TELESCOPE FLOCKING

This article explains how I flocked some inner parts of my telescope that I supected that could produce internal reflections. The goal of this implementation is mainly to improve the flat field calibration of the subexposures. It is an economic modification and the materials are easy to find, but please note that the task can be quite difficult if not executed correctly, and the benefits cannot be guaranteed, so you must decide if you really need to flock or not at your own risk.

background

I had reasons to consider that my telescope could benefit from this modification. The most important is the improvement of the flats. Flats are really difficult to manage, even when a good flat field device is used. I had some gradient problems, and I suspect that they can come from non-flocked surfaces. I’m not sure about this statement, but in any case it is a modification that would not degrade the quality of the images (if executed correctly). I planned to disassemble the whole telescope to clean, collimate and make other modifications so it was a perfect opportunity to make this modification too.

analisys

First step was to make an analisys of the internal reflections before flocking. To do that I made a serie of photographs through the optical system, without the camera, while it was iluminated with the flat panel. In each image, one component was removed. That helped me to realize which components were producing reflections.

It’s important to note that some of the internal surfaces of the Borg were already flocked by factory.

First image, the default configuration, including optics:

The image is completely overstretched for a better visualization, but the reflections are quite dim to the eye. In any case, there are 2 noticeable reflections near the rear end of the scope, that can came from the focuser, the rotator, or the drawtube. In addition there is a third reflection near the lens cell that seems composed by at least two rings.

Second step, tube assembly without optics.

One of the outer reflections has disappeared. This is good as it means that one of the main reflections come from the optics. The optics of this telescope is composed by 3 groups (Front lens, front reducer and read reducer). The problem with the optics can’t be the front lens, because it is at the end of the «tunnel». I checked also that the rear reducer has almost no reflecting surfaces so the front reducer is the first candidate.

Now, I start to remove components from the assembly, first, without the Canon EOS adapter:

No change…

Next, removing the rotator.

The remaining ring has disappeared so the reflection should come from this component. The slightly illuminated outer ring is the focuser thread. It is hidden by the rotator so it’s not a problem. looking at the end of the tube, the last reflection seems to be splitted in several reflections.

Next step, removing the focuser:

The outer reflection is produced by the front end of the drawtube (the tube drawtube, not the focuser drawtube) so this is another component to fix.

In addition, as commented in the first image, the reflections near to the lens cell, seem to come from different surfaces. It is expected that they will be more discernable in further steps.

The next step is to remove the drawtube and also the main tube, so only the objective tube is visible:

We can see 3 reflections clearly separated.

The outer one is the inner sufface of the thread that connects the objective to the main tube. A visual inspection shows clearly that this is a surface to fix.

The middle ring are from internal steps machined into the objective tube. This surfaces could be flocked as well but after an inspection I found it that their surface would have very low adhesion to the flocking sheet. In addition to this, I suspect that they were machined to reduce the vigneting, so I’m afraid to increase it. Taking this into account, I prefered not to flock it and see how the optical system works.

Finally, we can see a very bright ring just beside the lens cell thread. It is a surface that was not flocked by factory and I considered important to flock it.

development

CAD MODELING

This modification is included in a bigger optimization project and one of the things I did was to disassemble all the telescope into its inidividual components and measure every piece with the maximum accuracy that I could. While measuring, I modeled the pieces so I have an exact 3D model of my imaging rig. This allows me to face any modification with the security of having testing it in a simulation. In this case I could use the assembly to analize which surfaces must be flocked.

CUTTING TEMPLATES

Another advantage about modeling all the telescope is that I can calculate exactly the size of every piece of flocked sheet. Once calculated, I drew the shapes and printed them to be used as a template to cut the flocking sheet. If a CAD assistance is not possible, you should measure the diameter of the surface to be flocked and use the perimeter formula (L=2·Pi·r) to know the lenght, and measure the width too.

This method worked great and the flocked pieces fitted almost perfectly into the telescope components.

FLOCKING MATERIAL

The flocking sheet used is the D-Fix Black Velvet.

Is a cheap and easy to find product, that performs quite well. In the following lik there’s a comparison against other products. It isn’t an astronomical review, but in my opinion is valid enough.

http://projectiondream.com/en/protostar-dc-fix-triple-black-velvet-devore/

CUTTING TOOL

Another aspect to really consider if you are looking for the finest result is to use a good cutting tool. I choose a rotatory cutter becasue it need much less force to be operated, and this is key, in my opinion, to cut long and narrow pieces.

RULER

As importent as the cutting tool, is the ruler. A steel ruler is really recommended.

PATIENCE

Don’t try to achieve a high quality flocking making it fast. Every of this steps should be executed as accuratelly as possible and this takes time.

implementation

In the following images you can see the results of the implementation:

The objective tube had a lack of flocking just aside of the lens cell thread. In adittion, the inner surface of the lens cell was flocked also (not shown in the image).

Objective Tube (The right thread attaches to the lens cell)

There was also another surface that greatly reflected that was the inner surfacte of the obective thread. It can be seen as the innermost black circle in the image below.

Objective tube, viewed form the lens.

The next image shows the main tube which was already flocked by default, but not completely, so it was necessary to add some flock in the end of the tube. This is not comlpetely necessary, as the added flock is hidden by the drawtube, but in any case was flocked, just in case… The result of this surface flocking was not perfect, you can see a big gap on the upper side of the tube, of about 1mm, but as it is hidden, I didn’t reworked it.

Main Tube

The next component to be flocked was the drawtube (sorry, no image of it), but only the end of it was needed to be flocked. The rest of the drawtube was already flocked by factory. The origin of this non-flocked surface is that there was a piece attached to the end of the drawtube to keep the drawtube centered. I decided to remove this piece because I installed a different locking/adjustment on the main tube (the four screws that can be seen in the main tube image above), so this new surface was revealed (at it wasn’t flocked).

Next component to flock was the focuser drawtube. In addition, the front flange of the front reducer was flocked also. Note the slightly visible line on the bottom right of the focuser drawtube, it is the joint of the flocking piece. This is the level of accuracy taht I normally achieved by using a CAD calculated template (very good, in my opinion).

Next component, the rotator/filter holder, that is attached to the focuser. A visual inspection revealed that the ring seen in the analysis was produced by the inner surface of the thread that attaches to the focuser (the farther flocked surface in the image). Apart of this, the surface right beside of the rear focuser was flocked too. In the image can be seen also two surfaces that haven’t been flocked. One is the housing of the filter holder, that I choose to skip to maintain its functionallity, and I will check how it works, and the other is the housing of the rear reducer, so is a hidden surface.

And finally, the Canon EOS adapter, that is almost hidden by the rear reducer, but I preferred to flock it, to avoid the possibility of a reflection produced by the filters of the camera.

CLEANING

Once all the pieces are flocked, it is important, in my opinion to take care of the fiber losses to avoid them to mess the optics. To clean any fiber I used a vacuum cleaner and carefully cleaned all the pieces. In order to avoid to hit and mark the flocked surfaces I flocked the end of the vacuum cleaner too. Maybe you will think that this is too much care, but taking into account that the idea is to not to open the telescope again in several years, I think that all precautions are welcome.

STORAGE

After cleaned, the pieces were stored into plastic bags to avoid any environmental dust to mess them.

post-flocking analisys

Once the telescope was assembled, I made the same photograph through the optical system, to check the improvement:

The improvement is clearly noticeable. There is a dim ring that a suspect that can come from the edge of the drawtube, but in any case seems to bee dim enough.

Flats doesn’t seem to be so much different, maybe the effectiveness of the flock will be noticeable only when a harmful scattered light reaches the optical system, or maybe is simplest, and the flocking procedure didn’t improved nothing. In any case, it will be necessary to check it with real images and see how well are them calibrated with the corresponding flats.

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