Focuser colour

[david_taurus83]

Hi all. There have been a couple of topics posted lately about flats either causing gradients or enhancing vignetting rings in images. I'm having the same issues. I'm also having issues with star shapes which I think are down to the Baader MPCC I'm using. So I was inspecting the focuser last night and I realised that the inside of the focuser tubing isn't black! It's quite reflective in the pictures. Would flocking help to reduce reflections or is there a certain paint that's recommended to use?

 

Also attached is 2 pictured showing the CC fitted. The first picture is the CC just held flush with the drawtube as it should sear. The second is with the brass retaining ring tightened. Due to the lip on the CC by the spacing ring, I believe that when the brass ring is tightened, it's half on the lip and half in the recessed part and forcing the CC to tilt back out slightly. Would this be enough to induce funny shaped stars? I'm on my phone at the moment so can't post any subs from Saturday night. I ran CCD inspector on a sub last night and it said tilt was x0.1" and y0.5". Collimation was around 5" error. I believe this is pretty good so it points back to CC tilt or spacing?

 

Thanks

[Oddsocks]

The inside of your focuser tube is painted with a non-reflective paint, even though it isn't black. as long as it is non reflective then in theory it should be ok, but.....

Most problems are caused by plain black anodised aluminium surfaces, which although they look black in visible wavelengths they become white or silvery to infra-red wavelengths.

Flats can be difficult with reflectors as the only light you want to reach the camera when you take the flats must take the same path as the photons from the target, only via the mirrors, but when you use an artificial light source, such as a LED panel, directly on the front of the OTA, some light will illuminate the entrance to the focuser tube directly, reflect along the focuser draw tube and reach the camera, getting there not via the mirrors, and this will contaminate the flats leading to incorrect vignetting correction.

The same applies when trying to do sky flats during daylight with a white fabric diffuser over the OTA or pointing the telescope at a lit wall indoors etc, anything that allows some stray light to enter the focuser draw tube independently from the principle optical path will contaminate the flats.

And this highlights a big difference between taking flats with a refractor, where, as long as the OTA is well baffled and the focuser doesn't leak light around the draw-tube, then the only light reaching the camera must get there via the optics, but with a reflector and that big sidewall opening for the focuser, just below the entry to the OTA, any stray light can enter the focuser draw tube and mess up the flats.

So, yes for a reflector, flocking the entry portal and draw tube to reduce this non-focal-path light source will help, flocking paper works well, as does matt-black paint that uses black pigments for it's colour rather than dyes which don't work across the full colour spectrum. 

The usual recommended stuff is matt-black high-temperature barbecue paint from DIY stores like B&Q, it comes as a tin of brush-on paint or an aerosol. When buying it note that they do a gloss finish and a matt finish, obviously you need the matt finish. If they are out of stock of the brush-on stuff then the aerosol version can be used but only by spraying a good amount into a clean, empty glass jar, well away from the telescope, and then transfer to the focuser draw tube and entry portal with a brush.

When you think about it, it should be possible to shield a light source for flats so that the only light reaching the entrance to the OTA is travelling in long straight lines. One way to do this is just to use a long, cardboard box or tube, open at both ends and either flocked or painted matt-black inside with black-board paint, say at least 0.75 mtrs long. It should be around one and half times the diameter of the OTA. In a darkened room line up OTA centrally in one end of the tube or box with the diffused flats light source at the other. Any non-parrallel light travelling down the tube will be absorbed by the flocking and mostly, the only light entering the OTA will be travelling straight down to the mirror and should avoid entering the focuser portal allowing you to take uncontaminated flats. 

This kind of solution was fairly common at one time for reflectors with the 'old' boys but seems to have gone out of common use these days, maybe that is just down to mobility.

The loose fitting of the Baader flattener in Skywatcher focuser draw tube is another common issue, besides the bad effect on tilt and flattener performance across the field, if you turn the OTA to a different angle for the flats to that for the target then the flattener can swivel and twist under the changing direction of gravity forces acting on the camera and alter the camera angle enough to ruin the flat.

This has been a long standing weakness in the design of the Skywatcher focuser draw tube where wide manufacturing tolerances combined with the safety undercut in the flattener body does not allow the flattener to be clamped evenly when you tighten the brass compression collar. To be fair though, this problem is spread across other manufacturers just as frequently, with no common standard for sizing and manufacturing tolerances it is hardly surprising that different manufacturers components fit together poorly.

A cheap solution is to experiment with thin collars cut from the wall of an aluminium drinks can, or the thin, transparent, hard plastic sheeting used in food packaging, wrapped as a narrow strip around the front part of the flattener so that when the flattener is inserted into the focuser draw tube the nose is a tight fit and when the compression ring is tightened the flattener can not tilt. It will take a bit of experimenting to find the right thickness of material that fills the gap between flattener nose and focuser draw tube but once found it can be glued in place.

If a collar around the nose of the flattener proves difficult consider using three equally spaced 1cm wide and ~5cm long strips of aluminium drink-can wall, arranged longitudinally inside the focuser draw tube, beginning just behind the compression ring and used to reduce the diameter of the inside of the draw tube. Again, these can be glued in place once you find a good thickness of robust material to take up the slack between focuser draw tube and flattener body along the flatteners full length.

A colleague tried drilling and tapping the flange at the end of the focuser draw tube and adding two more thumb screws at 120 deg spacing so that the compression ring can be tightened equally all around but in practice it didn't stop the nose of the flattener lifting, the difference between the internal diameter of the draw tube and outside diameter of the nose of the flattener was too large (nearly 2mm) and in the end he used the clear, hard plastic film from a Sainsburys cheese-cake slice as a liner for the inside of the draw tube to make the flattener a tight fit, together with the extra thumb screws in the flange of the draw tube it made a very rigid coupling that could not twist.

Images posted below explain the concept.

A more expensive option is to use a Hotech SCA flattener instead of the Baader flattener. The Hotech SCA flattener has rubber rings set into it's body that expand to fill the gap between focuser draw tube and the flattener body and force the flattener to remain on-axis inside the focuser. I have one of these in my collection of flatteners and it works well as long as the f/ratio limits of its design are respected.

https://www.firstlightoptics.com/reducersflatteners/hotech-sca-field-flattener.html

As I sense you're beginning to drift off I think this is good point to finish 

William.

 

[david_taurus83]

Hi William. Thanks for taking the time and posting such an excellent response. The idea of the light source for the flats reflecting off the focuser drawtube makes perfect sense. I'm surprised I've not read that anywhere but I am still a newbie to this. I have access to plenty of polypropylene pipes and ducting at work so if I needed a metres length of 300mm dia pipe I could flock the inside to create a light tunnel for flats. What an excellent idea, thanks! Though, I have bought an Artesky flats panel and tried it out last night. It does get very dim at it's lowest setting and I would be surprised if it was bright enough to light up the drawtube. I was using a Canon 50mm lense last night and it needed a 3.5 second exposure on the lowest setting to get a flat histogram in the middle. Need to try it out on the telescope yet. Regarding the drawtube and CC. There is very little play between the tube wall and front of nosepiece. I doubt I'd fit aluminium shims in to be honest. I have some flocking paper arriving today from FLO and i will get that sorted tonight. I think that should reduce to tolerance quite well. I've had another look at the CC. I've found that if I position the nosepiece partially and tighten the brass ring in the cut out, I can then push the nosepiece in a bit more so the end of the cut out sits flush with the brass ring. This looks much more square and doesn't lean back out of the drawtube. Hopefully will get some better results Friday night!

 

Thanks.

[Oddsocks]

11 hours ago, david_taurus83 said:

It does get very dim at it's lowest setting and I would be surprised if it was bright enough to light up the drawtube

For a standard LED flats panel varying the brightness has no effect on the distribution of photons leaving the panel, only the rate at which they leave. Increase the brightness and the rate of photons per second increases, lower the brightness and the rate of photons per second decreases.

When you take your flats you still need the same number of photons to reach the camera, if you decrease the brightness the camera exposure time has to be longer, if you increase the brightness the camera exposure time has to be shorter.

So far as the distribution of photons entering the OTA is concerned there is no difference. Exactly the same number of unwanted scattered photons will enter the focuser port during a long exposure with a low brightness setting as they would for short exposure with a high brightness setting.

Things would be little different if we were to change the energy of the photons leaving the flats panel (such as shifting the colour between the red and blue ends of the energy spectrum) but in this case we don't do that, we just alter the rate at which the photons are produced.

Flocking or painting the focuser entry port at the side wall of the OTA as well as the inside of the focuser draw tube should help enormously.

Be careful not to be tempted to increase the exposure time for the flats to too long a time, if you do this you will need to start taking matching time dark frames for the flats and then the whole business of image calibration becomes a lot more complicated.

HTH.

William.