18.02.2020 The Baader D-ERF and reworked cell are back!

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Tuesday 18th: 40F, gales and heavy showers.

 Fullerscopes Telescope Mountings: 13.01.2020 160mm Baader D-ERF & T-S filter cell.

I started off by shortening the collimation "push" screws on the iStar 150mm f/10 H-a objective. This was to allow more depth on the nose of the objective cell for the D-ERF holder [cell] to clamp onto. 

The 160mm filter x 10mm thick was a close fit in the holder. It took some time to settle properly into the recess provided. Only then could I screw in the filter retention/ lock ring. Leaving only the hint of a rattle to avoid pinching the precision, optically surfaced glass.

Unlike my 90mm D-ERF the 160mm seems quite symmetrical in its reflectance. With a deep red cast to anything seen through it. Both filters have a penciled arrow to ensure the correct side faces the sun. From memory, the 90mm filter looked much more reflective on one side than the other. I shall have to check this.

The rear of the filter cell is lined to avoid scratching and fits snugly onto the front of the 150mm, iStar, objective cell. With 25.5mm /1" of the objective cell safely nestled inside the filter cell. This is perfectly adequate for security. Though it could be increased further by shortening the collimation push screws by a few more millimeters. The limitation is whether the lens cell is lifted clear of the countercell by the push screws. If not, then no collimation will take place.

The rear edge of the filter cell stops up against the heads of the collimation screws. Which sets the maximum possible insertion of the objective cell.

The telescope, objective lens collimation will be carried out with the D-ERF cell removed. Allowing free access to the M4 collimation screws. Once collimation is achieved, the full aperture filter, in its cell, can be safely replaced to protect the instrument and its user.

The three, large, nylon, clamping thumbscrews provide all the security one could hope for. I was easily able to drag the entire 6" telescope along the bench. Simply by holding the clamped filter cell rim with my fingertips. Under normal circumstances the filter will never find itself "face down."

The 160mm D-ERF filter and its cell weigh almost exactly 1000g or 1kg. The finish of the cell is very satisfying and could grace any telescope without detracting from its appearance.

Different sized D-ERF filters up to 180mm/ 7" and matching cells are available to order from Teleskop-Service. Cells can be made much larger, to order, but Baader does not offer a larger filter than 180mm. Cell manufacture is dependent on purchase of a D-ERF. Though these filters are widely available form other dealers, only Teleskop-Service provide a cell making option. [As far as I know.]

Airies Instruments.Co, of the Ukraine, makes "Triband" ERFs and cells for much larger telescopes and even for SCTs. [Priced accordingly!]

The last images [left and  right] were taken with flash. While the images with the turquoise appearance of the D-ERF are taken in natural light. The colour and apparent depth of the multiple reflections from the numerous coatings are interesting. The depth is only an optical illusion. Not unlike holding two mirrors, facing each other, to see ever smaller reflections going off into infinity.

The purpose of the D-ERF is absolutely vital and will help to reduce the increased heat load, from the large aperture objective [150mm] on the PST etalon and filters. Two further [2" Ø] filters will help the cause by blocking unwanted UV/IR. The PST etalon is normally used with a relatively tiny 40mm objective. An area difference of 12674 mm^2 to 1257mm^2 in the case of a 150mm to the PST's 40mm. An area and heating increase of 14:1!

WARNING! Failing to use a D-ERF on a modified, H-alpha telescope, pointed at the sun, will cause instant blindness of the user at the eyepiece! It will probably destroy the following optical H-alpha components and any cameras fitted at the eyepiece end! 

The D-ERF is a highly specialized [and expensive] filter intended for H-alpha observation and imaging. It still requires further optical components for the telescope to function at H-alpha wavelengths. AND to greatly reduce the heating effect of the focused sun!

Without all of these vital components a telescope objective lens can do considerable damage and cause permanent and devastating personal injury! Building solar telescopes requires considerable knowledge, expense and responsibility. There are no shortcuts! Nor cheap "make-do" components. Take no chances or you will lose your sight! 

For white light, solar observation, with any telescope or binocular, an approved, full aperture solar filter or filters[s] MUST be used instead of a D-ERF.

Leave NO telescope or binoculars unattended in daylight! You cannot possibly foresee somebody unknown trying to view the sun through the telescope or binoculars. Nor do you want to be responsible for "accidentally" blinding your own family members! Warn them repeatedly of the dangers of looking at the sun! Just because it was overcast when you left the telescope "for a moment" does not mean the sun cannot make an appearance!

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