Astrodon 1.25" Filter Cells

Removing Filters for Cleaning for 1.25" Filters and Replacing o-rings

Astrodon 1.25" Filter Holders

Astrodon 1.25" filters most commonly come mounted in 1.25" cells that are standard in astrophotography.  They screw into the popular filter wheels made by SBIG, QSI and other companies.

From the introduction of Astrodon filters years ago, we have always mounted our filters in these cells with an o-ring and a metal retaining ring.  Early cells had a press fit metal ring that made it difficult to remove so that the glass could be removed and cleaned.

We used this design to make sure that a metal ring is not screwed down and rotated against the thin optical coatings of the filter and that contact is made only by the force exerted on the o-ring.

In the past 3 years we replaced the press fit ring with a threaded screw-in ring. These parts are shown below in Figure 1.  The screw-in ring is made with two opposing notches that can be seen below, allowing easier removal.

Traditionally, we used a black polymer o-ring with a size designation of 022.  This o-ring has an outer diameter of 1 1/8" and an inner diameter of 1".  The thickness of the o-ring is specified at 0.07".  Sometimes, the thickness of this o-ring prevented the screw-in ring from screwing into the cell completely.  The top of the retaining ring was slightly above the top of the cell in some cases.

Figure 1.  Astrodon Filter Cell Parts

Occasional Problems with SBIG Filter Wheels and Solution

Apparently, the internal height dimension of SBIG filter wheels is very tight and this screw-in ring occasionally scraped the ceiling of the SBIG filter wheel if it was above the top of the cell holder.  Please realize that this is a rare occurrence considering that there are hundreds and hundreds of Astrodon 1.25" filters in SBIG filter wheels used over the years.  Nevertheless, we are committed to resolving any outstanding issues.

As of April 2011, we are now supplying a different o-ring to solve this problem for ALL 1.25" mounted filters. It is a metric specification of M1.5 x 25.5 with an inner diameter of 25.5 mm, outer diameter of 28.5 mm and a thickness of 0.059".  As a result, this thinner o-ring allows the screw-in ring to be completely located below the height of the cell.

Please contact Astrodon from this web site for o-ring replacements if you have this problem.  Please follow the procedure below to replace the o-rings and/or clean the filters removed from the filter holders.

 Remvoing the Filter for Cleaning or Replacing the o-ring

To remove the screw-in retaining ring, we suggest using a blunt (non-sharp) object that will fit into the notch. We use a 0.05" hex driver.  To ensure that you don't slip and scrape the coating of the filter, we recommend that you put your thumb against the outside of the cell and at a low angle put the tip of the hex driver in the groove and slowly turn counterclockwise. Remove the screw-in filter, as shown in Fig. 2.

Figure 2.  Unscrew retaining ring with 0.05" hex drive in notch.

Once removed, set this screw-in retaining ring aside.  Next remove the o-ring with a toothpick.  You can even use your fingernail, since the filter will be cleaned anyway. Set the o-ring aside as well. 

You can remove the filter by placing a paper towel over the filter cell and turning it over, for example. 

We clean our filters using KimWipes, a lint-free laboratory wipe, and 91% isopropyl alcohol (IPA). We purchase IPA at the local pharmacy, as shown in Fig. 3.

Place the KimWipe on a clean surface and wet a 1" area near the top.  Blow off any dust on the filter, place the filter on the wetted area, drag the filter to the dry area of the KimWipe without applying any pressure and lift off. Repeat for the other side of the filter.  You can use finger cots or thin surgical (disposable) gloves if you like.  If you are careful, you can simply use your fingers as you will only be touching the edges of the filter.

Figure 3.  Cleaning the Filter

Place the clean filter back in the cell, take the metric o-ring and press it down in the cell with the 0.05" hex driver, as shown in Fig.4.

Figure 4.  Put metric o-ring into cell on top of filter

Place the screw-in retaining ring on the filter holder and screw in clockwise (reverse of Fig.2) to tighten. 

Figure 5. Correctly Installed with Screw-in Retaining Ring Below the Surface of the Filter Holder 


Notice that the top of the screw-in retaining ring using the new metric o-rings is below the top of the filter holder as shown in Fig 5.  This should eliminate any interference in SBIG filter wheels, provided that you screw the entire filter holder completely into its threaded hole. 


Figure 6. Incorrectly Installed with Screw-in Retaining Ring Above the Surface of the Filter Holder 


Notice that the  top of the screw-in retaining ring in Figure 6 is above the top of the filter holder. This is incorrect and should no longer happen with the new metric o-rings.