4 replies to this topic

[Dangoldber31]

Would you ever use two filters simultaneously, provided there is overlap in the target wavelengths? ie. not an Ha on top of an Oiii, which would just block everything. 

 

Two examples

• You are imaging Ha with on a monochrome camera in an area with a lot of light pollution (Bortle 8 or 9). You have a 7nm Ha filter and also have a 3nm dual band Ha/Oiii filter. With just the latter, you are getting Oiii data you don't want.. With just the former, you are letting in more unwanted light via the wider bandpass. With both used simultaneously, I would expect the result to be equivalent to a 3nm Ha filter, albeit with somewhat reduced peak transmittance. . 
• You have 2 identical filters that are rated OD3.5 (or 4, 4.5, take your pick). I would expect using them together to significantly reduce (in percentage terms, at least) light transmission outside the target wavelength, again with some cost to peak transmittance. The loss of peak transmittance is much larger in absolute terms than the second layer of reduction in light blocking, but perhaps this could be a worthwhile tradeoff in a Bortle 8 or 9?

Other than reduction of peak transmittance, halos seems like a major potential negative. Or perhaps I am missing something else?

[Brian Carter]

No, I would never do that.  Use filters one at a time like they are designed to be used.

[Zambiadarkskies]

With my AT 80ED doublet I used to stack filters.  I kept a L3 astronomik ir/uv cut filter permanently in the imaging train (there is a filter thread at the front of the field flattener).  I would even leave the said L3 in there when using my L-Ultimate filter as I found out that doing so killed the halos that would otherwise be present on smaller reddish stars.  It is a slightly odd use case.  

 

I suppose I still stack filters, in that I use a 2600mc which apparently has ir/uv cut in the sensor window.  However I still use a decent quality separate (usually an L2 or L3) filter in my drawer when broadband imaging.  I like most ZWO products but I really don't like their filters so I prefer my own choice in the drawer.   

[steveincolo]

With my AT 80ED doublet I used to stack filters.  I kept a L3 astronomik ir/uv cut filter permanently in the imaging train (there is a filter thread at the front of the field flattener).  I would even leave the said L3 in there when using my L-Ultimate filter as I found out that doing so killed the halos that would otherwise be present on smaller reddish stars.  It is a slightly odd use case.  

 

I suppose I still stack filters, in that I use a 2600mc which apparently has ir/uv cut in the sensor window.  However I still use a decent quality separate (usually an L2 or L3) filter in my drawer when broadband imaging.  I like most ZWO products but I really don't like their filters so I prefer my own choice in the drawer.   

I did this with an L3 Astronomik as well, but inserted into my 533MM, to help with blue bloat for my AT130EDT.  Eventually I switched to Astronomik Deep Sky RGB filters, which allowed me to move the L3 into my filter wheel.

[joydeepb]

I have a 2" Optolong UV-IR filter permanently installed in the filter cell of the scope, and stack it with the filters in my CFW. The reason to do this was to eliminate the halos in the Oiii filter (Antlia, 2.5nm Ultra),. It doesn't seem to hurt the other filters noticably, but it significantly reduces the halos when using the Oiii filter.