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Old 06-08-2017, 09:28 PM
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skogpingvin (Bill)
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Newbie narrowband questions

Hi experts

I'm a triplet refractor user but only I image with a DSLR (Pentax K3-II) so my images are a bit on the meh side. I was speaking with a narrowband images who told me that a Ha filter can actually work on an unmodded DSLR (if you take sufficiently long exposures) and the results when blended back in with the DSLR's image look pretty good.

My question is how would I do this? Where would the filter go? I've got a focuser that ends in an M68, going to a flattener that ends in an M48, which goes to a home-modified Pentax t-mount. The t-mount has no spare length in the train due to insufficient back-focus from the flattener, so I can't put a filter there. Is there anything I can do? Anywhere else I can put a Ha filter?

Or am I going to start asking for beginner-level CCDs in the second-hand equipment section?

Many thanks

Bill
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Old 06-08-2017, 09:44 PM
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SimmoW (SIMON)
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Hi Bill, there are a few producers of clip in filters that insert between the sensor and bayonet mount, like my favourite http://www.astronomik.com/en/clip-fi...ml?___store=en

But they're only for canons and some do Nikon. You may need to jump ship as canon seems far better supported generally. Bit if you are already thinking narrowband, I'd recommend a cheap mono to try out or a one shot colour ccd or CMOS that will be more sensitive
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Old 06-08-2017, 09:48 PM
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skogpingvin (Bill)
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Originally Posted by SimmoW View Post
Hi Bill, there are a few producers of clip in filters that insert between the sensor and bayonet mount, like my favourite http://www.astronomik.com/en/clip-fi...ml?___store=en

But they're only for canons and some do Nikon. You may need to jump ship as canon seems far better supported generally. Bit if you are already thinking narrowband, I'd recommend a cheap mono to try out or a one shot colour ccd or CMOS that will be more sensitive
Yeah, I agree that a clip-in would be nice, but it doesn't exist for Pentax, and if I'm going to change cameras it'd be to a proper astronomical CCD. I'm looking for a cheap way of inserting a Ha into the train without changing anything else. I made the decision to go Pentax in the late 1970s, so I'm way too far in to change now!

Bill
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Old 07-08-2017, 02:15 PM
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astronobob (Bob)
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Originally Posted by skogpingvin View Post
Hi experts

The t-mount has no spare length in the train due to insufficient back-focus from the flattener, so I can't put a filter there. Is there anything I can do? Anywhere else I can put a Ha filter?

Many thanks

Bill
Hi Bill,
I'm no expert, but my 1st thought on this is: when you say insufficient back focus, Im just wondering how much is insufficient you mention, because a filter, I'm thinking a filter 'some screw in types is/can be only a few mm thick ?
So - I assume your focus train is fully & totally extended with absolutely no more room to extend, not even a few mm ?

Just for any other 'onlookers' Can you post a 'close-up' pic of your Focus train, maybe a hack with the actual focuser that may give a few more mm ?

But as said, Im no expert, and also not sure of the size shape of the filter needed, but there should be a thin screw in version, ?

Just added my Bobs worth
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Old 08-08-2017, 07:48 AM
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skogpingvin (Bill)
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Hi Bill,
So - I assume your focus train is fully & totally extended with absolutely no more room to extend, not even a few mm ?

Just for any other 'onlookers' Can you post a 'close-up' pic of your Focus train, maybe a hack with the actual focuser that may give a few more mm?
That sounds like a good idea. I don't have a photo of the whole train from the OTA to the DSLR - I'm at work at the moment, but I'll get one up when I get the chance. But it's not the focuser, the "fully extended" problem that I have is that I'm really pushing for space between the flattener and the DSLR. This one I do have a photo of: https://www.flickr.com/photos/skogpingvin/35614038593/

The DSLR is on the left, then there's the t-ring attached, then the stepdown (with the M42 to the left and the M48 to the right) then the flattener. This part of the train is all fixed together with threads - obviously the focuser is on the right of the flattener. Even with this setup, I'm getting streaky stars in the corners, so I need to get the DSLR closer to the flattener. (Believe me I've taken heaps of test shots - lengthening the train makes it way worse. I think I need to compress the flattener-sensor distance by about 0.5 or 1.0 mm).

I've designed an adapter that will allow me a couple of extra millimeters and a good mate with a lathe is going to make the part up in the next couple of weeks. There's just no way I can get MORE equipment in there and get a decent photo.

The question therefore is where can I sneak a filter in - I'm thinking it might be possible up in the part of the train where the focuser actually adjusts the length - but can you get a screw-in Ha filter in an M68 (and not have to sell a kidney to afford it)?

Bill
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Old 08-08-2017, 08:43 AM
glend (Glen)
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You can buy standard Ha filters that screw into the front of your T-Adaptor. This is easy for all makes of camera as they all need a T-Adaptor to fit a scope focuser. Clip-in filters work but might be hard to find (or expensive) for the Pentax.

Concerning exposure for Ha, a typical DSLR has fewer red detecting pixels and the internal cut filters reduce the amount of Ha spectrum.

https://en.m.wikipedia.org/wiki/Bayer_filter

So yes you need longer exposures to capture Ha, and your green and blue pixels will get nothing through an Ha filter. The reason folks modify DSLRs, by removing the internal blue cast filter, is that it allows about 4x to 5x as much Ha capture.
So for an Ha image sub exposure i would suggest you start with 5x the amount of time required for a standard exposure. You can start out on targets that are great sources of Ha like Eta Carinae, and the Lagoon Nebula, and experiment with the sub lengths required for your camera through the Ha filter. Obviously, the longer exposures require good mount tracking and guiding.
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Old 08-08-2017, 04:16 PM
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Yes Bill, I see that your Variable 'airgap' extender is looking rather close to being fully retracted - good piece of kit too. This distance set is achieving your desired flat field I assume ? If so, then agree, would find it near impossible to fit a filter in there without altering the air-gap, thus impinging on the flat field .
Only other option is at the scope end of the fucus tube, ??
Not sure what the go is there tho, concerning if thread is available and/or what size - and only providing you have few mm of in-focus travel to work with to compensate for the filter in situ ?

I was just reading & watching vids about these Variable extensions last night 'need one myself' and there are a few types out there, some have a smaller spacing when set to minimum than others ?
Not sure if may be an option tho
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Old 08-08-2017, 05:35 PM
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Originally Posted by astronobob View Post
Just for any other 'onlookers' Can you post a 'close-up' pic of your Focus train, maybe a hack with the actual focuser that may give a few more mm ?
OK, here it is: www.flickr.com/photos/skogpingvin/36439552055

There's plenty of travel left in the focuser - it's focused on infinity about where it is in the photo. If I could get a filter in there somehow that'd be cool. The flattener screws into the focuser using an M68 (I think) thread.

Bill
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Old 08-08-2017, 07:34 PM
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skogpingvin (Bill)
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Yes Bill, I see that your Variable 'airgap' extender is looking rather close to being fully retracted - good piece of kit too. This distance set is achieving your desired flat field I assume ?
Close, but not quite there, Bob. I think I need about another half or one mm. I'm having a mate with a lathe make a part that will enable me to shave a mm off the front face of the t-ring and snuggle the DSLR that little bit closer to the flattener.

One of these days I'll upgrade to something like a ASI1600MM-cool to go with the filter wheel I've already got, so I can get big-grown-up Ha photos. When that happens I'll be back looking at back focus (not looking forward to it, but I think you need a spacer rather than to shorten it). Just don't suggest an OAG!

Bill
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Old 08-08-2017, 10:39 PM
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Close, but not quite there, Bob. I think I need about another half or one mm. I'm having a mate with a lathe make a part that will enable me to shave a mm off the front face of the t-ring and snuggle the DSLR that little bit closer to the flattener.

One of these days I'll upgrade to something like a ASI1600MM-cool to go with the filter wheel I've already got, so I can get big-grown-up Ha photos. When that happens I'll be back looking at back focus (not looking forward to it, but I think you need a spacer rather than to shorten it). Just don't suggest an OAG!

Bill
See how that goes then Bill, yep, hope it is enough
BTW, what is meant by OAG a) Off Axis Guide 'or' b) On Axis Guide
Apparently On - Axis is the flavour of the month at moment

All the best
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