Lense on a C11

[spacetff]

I bought this used C11 and it has this lense on the back of it.
Not sure what it is and guess it is some sort of reducer.
Has no markings on it apart from the Japan sticker. Just wondering if anyone recognises it and if it is a reducer is there a way I can tell or test it's value?
thanks

[astro744]

Could be a skylight clear filter to provide a dust seal. A reducer will change the field of view. You can check by viewing with and without this part.

[bojan]

Reducers are like objective (positive) lenses... you can try to project the image (of Sun, for example) on the piece of white paper ~20 cm away from lens.

If you can't find the focus, it is filter or protection as suggested by astro744 (thing on your pic looks like that, I cant see any "lensing" effects).

[spacetff]

When you look through it it does not magnify or change the view at all so must be the dust seal I guess. Thanks for your replies, much appreciated.

[spacetff]

Hi Astro744 so looks like it is the dust seal do you normally leave these in or out when using? I do mostly visual and only just startig to learn AP.

[astro744]

I have never used one and don't know how many others have one or use it often. It is a good idea if you observe in remote dusty locations especially if the wind picks up as you can change accessories after the rear cell without dust getting inside the telescope. The front corrector will likely collect as much if not more dust but at least it can be cleaned from the outside without dismantling the telescope (dismantle NOT recommended). Proper cleaning instructions need to be adhered to for front of corrector.

If in position it will ever so slightly reduce the light throughput, perhaps imperceivably. The test for this is to observe faint stars or galaxies at the limit of detection with and without the filter in place.. It may also vignette if the clear aperture of the filter is less than the rear cell opening but again this may likely not be perceivable.

[astro744]

Here is some info on this type of filter.

https://www.worthpoint.com/worthopedia/celestron-sct-telescope-rear-cell-245318671

https://www.bhphotovideo.com/c/product/202122-REG/Celestron_93621_Skylight_Filter_Dus t_Seal_for.html

You may also find other references elsewhere.

[spacetff]

Thanks for the links, much appreciated