M45

[MartinB]

8 hours ago, Tommohawk said:

If the issue were simply that the sensor is exposed to a greater amount of signal with blue light then this would be so - but the issue is partly that the light at the blue end of the spectrum is defocussed, and especially with very short wavelengths. Have a look at this, from Olympus life-science's website:

This probably isn't a typical telescope achromat but it makes the point - light below 450nm wavelength has a huge shift in focal length. A similar example is given at Thor labs here. This illustrates nicely the real problem with light at 380-420nm - its not so much the additional energy, but more about the blur. The same probably applies to luminance filters which transmit at shorter wavelengths when used with achromats.

I'm not sure I understand.  I appreciate that the blue can have a different focal point to the rest of the visible spectrum but have assumed that this is addressed by focussing each filter individually.  Are you saying that there is a shift in focus across the blue itself?  If this is the case I guess it would be important to check the band pass of your blue filter and see if there is a filter with a more restricted pass (as Merlin has previously suggested).  However, this would definitely not be an issue with an OIII filter.  I haven't encountered this problem with my own ASI 1600MM.

[Merlin66]

Yes,

the focus varies dramatically from the UV to the blue in any refractor.

Independent of the filter, obviously a narrow bandwidth would limit the focus change

[MartinB]

I have  just checked the band pass graphs for zwo (optimised for the asi 1600) baader, Astronomic, and Astrodon.  They all show a similar band pass of 380 to 520-40.

[Tommohawk]

9 hours ago, Adreneline said:

I was also wondering about one of these which claims to "cut out both UV rays below 390nm and IR rays above 700mn

Interesting that the UV filter helps - would be interesting to see the transmission curve for this, also the one with the IR / UV block. I rejected the idea because its another element in the system with more potential reflections etc, but maybe it is worth exploring further.

Worth bearing in mind that a UV/IR block is the same as a LUM filter -  astronomic refer to their LUM filters as UV/IR block filters. Hoya curves are published but only for some products - again, might be worth exploring especially if it would double as a LUM filter. Bit hopeful maybe! 

[Tommohawk]

1 hour ago, MartinB said:

Are you saying that there is a shift in focus across the blue itself?

Definitely!!

1 hour ago, MartinB said:

If this is the case I guess it would be important to check the band pass of your blue filter and see if there is a filter with a more restricted pass (as Merlin has previously suggested)

Again yes, and this is the point I made in my earlier post! (No post numbers anymore??)

1 hour ago, MartinB said:

However, this would definitely not be an issue with an OIII filter.

Totally agree - the problem with haloes on NB filters is quite different - I think this has more to do with the limitations of dichroic filters.

1 hour ago, MartinB said:

 I haven't encountered this problem with my own ASI 1600MM.

Maybe you have an Apo, not an achromat?

1 hour ago, MartinB said:

I have  just checked the band pass graphs for zwo (optimised for the asi 1600) baader, Astronomic, and Astrodon.  They all show a similar band pass of 380 to 520-40.

The Astromomik Deep sky filter clips at 420 ish - see my earlier post, but I agree the others all seem similar.

[Adreneline]

19 minutes ago, Tommohawk said:

its another element in the system with more potential reflections etc,

Thanks Tom. This was also a concern for me but so far I cannot see any evidence of problems; what it has done is to put the G and B focus positions (for my 200mm) in the same place. For me (with my belt driven focus system and associated backlash) it is helpful to know which filters are at the same focus position and hence the order in which to collect subs, e.g. Ha, OIII, SII makes no sense!

From my humble M45 offering this has turned into a really interesting thread - many thanks to all who have contributed.

Adrian

[MartinB]

Sorry Tom and Adrian, I must learn to read threads more carefully!

I have gathered a few images with my ASI 1600 using a Canon 200mm at F2.8 most of which are narrowband but I did an HaRGB of Auriga DSOs.  I was quite pleased with the stars.  There were some very small blue halos on brighter stars but nothing out of the ordinary compared with other cameras I have used.  However, there were no especially bright stars in the image and most of the stretching was done in the Ha channel so the stars had an easy time of it.  I am planning a widefield Orion which might show up more problems!