Narrowband filter query

[Bobby1970]

I am considering a duo or tri band filter to have a go with my ZWO 178MC. 

 

My budget limits me to either the ZWO Dual band, or the Altair Triband. Both can be had for sub £90 which for me, makes it worth a punt. 

 

Will there be much difference between the two? Not sure which to go for, I have a few bits of ZWO gear and find it very good tbh. 

 

Ideally I want the filter to perhaps allow some imaging time when the moon is up and about. 

 

Any advice or experience will be gratefully received. 

 

 

Edited February 24, 2021 by Bobby1970

[TerryMcK]

I have an Altair tribander and find it good especially under the moon - not sure it blocks out LED lights as well as my IDAS D2 does. I actually have the 2" one which is a bit more expensive than the 1.1/4" one which judging by the price you are going for. 

Obviously when shooting deep sky you wouldn't aim within 20 degrees of a full moon but it copes outside of those limits well.

[Bobby1970]

44 minutes ago, TerryMcK said:

I have an Altair tribander and find it good especially under the moon - not sure it blocks out LED lights as well as my IDAS D2 does. I actually have the 2" one which is a bit more expensive than the 1.1/4" one which judging by the price you are going for. 

Obviously when shooting deep sky you wouldn't aim within 20 degrees of a full moon but it copes outside of those limits well.

I read that the Altair Tri band has caused halos around stars on some occasions. Is this something you have experienced?

 

Thanks

[TerryMcK]

Yes indeed. I seem to recall it was prevalent in the OIII and HII spectrum as they are covered in one wider band. Ha seems to be ok.

It only affects larger stars though. Any filter like this is a compromise of course and halos can be reduced in processing.

 

I do get better results with the IDAS D2 it has to be said. But I am surrounded by LED light pollution.

[Bobby1970]

So, in simple terms, what is the actual difference between the ZWO Dual band and the Altair Tri band? 

 

[TerryMcK]

I've no idea what the ZWO filter is like as I don't have one nor ever seen one. But from extrapolating the data that is available:

From the look of its description on the ZWO website it looks like it is designed to pass Ha and OIII. It has a pass band (normally quoted in nm) of 15nm for Ha and 35nm for OIII. 

 

Whereas the Altair one is designed for Ha and OIII/Hb hence the triband name. The Altair one is quoted as 12nm for Ha and 35nm for OIII/Hb

 

 

What the differences are in practice are difficult to determine from the specs as they are very similar. The Altair one is narrower in Ha so may produce better results it also has UVIR block and AR coating. The ZWO filter does not state these coatings.

 

They are even the same thickness glass so might share common manufacturing.

 

Hope this helps.

 

 

 

 

Edited February 24, 2021 by TerryMcK

[Bobby1970]

Slightly on a tangent here. Can a 2" filter be used on the front of a DSLR lens? With a suitable adaptor? Or is it too far from the sensor?

 

 

[TerryMcK]

Yes it can. I use a Canon 50mm f1.8 prime lens on one of my DSLRs to take ultra wide field. I use an M48 to M49 adapter to fit a filter. The M48 is female and the M49 is male. A bit expensive for what it is but it works well. Very little vignetting around the edges that is sorted with flats.

If your front element is more than 50mm diameter I would use a clip in filter instead. However if you have a Canon EFS body you have to use an EF lens as the rear of an EFS lens sticks into the body too far. An EF lens has enough clearance to not foul the clip in filter.

However focusing is another matter. I use a tiny Bahtinov mask that I have fitted into a plastic mount that pops on the front of the lens. You have to take 6 to 10 second snaps and have a look at the computer screen to see the familiar pattern. The difference between focus and slightly out is a hair of a turn on the focusing ring!
 

 

[Bobby1970]

Hmmm. I'm a bit torn now. 

My scope has a 2" Focuser, with an adaptor that steps down to 1.25". 

 

So would a 2" filter screw into the front of the 2"-1.25"adaptor?

 

Then I still insert my ZWO cam into the back of the adaptor as I do now? 

 

Just wondering if I can find a solution where a 2" filter could be used in the scope and on a lens. I have a Canon 450d with the 18-55mm kit lens but also some old M42 lenses I have used sometimes with it. 

 

@TerryMcK, is focussing difficult because everything is much dimmer?

 

 

Edited February 26, 2021 by Bobby1970

[TerryMcK]

On 2/24/2021 at 10:05 PM, Bobby1970 said:

is focussing difficult because everything is much dimmer?

Yes you need to have good eyes to focus or a laptop. My DSLR for astro is an astromodded 400D that doesn't have live view. The screen is tiny so you have to use a laptop with something like APT on to view the focus images. Also with a conventional lens, when at almost infinity, it is difficult to get focus as I find there tiny movements between focus and just slightly out. On my 50mm f1.8 lens I stop down to about f2.5 because it produces strange shaped stars at the edges when wide open. Once I have got focus I use some insulating tape around the focusing ring to stop it from moving. I much prefer using a telescope but for the odd ultra wide field with a DSLR I can put up with getting it just right. The results can be spectacular though so it is well worth having it in the arsenal. The step down adapter is "STEP DOWN ADAPTER 49MM-48MM STEPPING RING 49 TO 48MM 49-48 STEP DOWN RING" from eBay and cost £5.75 The Bahtinov Mask I use is 52mm but I made a quick release plastic adapter to mount it on the front of the lens.  https://www.secure-eshop.com/epages/www_astrograph_net.sf/secf8ab3ce049/?ObjectPath=/Shops/www_astrograph_net/Products/AGBM52 at £35 - Expensive but it works   Nothing in astro is cheap 😉