Canon 70D with BackyardEoS and found focusing results were generally good. Since moving to the Atik I feel focus has not been so precise, but maybe it is something else altogether different and I am barking up the wong tree.

I did have a major issue with the draw tube slipping on my 80ED but in the end I solved that with a strip of high quality insulating tape - there was clearly a need to change the coefficient of friction rather than apply an ever greater force normal to the focus rod. Again that issue only surfaced when I hung the Atik on the end of the scope; it weighs more than the 70D and with the scope near vertical just pulls the draw tube out of the scope body.

I seem to spend more time solving problems than taking images!

The joy of AP.

Thank you again for your help and advice.

Adrian

[Waldemar]

Slipping draw tubes are a disaster. I found the best solution is to replace the focuser with a FeatherTouch. It may cost even more than you paid for your scope, but is really worth it... 
Using tape or other temporary solutions most of the time means you loose perpendicularity (if it was there that is...)

Yep... solving problems is a huge part of ap... part of the fun as well 

[Oddsocks]

1 hour ago, Adreneline said:

Again that issue only surfaced when I hung the Atik on the end of the scope; 

A simple, much overlooked and inexpensive solution to slipping Crayford focusers just requires a bit of elastic rope or "Bungee", also called "shock cord" by the sailing fraternity.

The elastic rope is slipped below the OTA and behind the first or second tube ring, both ends bought together and tied behind the camera or filter wheel and adjusted in length until the pull exerted is roughly half the weight of the camera.

When the tube points straight up the effective weight of the camera is reduced by a half and the Crayford won't slip, as the tube points towards horizontal the maximum pull towards the OTA by the elastic is again only half the weight of the camera. At 45deg elevation the camera is exactly balanced and appears weightless.

When selecting a suitable elastic rope choose a thinner rope and if necessary use a few loops around the OTA/camera in preference to a thicker rope with a single loop. With a double loop it is easier to adjust the length and arrive at a suitable counter-force and the pull won't change by much over the travel range of the focuser.

Tying the rope around the second tube ring instead of the first means a longer loop is used and gives even finer control of the counter-force applied.

Once done for the session and packing up just leave the rope tied to the OTA and slip it off the back of the camera.

Elastic rope is more elastic in warm weather so depending on the rope thickness and length you may need to slacken the rope in winter or shorten the rope in summer.

Cost? ~ £3- approx. Yacht chandlers carry the biggest ranges of thickness, DIY stores carry the smallest ranges but are cheapest.

Have been using this for years on a mobile setup that uses a Crayford with heavy camera and an AO unit, close to 2kg total weight and it never slips when pointing up towards the zenith.

[Adreneline]

The tape solution seems to work well - so far! I just applied a length directly on top of the machined flat on the draw tube. The tape is 0.1mm thickness (measured with my trusty vernier) but it dramatically increases the coefficient of friction between the focuser spindle and the draw tube. The FeatherTouch may have to be on my 'if only I could afford it' list - it's a long list!

I have to say I had not thought of using a length of elasticated rope to support the camera, however, I do use a bungee cord to help support and contain the wires trailing from the various bits of kit attached to the scope. If the tape fails to deliver a long-term solution I'll consider the rope solution.

Thank you again for the helpful suggestions.

[andyo]

I measured mine from B which worked ok for the small sensor in the atik 314 but since upgrading to the 460ex I have noticed curvature in the corners (elongated stars).So am now wondering if Filroden is right about measuring from A which would mean a 31mm extension would be required as there is already an 11mm on the reducer.

[Adreneline]

I am very grateful for all the help / advice received but I still do not really understand why there does not appear to exist definitive information published by SkyWatcher as to where exactly the requirement for 55mm spacing is measured from and to. If you look at other manufacturers reducers many give nice, easy to understand drawings showing exactly what is required. I must be missing something somewhere - I am clearly not typing the right thing into Google!

On a separate, unrelated thread, I was advised not to use the R/F at all in the image train; at least then I would not have to worry about whether I was using correctly

Thanks again AndyO for your input and advice.

Adrian

P.S. I've attached a copy of the documentation supplied with the R/F - there is no mention of 55mm or any other dimension! I like the way it says "Now, you're ready to take pictures!". The question is - 'of what?'

[michael8554]

Hi Adrian,

Regarding the figure of 55mm for the spacing.

Your user guide shows a Canon DSLR attached to the FR.

The back focus on current Nikon, Canon, Sony etc DSLRs is in the range of 44mm to 46.5mm.

The "Synta" DSLR-to-M48 adapter is probably 10mm thick, giving a working back-focus of about 55mm from the FR.

You'll have to guess how far the M48 adapter screws onto the FR, and measure from there, but it looks like 55mm plus or minus a couple of mm is acceptable if any DSLR can be mounted.

Michael

 

[Adreneline]

Thank you Michael. I am fairly convinced I have my spacing correct for the Atik 414ex - I was just looking for confirmation re. my original photo at the top of this thread and my working premise that 'B'-'D' was 55mm. Thanks again.

Adrian

[andyo]

yes I am sure 55mm is the correct spacing the question really is as Adrian says where is the measurement taken from A or B Skywarcher should make this clear

[andyo]

from what Michael says the measurement of 55mm must be taken from B and not A as the screwthread on the reducer is 10mm upto point B which accounts for the 10mm on the DSLR to M48 adapter.

12 hours ago, Adreneline said:

 

On a separate, unrelated thread, I was advised not to use the R/F at all in the image train; at least then I would not have to worry about whether I was using correctly

 

 

 

 

Interesting to know why you where advised not to use a R/F at all.I wonder how much difference a flattener makes on a sensor that is not full size.For example on the smaller sensor in the Atik 314L would it make any difference as the curvature only occurs in the far corners on a full size sensor.Also with a reducer does it truly change the f ratio to a faster scope as it doesn't significzntly alter the scopes actual focal length or aperture size

[Adreneline]

Hi andyO,

I had posted an image of M81-M82 - what I considered to be my first success with a osc camera - and it was suggested I could "lose the R/F so the image was less cropped" - as it was I could only just fit both targets into one image. The focus on that image was not great (an ongoing issue I am gradually sorting out) and so a few more slightly more out of focus stars would barely have been noticed!  I'm still not sure I understand the benefit though regarding cropping.

I think I might be inclined to take a few images with and without the R/F and see just how much difference it is making over the image as a whole with my 80ED. When I started I used a Canon 70D and the importance of spacing was way above my head - I just connected it all together as per the included instruction sheet and focused up in BYEoS as best I could. It is only since I started using the 414ex combined with a UV/IR filter (as shown at the top of this thread) that I was made aware of the importance of spacing - hence the purchase of the Baader VariLock and this Thread.

Thanks again for all your advice and comments - all food for thought.

Adrian

[andyo]

5 hours ago, Adreneline said:

Hi andyO,

I had posted an image of M81-M82 - what I considered to be my first success with a osc camera - and it was suggested I could "lose the R/F so the image was less cropped" - as it was I could only just fit both targets into one image. The focus on that image was not great (an ongoing issue I am gradually sorting out) and so a few more slightly more out of focus stars would barely have been noticed!  I'm still not sure I understand the benefit though regarding cropping.

I think I might be inclined to take a few images with and without the R/F and see just how much difference it is making over the image as a whole with my 80ED. When I started I used a Canon 70D and the importance of spacing was way above my head - I just connected it all together as per the included instruction sheet and focused up in BYEoS as best I could. It is only since I started using the 414ex combined with a UV/IR filter (as shown at the top of this thread) that I was made aware of the importance of spacing - hence the purchase of the Baader VariLock and this Thread.

Thanks again for all your advice and comments - all food for thought.

Adrian

Losing the focal reducer does not crop the image or the image will not become "less cropped". A focal reducer is supposed to reduce the focal length of the scope and therefore increase the field of view. Personally I do not see any advantage to cropping the image unless cropping out bad data. I suppose you could enlarge and then crop to get a close up of a feature if the resolution was good enough.

 Field curvature is an aberration that most refractors suffer from, basicaly the optical plane of a refractor is curved and the sensor is flat so this has the effect of elongated stars towards the outer edges of the image a field flattener supposedly corrects this aberration. The skywatcher focal reducer is also a flattener.

If you wish to see the difference in the field of view of your set up both with the reducer and without the reducer I recommend blackwater skies which will let you do this it is very straight forward to use and a great tool for planning future targets, it can be found here

http://www.blackwaterskies.co.uk/imaging-toolbox/

P.S I have emailed skywatcher support to get a definitive answer to your initial post when they reply I will post there response

 

 

 

[andyo]

On 13/04/2017 at 14:02, Adreneline said:

Some articles infer it is between the final lens element 'A' and the sensor line 'D' and some indicate it is between the end face of the F/R body 'B' and the sensor line 'D'.

If it is between 'A' and 'D' how do I know where 'A' is?

Hi

Skywatcher did not respond to my email so I contacted Bern at modern astronomy where I bought my focal reducer from and he can give a definitive answer that it is indeed B from where you take the measurement from. I am pleased about that as I not long ago had a custom made adapter for this spacing.

[Adreneline]

Hi andyO

Thank you very much for getting to the bottom of this little issue. Disappointing that SW cannot respond though. At least I can now be sure the spacing is not contributing to my focus issues, which I am hoping I have now solved. If only we could get a glimpse of the stars here in the East Midlands that would be good!

Many thanks again for all your help, advice and support.

Adrian