Filter size

[dodgerroger]

Hello folks, I’m after some advice on the minimum filter size for a Sharpstar 13028hnt paired with an sx694 camera? On reading up on f ratio to filter size I believe my current 1.25” filters will be to small. Thank you 

[michael8554]

The Sharpstar 13028hnt has a 44mm image circle

The sx694 camera diagonal is only 15.98mm .

A 1.25" Filter has a diameter of about 30mm.

If the filter is mounted close to the camera will that be okay ?

Need to get a bigger boat camera to do that scope justice !

Michael

[Oddsocks]

It's just simple geometry.

Based on the clear aperture of a 1-1/4" filter being ~26mm diameter (typically, mounted 1-1/4" filters have a clear aperture in the range 26mm-28mm depending on manufacturer), the focal length of the OTA is 364mm, the diameter of the mirror is 130mm and the diagonal measurement of the sensor (12.49mm x 9.99mm) is ~16mm, then the filter mount will begin to vignette the sensor when the front face of the filter is at a distance greater than ~31.7mm from the sensor.

If the filter wheel/filter mount is closer to the sensor than ~31.7mm then no filter vignette will occur.

In practice, the max' distance from the front face of the filter to the sensor will be smaller than calculated because the filter mount has raised edges on the skyward side and we don't have those dimensions.

Note: diagram omits secondary for clarity, ray paths are still valid

 

[TiffsAndAstro]

Apologies for jumping in here, but after reading this I wonder if I could solve or reduce the vignetteing I seem to have from my 1.25" filter by swapping my filter drawer with the spacer I have between it and my camera? I have a imx533 with approx 15.5 diagonal. It should reduce the vignetteing if nothing else?

[Oddsocks]

58 minutes ago, TiffsAndAstro said:

Apologies for jumping in here, but after reading this I wonder if I could solve or reduce the vignetteing I seem to have from my 1.25" filter by swapping my filter drawer with the spacer I have between it and my camera? I have a imx533 with approx 15.5 diagonal. It should reduce the vignetteing if nothing else?

That depends on whether the principle vignetting you are seeing in your images is primary vignetting from the main lens or mirror, which is an inescapable law of physics, or secondary vignetting caused by e.g, baffle tubes included in the OTA design, focuser draw tube diameter and length, flattener (if present), diameter and length of the mounting components used to attach the camera to the OTA, diameter of the camera port, etc., etc., or, largely caused by the undersized clear aperture of any object placed in the way of the focal path, such as a filter.

Many OTA's have some combination of primary and secondary vignetting affecting the camera image.

Moving the filter closer to the sensor will reduce secondary vignetting in any convergent optical path where the principle cause of vignetting is an undersized filter and filter-holder.

Just how much the vignette will reduce by will depend on the geometry and mechanical layout of the entire OTA, and can be roughly determined with a simplified ray-path diagram drawn on graph paper, so long as you know the dimensions of the various components used in the assembly.

If it's not too difficult to reverse the mounting sequence of filter holder and camera spacer, without affecting total back-focus distance for any flattener/reducer that you might be using, and bring the filter/filter-holder closer to the camera sensor is worth trying.

[TiffsAndAstro]

3 minutes ago, Oddsocks said:

That depends on whether the principle vignetting you are seeing in your images is primary vignetting from the main lens or mirror, which is an inescapable law of physics, or secondary vignetting caused by e.g, baffle tubes included in the OTA design, focuser draw tube diameter and length, flattener (if present), diameter and length of the mounting components used to attach the camera to the OTA, diameter of the camera port, etc., etc., or, largely caused by the undersized clear aperture of any object placed in the way of the focal path, such as a filter.

Many OTA's have some combination of primary and secondary vignetting affecting the camera image.

Moving the filter closer to the sensor will reduce secondary vignetting in any convergent optical path where the principle cause of vignetting is an undersized filter and filter-holder.

Just how much the vignette will reduce by will depend on the geometry and mechanical layout of the entire OTA, and can be roughly determined with a simplified ray-path diagram drawn on graph paper, so long as you know the dimensions of the various components used in the assembly.

If it's not too difficult to reverse the mounting sequence of filter holder and camera spacer, without affecting total back-focus distance for any flattener/reducer that you might be using, and bring the filter/filter-holder closer to the camera sensor is worth trying.

ty for all this. ill try just swapping the order so filter holder is next to camera, then spacer, then flattener. 

[dodgerroger]

Thank you all for your replies, it is mounted using an SX midi filter wheel so is as near as it will get. I think it is less than 31mm. Money does not allow me to upgrade to a larger sensor camera and really struggled with collimation, tilt and a general lack of time to fettle the scope last year