Doh !!

[michael8554]

I was attaching a newly aquired ZWO ASI 120MM camera to a 300mm F4 M42x1 Pentax/Practika threaded lens.
I'd bought a set of 3 Pentax macro extension tubes 7mm, 14mm and 28mm long, £4.81 on the bay.
I attached the 28mm extension to the lens, then attached a M42x1 to M42x0.75 (ie T-2) adapter (£3.40), then screwed on the ZWO.
As recommended many times to beginners, tried it out during the day on a landmark a mile away.
The lens was in focus at about the 50ft mark on the scale, leaving a bit in reserve for true infinity on stars.
I fitted it all to my telescope, then cleared up, but couldn't find where I'd left the 7mm extension to put in the box with the unwanted 14mm.
Later on I tried out the lens and camera when it was dark enough to see Altair.
Not a hint of a star on the display.
Lens cap was off, exposure upped to 4 secs, gain to max (new to me camera remember, so no idea of standard settings.)
Still nothing.
Torch shone down lens gave a white display.
Hours later I swung the scope to point at a lit house window and got an image, focus on that was way down the scale.
Swung back to Altair and finally got a very out of focus image, I mean more than filling the frame.
Focused up, setting was at about 15 feet on the scale - very strange.
Next day had a look at the setup again (you know where this is going right?)
Found my missing 7mm extension - screwed onto the 28mm extension on the lens.

Michael

[triton1]

We have all had "lost the plot moments" where the brain apparently goes smooth.

[michael8554]

"brain gone smooth", I iike that Jonn, very quotable.

Final observation, I found these cheap M42x0.75 and x1.0 threads shouldn't but did happily thread onto each other, not a problem in the end when tightened up.

Michael

[Gina]

Yes, I've found that too   Probably shouldn't do it but if it works...

[SilverAstro]

On 09/09/2016 at 00:28, michael8554 said:

Found my missing 7mm extension - screwed onto the 28mm extension on the lens.

At least it wasn't lurking in the carpet to do you a mischief

Have you had a chance at any stars since ? Did it all come good / focus up in the end   7mm sounds quite a lot !!  Approx how many mm does the T2 add to the path ?

[michael8554]

Thanks SilverAstro

Yes had a look at M57 and NGC 7331, found I needed to stop down my F/4 Guider lens to F/5.6 to stop stars saturating on 2 sec exposures, which I use to stop the guider chasing seeing ( though I read tonight that seeing might only have a 1/2 sec period).

Not one "lost star" all night, which happened fairly regularly with my previous Long Exposure SPC900NC.

Rough measure just now, the M42x1.0 to M42x 0.75 adds 4mm. 

28 + 4 + 15 (ZWO flange to sensor) is still too long for a Pentax lens, but focus at between 50 and infinity on the scale is close enough.

Michael

[SilverAstro]

39 minutes ago, michael8554 said:

Rough measure just now, the M42x1.0 to M42x 0.75 adds 4mm. 

28 + 4 + 15 (ZWO flange to sensor) is still too long for a Pentax lens, but focus at between 50 and infinity on the scale is close enough.

Thanks for the info. I have no firm plans yet but was following Gina's experiments and wondering "what if ",  as one does

So I had got as far as 45mm for the pentax flange to focus, -6.5 for the "back focus" ? in the spec for the ASI1600MC giving me a gap of (45-6.5)= 38.5 to fill  so 32 (28+~4) would not do it, but I think I may have misunderstood this back focus thing (flange to sensor would make more literal sense but they dont quote that !) or including the 7mm might be close  > back to googling >

 

[Gina]

6.5mm for the 1600 is without the F-F adaptor ring - it's 17.5mm with it.

[SilverAstro]

2 hours ago, Gina said:

6.5mm for the 1600 is without the F-F adaptor ring - it's 17.5mm with it.

Oh thanks, hmmm  what is an F-F ring I wonder *, I have just seen that there is a user manual, I had better download that.

I'm not set on this sensor ( yet ! but while I watch Gina's experiments who knows what might happen , , ) just using it as a vehicle to see if I understand this brave new sensor world ( which I dont obviously ) and what a lot of them there are !

* female-female thread adapter ?? ! and being a 2" eyepiece mounting at the same time ?  So, one then wonders why one would not simply use a M42x42F to M42x1.5 when adapting a Pentax lens,,, because they dont exist ?

 

[michael8554]

So long as your lens flange to imaging chip is MORE than 45mm, as was my case with the extra 7mm spacer, the lens will focus at infinity, but not on the infinity mark on the lens, but somewhere less.

Does that matter? Not for Astro, but would for closeup daytime photography .

If your spacing is LESS than 45mm it will not focus on infinity at all.

Actually the Pentax M42 distance is 45.46.mm

My final setup is 47mm, but being a little bit perfectionist I plan to file 1mm off the camera end of the 28mm, not 2mm in case I accidently go under 45.46mm total.

The 17.5 that Gina specs is with the glass window T2 spacer, I wouldn't run without that myself.

In your case, 28mm Extender + 4mm M42 to T2 + 17.5mm = 49.5.

That might just work, remember 47mm worked for me with the extra 7mm, with infinity focus way down the scale.

Without the 4mm converter it's 45.5mm (remember I found the Pentax M42 would screw into the T2 on the camera)

That might or might not be exactly right, depending on how accurate my measurements have been - you can always add a spacer cut from thick card or a plastic floor tile on one of the threads to get on 45.46 or over.

Michael

[Gina]

1 hour ago, michael8554 said:

The 17.5 that Gina specs is with the glass window T2 spacer, I wouldn't run without that myself.

The spacer is nothing to do with the glass window - that is in the main part of the camera.  The spacer is simply a tube with a T2 thread on the inside to act as a gender changer (F to F) and the camera can be used perfectly well without it if you have a female T2 threaded hole for it to go in (as in the ZWO filter wheel).

[SilverAstro]

56 minutes ago, michael8554 said:

The 17.5 that Gina specs is with the glass window T2 spacer, I wouldn't run without that myself.

with infinity focus way down the scale.

Lots of interesting suff there for me to inwardly digest, many thanks for that.

and yet more details for me to google up, like this glass window thing, new to me does she !

oh qrx, Gina typeth as   I  do  , , , 

 

[SilverAstro]

Oh gosh, I'll never get my head round all this, I think I will need to employ an assistant LOL!

[Gina]

Maybe THIS will help   I you want more info on the ASI1600MM-Cool, I'll be happy to help in any way I can - including photos

[SilverAstro]

37 minutes ago, Gina said:

Maybe THIS will help   I you want more info on the ASI1600MM-Cool, I'll be happy to help in any way I can - including photos

Thanks yes been there ( to the colour one actually ) where I spied that there is  a manual and pdf that I should have downloaded and when you prompted earlier about the FFring I saw a little grayed out pic that when clicked upon showed that 11mm ring to be a gender changer etc, so let that be a warning to any others that pass this way, click on everything !

thanks   will they be  LRGB as well ?

So this glass window will be part of the (double?) glazing or whatever, that I imagine will be needed to stop the sensor frsting up ?

[ OT  Last time I was into this it was gas-hypered films and frozen CO2 and 2" optical plugs to keep the film ice free Jack Nicklous(sp?) in Canada was TheMan ,, I think I may have time warped him in to  golfer ?  ]

[RichLD]

2 hours ago, michael8554 said:

So long as your lens flange to imaging chip is MORE than 45mm, as was my case with the extra 7mm spacer, the lens will focus at infinity, but not on the infinity mark on the lens, but somewhere less.

Does that matter? Not for Astro, but would for closeup daytime photography .

If your spacing is LESS than 45mm it will not focus on infinity at all.

Hi there

I always find it's the other way around - I can't achieve infinity focus if the register distance is more than recommended for the lens. When the register distance is shorter than recommended the I find infinity focus is way down the scale at 3m for example. In daytime photography, I use extension tubes for macro work and cannot achieve infinity focus. Not wishing to be Mr Disagreeable, just wondered if I was missing something? 

Cheers

Rich

[Gina]

If the distance is too great you can always add extension tubes or Delrin washers - to little and, yes, you have a few mm available by focussing on a shorter distance but beyond that you're stumped!

[michael8554]

6 hours ago, RichLD said:

Hi there

I always find it's the other way around - I can't achieve infinity focus if the register distance is more than recommended for the lens. When the register distance is shorter than recommended the I find infinity focus is way down the scale at 3m for example. In daytime photography, I use extension tubes for macro work and cannot achieve infinity focus. Not wishing to be Mr Disagreeable, just wondered if I was missing something? 

Cheers

Rich

Hi Rich

I agree, it does seem to be counter-intuitive that more extension brought  the infinity focus point down the scale, but with 7mm too much flange distance, infinity was at about 15 on the scale.

With 7mm removed, infinity focus is at 50 on the scale.

Michael

[RichLD]

7 hours ago, michael8554 said:

Hi Rich

I agree, it does seem to be counter-intuitive that more extension brought  the infinity focus point down the scale, but with 7mm too much flange distance, infinity was at about 15 on the scale.

With 7mm removed, infinity focus is at 50 on the scale.

Michael

Many thanks for the response Michael, I'll have a go at this myself - it would certainly help with some of the spacing issues I've had! That's what I love about this forum, you're always learning something new!

Cheers

Rich 

[michael8554]

Hi Rich

I'm going to get another M42 lens out, attach the 7mm extender from hell, pop onto my eos with bayonet adapter, and prove myself wrong!!

Michael

[SilverAstro]

1 hour ago, RichLD said:

Many thanks for the response Michael, I'll have a go at this myself - it would certainly help with some of the spacing issues I've had! That's what I love about this forum, you're always learning something new!

Seconded and thirded   and especially from threads like this where something went wrong ! So a big "Well done" and thanks to Michael.

I did not realise previously how large the range of "adjustment" available was

 

[michael8554]

Well Rich, I said it was counter-intuitive, you'll be pleased to know you were right !!

That is, you can probably reach infinity focus if the flange spacing is LESS than the correct figure.

On the Doh! night I definitely had the elusive 7mm extender from hell on, so perhaps I actually had used the third extender in the kit, the 14mm M42x1, instead of the 28mm M42x1 :

7mm M42x1 + 14mm M42x1 + 4mm M42x1 to T2 + 15mm ZWO =40mm

Or even 7mm + 14mm + 15mm = 36mm

Which are both  LESS than 45.46mm.

Apologies to everyone who has been hanging on my every word (yeah right).

So SilverAstro, for you:

14mm M42x1 +7mm M42x1 + 4mm M42x1 to T2 + 17.5 ZWO = 42.5mm

Which is LESS than the correct flange distance, so should focus on infinity, but not on the infinity mark on the lens.

But what do I know.......

Michael

[RichLD]

No worries Michael, thanks for saving me the time of testing your theory by posting! Optics can have some unusual quirks and I'm certainly no expert on them!  Glad you got to the bottom of your problem 

All the best

Rich

[SilverAstro]

2 hours ago, michael8554 said:

So SilverAstro, for you:

14mm M42x1 +7mm M42x1 + 4mm M42x1 to T2 + 17.5 ZWO = 42.5mm

Thanks so very much Michael, all most interesting / helpful /thought provoking even

Maybe one cloudy day I'll build me an optical bench as well

[Demonperformer]

3 hours ago, michael8554 said:

Well Rich, I said it was counter-intuitive, you'll be pleased to know you were right !!

That is, you can probably reach infinity focus if the flange spacing is LESS than the correct figure.

On the Doh! night I definitely had the elusive 7mm extender from hell on, so perhaps I actually had used the third extender in the kit, the 14mm M42x1, instead of the 28mm M42x1 :

7mm M42x1 + 14mm M42x1 + 4mm M42x1 to T2 + 15mm ZWO =40mm

Or even 7mm + 14mm + 15mm = 36mm

Which are both  LESS than 45.46mm.

Apologies to everyone who has been hanging on my every word (yeah right).

So SilverAstro, for you:

14mm M42x1 +7mm M42x1 + 4mm M42x1 to T2 + 17.5 ZWO = 42.5mm

Which is LESS than the correct flange distance, so should focus on infinity, but not on the infinity mark on the lens.

But what do I know.......

Michael

Hopefully you have discovered this before filing that "extra" mm off the 28mm extender?