Sensor / reducer spacing question

[Tommohawk]

Hi  folks - quick question please.

I'm trying to sort the spacing from TS 0.79 reducer to sensor, using ZWO filter wheel and ASI1600.

From the TS website it  looks like I'll need 64mm spacing - but not sure which end of the T thread they mean when they say

"Back Focus - distances from the T2-thread to the camera sensor"

Do they mean the far end of the thread, or where it starts at the body of the reducer? Hope that makes sense!

thanks Tom

 

[Tommohawk]

Quick follow on - if anyone could check my sums I'd be grateful!

♦ Focal Length to 350 mm .... 70 mm
♦ Focal Length to 420 mm .... 65 mm
♦ Focal Length 450-500 mm ... 61 mm
♦ Focal Length 510-600 mm ... 58 mm
♦ Focal Length 610-700 mm ... 57 mm
♦ Focal Length 710-790 mm ... 56 mm
♦ Focal Length from 800 mm .. 55 mm

Scope has 432mm FL, so looks like I'll need approx. 64mm total spacing, increase to 64.5 to allow for filter increase in light path so 64.5mm 

FW is 20mm, sensor 6.5mm, so I'll need 38mm. ??? Agree??

also I have a 40mm adapter already - could I get away  with  this?

thanks!!

[Davey-T]

Can't be much help Tom, got a headache just thinking about it.

One day the shelf in my obs'y that's piled high with various spacers / adapters is going to collapse under the weight and I'll be found buried under them 

Good luck

Dave

[symmetal]

1 hour ago, Tommohawk said:

Do they mean the far end of the thread, or where it starts at the body of the reducer? Hope that makes sense!

From the flat end body of the reducer which the spacer will butt up against when screwed on.

Your distance calculation is about right. From the focal length spacing chart I would put it nearer to 63 to 63.5mm. Filters are usually 3mm thick so you would then add 1mm to the overall spacing distance (1/3 thickness). This comes to 64.0 to 64.5mm total. Subtracting your 26.5mm filter wheel and camera distances this comes to 37.5 to 38.0mm much as you said. 

You could try the 40mm spacer and see what the corner star shapes are like but I think it will be too long. I found that 0.5mm makes a noticeable difference in star shapes on my ZS-61(360mm) and ASI1600.

Alan

[Tommohawk]

3 minutes ago, symmetal said:

From the flat end body of the reducer which the spacer will butt up against when screwed on.

Your distance calculation is about right. From the focal length spacing chart I would put it nearer to 63 to 63.5mm. Filters are usually 3mm thick so you would then add 1mm to the overall spacing distance (1/3 thickness). This comes to 64.0 to 64.5mm total. Subtracting your 26.5mm filter wheel and camera distances this comes to 37.5 to 38.0mm much as you said. 

You could try the 40mm spacer and see what the corner star shapes are like but I think it will be too long. I found that 0.5mm makes a noticeable difference in star shapes on my ZS-61(360mm) and ASI1600.

Alan

OK thanks that  really helpful. These are 31m ZWO filters and I think only 2mm think. I assumed it would be measure from the main body but the I found the description ambiguous. Looks like yet another spacer than!! 

[Tommohawk]

1 hour ago, Davey-T said:

Can't be much help Tom, got a headache just thinking about it.

One day the shelf in my obs'y that's piled high with various spacers / adapters is going to collapse under the weight and I'll be found buried under them 

Good luck

Dave

LOL - yes I've got quite a collection, none of which are ever correct for the job in hand!

[symmetal]

1 hour ago, Tommohawk said:

These are 31m ZWO filters and I think only 2mm think.

You're right . I assume your filters are the new version which don't have the edge artifacts that my original 31mm filters have. I only fitted the masking rings from ZWO last week to hopefully cure the problem. The flats from the masked ones are certainly more of a normal 'flats' shape compared to the original ones which had multiple 'humps' so hopefully will be an improvement. 

I normally have to resort to the Baader variable spacers to get the spacing optimum.

Alan 

[Tommohawk]

1 hour ago, symmetal said:

You're right . I assume your filters are the new version which don't have the edge artifacts that my original 31mm filters have. I only fitted the masking rings from ZWO last week to hopefully cure the problem. The flats from the masked ones are certainly more of a normal 'flats' shape compared to the original ones which had multiple 'humps' so hopefully will be an improvement. 

I normally have to resort to the Baader variable spacers to get the spacing optimum.

Alan 

Hi Alan and thanks for your post. 

First thing, I have solved the spacer issue I think ... in theory anyhow. ZWO supply with the ASI1600 a 21mm long M/F T2 extender, an 11mm long F/F T2 extender and a 16.5mm long F 48mm/M -T2 extender.

So the combination of 21 and 16.5 would work nicely except that the F end is 48mm. But whilst having another firkle in my ZWO Box of Many Things, I found a 42 to 48 bush, so the F 48 becomes F42.

Perfect solution! Well, I've got to acquire the reducer and scope yet, but it should be good.... and it must be a sign that I really should buy it. 

NOW  what's all this about 31mm masks???? I bought my 31mm NB filter 23rd Jan 2018, and had some really unhappy issues with haloes and flats. I posted re this shortly after as I recall, but I didn't see any relevant posts, and assumes it was just me doing it wrong. The haloes aren't great, although I felt I could live with it - it was the flats that gave me real grief.

I've just looked and as you say ZWO now supply masks with the FW. I guess ZWO, or maybe FLO (who supplied the filters) would probably supply masks on request. That said, it's a bit of a fiddle getting the filters in place, never mind with masks over the top - that must be a right whatsit! If they are thin, there's no way they would lay flat. Someone mentioned taping the edges I think to help them stay flat - but doing that must be horrible, and keeping the filters clean whilst ding it near impossible. 

Having followed a couple of threads on this, am I correct in supposing that the problem is with light entering the edge of the filter? I read that someone blacked the edges but not sure if this gave any benefit -  that seems a better solution. Any thoughts Alan?

One further thought - if light leaks through via the edges, you have to suppose it would affect the lights as well as the flats, no? Of course the source is typically more intense when doing the flats, but much shorter exposure. So same proportion, no?

TBH I only got the 31mm filters because I'd hoped to image with something superfast, but if going with F5 or thereabouts I may as well have bought the 25mm version and saved some money!

[Tommohawk]

Follow on - for anyone (probably me at some point in the future when I've forgotten) following this with similar filter issues, here is a thread on CN which deals with this quite well.

[symmetal]

Yes, I bought the 31mm ZWO filters with the 1600 in Sept 2017 thinking it would be a benefit if I got a larger sensor in the future. However, after all the hassle with the edge problems I wish I'd just bought the 1.25" mounted ones instead. I saw the threads like you posted and enquired from ZWO about supplying some masks. They replied that they would send some to FLO with their next shipment to distribute. I got them a couple of weeks ago.

There are different masks for the 8 position wheel and the 5 position mini wheel which are really fiddly to fit trying to avoid touching the filter surfaces or the screwdriver slipping. Here they are on my 5 position wheel. 

As the 5 position filter wheel shares a common mounting screw between filters the masks have to overlap so don't sit flush with the surface.

If you bought the first version ZWO filters from FLO they should have a set of masks to send to you if you ask, and they still have some left.

4 hours ago, Tommohawk said:

Having  followed a couple of threads on this, am I correct in supposing that the problem is with light entering the edge of the filter? I read that someone blacked the edges but not sure if this gave any benefit -  that seems a better solution. Any thoughts Alan?

One further thought - if light leaks through via the edges, you have to suppose it would affect the lights as well as the flats, no? Of course the source is typically more intense when doing the flats, but much shorter exposure. So same proportion, no?

Yes I believe that light is reflecting off the filter edges or the 'corner' where the flat surface meets the edge. Newer version filters have the edge blackening carried over the 'corner' while the originals didn't. Some colours of the original RGB filters didn't have any edge blackening. I imagine 'painting' the edges with a suitable matt black should have the same effect but would be even more fiddly.

On the lights the problem usually seems to be most visible when very bright stars are in the frame, (or just out of frame) giving coloured 'flares' or halos. As the flats light source is not as bright (or pinpoint) as a bright star it doesn't give the same effect.

Checking my flats pre and post masks again more closely, I have to admit there is little difference between them. I thought the SGP histogram was a better shape when I took the post mask flats but that doesn't seem to be the case. I haven't processed some images I took after the masks yet so can't confirm the difference with actual bright stars.

Alan

[Tommohawk]

11 hours ago, symmetal said:

 

There are different masks for the 8 position wheel and the 5 position mini wheel which are really fiddly to fit trying to avoid touching the filter surfaces or the screwdriver slipping. Here they are on my 5 position wheel. 

As the 5 position filter wheel shares a common mounting screw between filters the masks have to overlap so don't sit flush with the surface.

If you bought the first version ZWO filters from FLO they should have a set of masks to send to you if you ask, and they still have some left.

Yes I believe that light is reflecting off the filter edges or the 'corner' where the flat surface meets the edge. Newer version filters have the edge blackening carried over the 'corner' while the originals didn't. Some colours of the original RGB filters didn't have any edge blackening. I imagine 'painting' the edges with a suitable matt black should have the same effect but would be even more fiddly.

 

Thanks Alan esp for the pic of the EFW. Cant remember the layout of the 8 posn one which I have - hopefully no overlap. Your masks seem to lie quite flat - are they plastic?

I've emailed FLO - hopefully they'll have the masks, though the masks seem to be supplied with the EFW, not the filters, and I got the EFW elsewhere - cant remember where. You'd think if they get the masks FOC theyd be happy to  oblige.

I don't recall any of my filters, NB or BB, having blackened edges and I wonder if it might be easier to do that rather than fit the masks. Take your point about the safety bevel needing blacking and that's likely to be main issue I suspect.  Either way its a horrible job and always heart in mouth re the screwdriver slipping. Also  I wonder if the masks still leave you with 31mm clear aperture or does it reduce it a bit?

Two of the new NB filters seem to have a different AR coating also, which makes it way easier to tell which way round they go if nothing else - I really struggled with this.

11 hours ago, symmetal said:

On the lights the problem usually seems to be most visible when very bright stars are in the frame, (or just out of frame) giving coloured 'flares' or halos. As the flats light source is not as bright (or pinpoint) as a bright star it doesn't give the same effect.

My flat source is a bright monitor panel and I think this is what causes the uneven flats - which  only occurs I think with the NB filters. Someone suggested using L flats for all, and this does seem to  work OK. Initially I tried doing flats with Ha and  that's where the problems arose -  presumably because the filter transmits so little light that the bevelled uncoated edges transmit proportionately more compared to the L flats. If that makes sense!

One big problem for me is that I only get to do AP occasionally - and in the big gaps between I forget all the idiosyncrasies of the equipment and processing techniques.  

[symmetal]

Hi Tom,

I don't think the 8 position filter wheel masks overlap as the 3 mounting holes are spaced evenly around the edge compared to the 5 filter wheel layout. I know this as FLO sent me the 8 position masks by mistake first time.  I suppose ZWO issue the masks with the filter wheel as they are wheel specific. If sent with the filters they would have to include both sets, though this wouldn't be more costly really, as they would then not have to issue them to people who bought mounted filters. I thought the masks were only needed with the original filters which I assume aren't sold anymore, so was surprised that they issue the masks automatically with new wheels. I'm sure FLO would send you the masks if you bought the filters from them. I sent the 8 position masks back to FLO as I thought it would be rude not to as they're free, otherwise I could have sent them on to you.  The masks are of a plastic material with one side smooth while the other side is textured. On the 5 position masks the only way to mount them is textured side facing the telescope if you're wondering which way to mount yours.

While fitting them I lifted the filters out to check if they were the right way around but to be honest couldn't see any difference in the reflectivity between the two sides on the LRGB. Holding a screwdriver in front to check the reflections as recommended in other posts made no difference, so I put them back as they were.

The other thing is the masks reduce the aperture a bit more. The 31mm diameter is reduced to about 29 by the original filter wheel mounting flange. Adding the masks the aperture is reduced to 27.5mm. I measured an 1.25" mounted filter and the aperture there is about 26.5mm. What a lot of hassle for 1mm. 

My ZWO Ha flats were a very weird shape and were also causing star halos so I swapped it for a Baader 31mm unmounted one. The Baader Ha flats were normal shape even without the masks.

Alan

[Tommohawk]

5 hours ago, symmetal said:

What a lot of hassle for 1mm

Couldn't agree more! And of course with the 31mm filters as you have to be careful not to muddle them especially the NB ones as they aren't marked and 2 of them are very similar in appearance- cant remember which.

Telling which way round they go isn't easy either - the reflections thing sort of works on the NB ones, but it isn't obvious. If I were ZWO I'd ditch the 31mm filters and just offer 25mm or 36mm.

I emailed FLO and they've kindly agreed to post me some masks - so that's one evening's entertainment sorted!?

[symmetal]

11 minutes ago, Tommohawk said:

I emailed FLO and they've kindly agreed to post me some masks - so that's one evening's entertainment sorted!?

Glad you got some from FLO ok Tom. Hope the fitting goes well for you and that they make a difference. ?

Alan

[Tommohawk]

Nyyaarrrghh.

Am off to Spain for my once a year imaging session on Friday - so set tonight aside to fit the masks. And looks like they're the wrong ones. I did check and there were 8 in the pack so I assumed correct for the 8 position wheel - but these wont fit. Look like the type for the 5 posn wheel. Mine have pretty much equally spaced holes, unlike the 5 posn version.

Pretty fed up with this now, especially as I've had to open the wheel and sensor again, so more dust inevitably.

Anyhow - I think I'm just letting off steam really, but can you confirm that the 8 posn masks are different? - looks like they must be.

[symmetal]

That's annoying Tom. ?

FLO had 2 types of mask sent to them from Zwo for the two filter wheels. There is an extra spare mask in each set. The first set FLO sent me had 9 masks with equal hole spacing which would seem to be the ones you need. The second set they sent contained 6 masks with the non-equal hole spacing for the 5 position wheel as in my image above.

It possible that whoever at FLO sent you the masks picked a pack, saw there were only 6 masks and added 2 from another pack not knowing there were two types of mask.

I dealt with Martin at FLO, so he's aware of the two mask types, but it's now probably too late to get you fixed by Friday. ? 

Alan

[ollypenrice]

Is this not something you could improvise yourself with a compass cutter?

Olly

[Tommohawk]

39 minutes ago, ollypenrice said:

Is this not something you could improvise yourself with a compass cutter?

Olly

Hi Olly. 

Thanks - I did think of a DIY solution but the mask can only have a annulus width of about 3mm and also needs holes through it to take the filter screws. I know some folk have tried it but 8 of these would take me a lifetime!

The main issue is with the NB filters so maybe I'll just try and find a BB target for this trip

[Tommohawk]

Footnote to all this - I got my masks from FLO double quick and all sorted for my trip - hooray!!

Unfortunately due to freaky high winds and cloud I didn't get a single sub - booo!

[symmetal]

I thought clouds only accompanied purchases but it seems to be included with freebies too. ? Hope you managed to do other things on your trip. ?

Alan

[Tommohawk]

28 minutes ago, symmetal said:

I thought clouds only accompanied purchases but it seems to be included with freebies too. ? Hope you managed to do other things on your trip. ?

Alan

LOL. Yes thanks got a day skiing in .. and oddly the usual howling winds at the top of Sierra Nevada were absent!

Also sampled quite a lot of local produce. (Hic!)