How do you know when spacing is correct?

[AndyUK]

Having never had to deal with spacing before, how can you tell when spacing is absolutely correct?

I have a TRF2008 and I'm hoping to be able to give it an outing tonight on the Equinox 80 - Apparently it requires 56mm from the backend of the TRF to the chip, which, with the help of some Delrin rings, I've managed to get to 56.2mm, but how important is that 0.2mm, and how can you tell when it's 100% right (or not?). I probably COULD get it to virtually 56mm dead, but the TRF spacing requirement is 56mm +/- 4mm, which seems to me quite a bit of leeway!

I've just taken a frame of a treetop about 100yds away, and I can seem to get it in focus, but there has to be more to it than that(?)

[themos]

I think that it's curvature of field that will betray the wrong spacing being used. You'd need some high-end software to measure it, I guess.

[AndyUK]

Oh... I guess that means something like CCDInspector... Unfortunately I've had a demo of that on this laptop and their demo licensing restriction won't let me have another demo (even for an hour... )

[Mike Hawtin]

Andy, I wouldn't put too much faith in CCDInspector, I thought it would be the ideal tool to aid in collimating my C6 hyperStar, I got it to read 0.0px tilt in X and Y with a total tilt of 0.0% and a collimation error of 0.0px but when I examined visually the frame that produced those results only one of the four corners of the frame had round stars! So for me it's back to the drawing board and yet another astro purchase that does not do what it says on the tin!!

[AndyUK]

Cheers Mike - I must admit, I can't want justify the expense of CCD Inspector for just the ocassional use... It's a little too over-priced for me. I guess it's a case of trial and error then and looking at the corners... (thanks!)

[AndyUK]

(Well, I think I might be close, but it's probably more appropriate that I continue this in the "help" section...)

[BlueAstra]

I'm using the TRF-2008 on my Meade 5000 ED80. With it screwed to the SXOAG+SXFW+SXVH16 I calculate I'm at 59.8mm, just in!. Stars look OK to me.

[AndyUK]

Cheers BlueAstra - I started mine at more or less bang on 56mm (Atik MFW, 314L+) and that seems to have worked with the Equinox 80. I would have been more concerned if it had been between 52-54mm as I'd have had to have bought a smaller extension!

[buzz]

I have found CCD inspector quite useful and just completed a similar exercise with the William Optics adjustable FF IV - I was able to compare flatness and tilt for each distance setting. I found the optimum to be within 0.5mm of the WO recommended setting. It tied in with the visual results too and it allowed me to find the source of some mis-alignment on the couplings.

You are welcome to send me some fit files of starfields and I'll measure them for you. If you have tracking issues - it will mess up the results.

regards

Chris

[AndyUK]

Thanks Chris - I'll send you a PM

[MikeWilson]

I have the same flattener and the same delrin rings...

Now with the QHY8L, I had the spacing more or less bang on. And I got some field curvature if I didn't take extra special care to tighten the focuser's three eyepiece retaining screws in carefully each time (slightly tighten one, move to the next and slightly tighten it - repeat until tight).

CCD Inspector was pretty useful to test this and I "wasted" several nights getting it right. Mr Hawtin is correct in to treat the results with a pinch of salt but I found that repeating a CCD Inspector test (you can even monitor a folder for the latest image and set up a batch imaging run to get a 'hands off' view) quite useful to see what's going on as you make iterative adjustments.

Since changing the QHY8L for the Atik 314L+... I know full well that I'm a couple of mm out (at least!) on the TRF-2008. And it doesn't seem to matter at all due to the smaller chip size of the 314L+.

So, I'd say - don't sweat <= 2mm with a 314L+ and a TRF-2008. APS-C needs to get pretty close and don't forget to include 1mm of back focus for your filters too (assuming the IDAS is screwed onto the front of the TRF-2008 - I have an Astronomik CLS here).

All the best,

Mike

[AndyUK]

Thanks Mike - That's very comforting to know...! I made a bit of boo-boo when I bought the IDAS as I was always using the 314L+ with its 1.25" nosepiece on the MN190, so I bought a 1.25" IDAS.

Of course when using the TRF with the Equinox 80, I don't use the nosepiece so I now know I should have bought a 2" IDAS and bought a 2" nosepiece adapter for the 314L+ when using the MN190

[MikeWilson]

Thanks Mike - That's very comforting to know...! I made a bit of boo-boo when I bought the IDAS as I was always using the 314L+ with its 1.25" nosepiece on the MN190, so I bought a 1.25" IDAS.

Of course when using the TRF with the Equinox 80, I don't use the nosepiece so I now know I should have bought a 2" IDAS and bought a 2" nosepiece adapter for the 314L+ when using the MN190

We've all done it before!

Just a note: You won't need a 2" adapter for the 314L+, the 1.25" nosepiece is itself a T-thread to 1.25" adapter so stick with T-threads from the 314L+ all the way to the back of the TRF-2008 and put the LPF on the front of that.

A lot of people say not to do that, but I'm going to stick with it for a while - I may lose a tad bit of sensitivity but the convenience of having it in the imaging train at all times more than makes up for it.

All the best,

Mike

[steppenwolf]

The only real test for spacing, Andy, is to look at your subs. I have CCD Inspector and it is excellent for pointing you in the right direction for faults but the real test us always on the sky and a good star field. 4mm is a lot of leeway but then I am used to the WO FR IV which is excellent (and easily adjustable) but a little finicky. Aren't those little Delrin spacers cool!

[AndyUK]

Aren't those little Delrin spacers cool!

Absolutely - It made life a doddle... especially coupled with a digital caliper I got off ebay for under £8!

Mike - I think we're talking about the same thing. I only use the 314L+ nosepiece with the MN190 (camera/filterwheel/nosepiece and the 1.25" LP filter screws into the end of the nosepiece).

When using the Equinox the nosepiece stays in the box (camera/filterwheel/spacer/TRF) - If I had a 2" LP filter, I'd screw it onto the end of the TRF. If I'd thought about at the time, the MN190 could have been camera/filterwheel/2" T adapter and a 2" LP filter could then have gone on the end of that...

With the MN190 I image with no filter in slot 1 and RGB in 2,3 and 4 (thus the LP filter on the end of the nosepiece is always in the imaging train), but with the Equinox I'm going to have to see how things pan out with the LP filter in slot 1 on the filter and shoot the RGB filters without it being in the path (and if there's a colour cast I'll probably need to get a 2" LP filter and a 2" T-adapter for the MN190 instead )

[MikeWilson]

Absolutely - It made life a doddle... especially coupled with a digital caliper I got off ebay for under £8!

Mike - I think we're talking about the same thing. I only use the 314L+ nosepiece with the MN190 (camera/filterwheel/nosepiece and the 1.25" LP filter screws into the end of the nosepiece).

When using the Equinox the nosepiece stays in the box (camera/filterwheel/spacer/TRF) - If I had a 2" LP filter, I'd screw it onto the end of the TRF. If I'd thought about at the time, the MN190 could have been camera/filterwheel/2" T adapter and a 2" LP filter could then have gone on the end of that...

With the MN190 I image with no filter in slot 1 and RGB in 2,3 and 4 (thus the LP filter on the end of the nosepiece is always in the imaging train), but with the Equinox I'm going to have to see how things pan out with the LP filter in slot 1 on the filter and shoot the RGB filters without it being in the path (and if there's a colour cast I'll probably need to get a 2" LP filter and a 2" T-adapter for the MN190 instead )

Interesting!

Yes, we're talking about the same thing or so it seems.

But we differ with your empty gap in the filter wheel. Are you re-focusing between subs? I'm not..

I have the "L" filter in the 1, with R, G and B following. (Then Ha, OIII, SII to make the full seven). I don't leave the gap because I want the clear glass in the 1 slot so that whatever filter I choose is generally parfocal with all the others.

That way I can automate LRGB imaging via the electronic filter wheel. Even if the filter wheel wasn't electronic, I prefer to capture one sub per filter before changing filters to increase the likelihood of getting a full set of subs for each filter on the one night.

I know several folks who use an IDAS filter in the 1st slot (your blank one, my L one) and no LPF filter otherwise. I think my light pollution is too severe to handle that, I don't currently regret having the CLS filter on the front of the TRF2008.

I've resigned myself to correcting colour casts in post processing. I usually try to get the highlights (the right hand side of the R, G and B histograms to match) as well as the shadows and that works out pretty well.

All the best,

Mike

[AndyUK]

Ah... I have a manual filter wheel with only 5 slots - My usual filter allocation in the wheel is none (or IDAS if using the Equinox) in slot 1 then R,G,B,Ha in the rest. If I want to shoot SII or OIII, I open the wheel and replace the Ha, but would probably only image either SII or OIII on one night.

I also don't trust "parfocal" - I have a focusing OCD (as well as the myriad of others ) and even when Bhatinov grabber is showing "critical focus achieved", I still like to get the actual value down to +/- 10 microns if I can...

Also I tend not to usually even attempt to capture a full set of LRGB images on one night. Depending on location / object (and time of the year), I might go for a night of luminance (using the LP filter only) pre-flip and then go for (say) all the R post-flip and then half G. The following session I might then try for even more luminance pre-flip and then go for all the B post-flip, and finish off the other half G... (but of course at this time of the year and heading into summer the number of sessions is going to increase... )

(As you say, any colour cast can be corrected in PS anyway, and 1:1:1 is a reasonable enough starting place anyway... but I was just wondering if I could get it better!)

[MikeWilson]

Ah... I have a manual filter wheel with only 5 slots - My usual filter allocation in the wheel is none (or IDAS if using the Equinox) in slot 1 then R,G,B,Ha in the rest. If I want to shoot SII or OIII, I open the wheel and replace the Ha, but would probably only image either SII or OIII on one night.

I also don't trust "parfocal" - I have a focusing OCD (as well as the myriad of others ) and even when Bhatinov grabber is showing "critical focus achieved", I still like to get the actual value down to +/- 10 microns if I can...

Also I tend not to usually even attempt to capture a full set of LRGB images on one night. Depending on location / object (and time of the year), I might go for a night of luminance (using the LP filter only) pre-flip and then go for (say) all the R post-flip and then half G. The following session I might then try for even more luminance pre-flip and then go for all the B post-flip, and finish off the other half G... (but of course at this time of the year and heading into summer the number of sessions is going to increase... )

(As you say, any colour cast can be corrected in PS anyway, and 1:1:1 is a reasonable enough starting place anyway... but I was just wondering if I could get it better!)

Hi Andy,

Interesting! I didn't spend too much time getting optimum focus on the ED80T CF as every time I tighten the focuser I invariably throw off the focus a little. I try up to about ten times to get in the right area but that Bahtinov Grabber application has given me an idea for autofocusing between shots.

It's time to start a new thread for that discussion as I'll probably have the FBI chasing me for hijacking this thread

The trouble is as you say - having a number of sessions to create an image and the possibility that it might leave one without a complete image.

Still, in the summer if going for nebulae - these are perfectly presentable in Ha only, or Ha + OIII and the SII is just a bonus

All the best,

Mike

[MikeWilson]

One more thing..

If the L is bang in focus but the R, G and B are just a few microns out - would that necessarily matter that the colour frames were slightly out of focus since they're going to get some heavy noise reduction/blurring during post processing anyway? (I appreciate that one would need a triplet refractor or a reflector to do this without refocusing).

Mike

[AndyUK]

One more thing..

If the L is bang in focus but the R, G and B are just a few microns out - would that necessarily matter that the colour frames were slightly out of focus since they're going to get some heavy noise reduction/blurring during post processing anyway? (I appreciate that one would need a triplet refractor or a reflector to do this without refocusing)

Hmmm - You've hit another one of my OCD's there...! I've decided (only my own decision - Not advising anyone else should do likewise!) that with the Equinox 80, due to it's reduced resolution with the 314L+ (c. 2.6 arcseconds/pixel), I'll always image unbinned as binning would reduce it to well below our "average" UK seeing. Therefore, as I'm imaging unbinned, I might as well get each channel as detailed as possible, just in case I decide not to take L at all (ie globular / open star clusters). You're right though that with Nebulae, and therefore taking L, possibly 100% critical focus may not be *quite* so important... but my focusing OCD won't let me take take images unfocused however true it might be!

(Just for completeness, I'm happy to 2x2 bin with the MN190 as its resolution with the 314L+ is c. 1.3 arcseconds/pixel, and c. 2.6 arcseconds/pixel is below the seeing limit that I've arbitrarily set... Again though, WRT focusing the RGB filters, the same as above would apply)

[MikeWilson]

Hmmm - You've hit another one of my OCD's there...! I've decided (only my own decision - Not advising anyone else should do likewise!) that with the Equinox 80, due to it's reduced resolution with the 314L+ (c. 2.6 arcseconds/pixel), I'll always image unbinned as binning would reduce it to well below our "average" UK seeing. Therefore, as I'm imaging unbinned, I might as well get each channel as detailed as possible, just in case I decide not to take L at all (ie globular / open star clusters). You're right though that with Nebulae, and therefore taking L, possibly 100% critical focus may not be *quite* so important... but my focusing OCD won't let me take take images unfocused however true it might be!

(Just for completeness, I'm happy to 2x2 bin with the MN190 as its resolution with the 314L+ is c. 1.3 arcseconds/pixel, and c. 2.6 arcseconds/pixel is below the seeing limit that I've arbitrarily set... Again though, WRT focusing the RGB filters, the same as above would apply)

Thanks for the stats - the MN190 is a good match for the 314L+ at 1.3/2.6 arcseconds/pixel. My ED80T CF gives 3.46/6.92 arcseconds/pixel which probably isn't pushing the seeing limits but I think it explains why my 2x2 binned RGB frames are giving me washed out star colours.

You've just inspired me to stop with the binning and shoot unbinned. Now that I think about it, there's not much point with such a small scope.

Anyway, it's another thread of yours that I've hijacked AND come away with something to think about for my next imaging session. So, uh, sorry AND thanks!

Best,

Mike

[swag72]

Another spacing question then - (Sorry Andy!!)

When calculating the spacing, do you need to add anymore mm for filters? I think I've read somewhere that you do.

**Edit** Answered in THIS thread - Doh!!!!!! 1mm extra it is then!

[terrym2002uk]

I find this "add a mm" biz quite interesting. I've also read a slightly different rule which is add a 1/3 of the filter thickness. But no-one ever seems to mention the thickness of the glass window in front the CCD camera ???

[MikeWilson]

I find this "add a mm" biz quite interesting. I've also read a slightly different rule which is add a 1/3 of the filter thickness. But no-one ever seems to mention the thickness of the glass window in front the CCD camera ???

That might be because a lot of cameras don't have a glass window in front of the CCD. I mentioned it on an earlier thread regarding the QHY8L which has a thick (1mm) IR blocking filter built in but the ubiquitous Atik 314L+ does not.

All the best,

Mike

[Interested]

Hi Andy,

I've thought about how to check spacing with my new setup (FLT 98 & IV flattener). Thought I might try zooming in once framed in Artemis then looking at the edge of the frame to see if the stars are round. Not had a chance to try it yet (since mid-March!) due to the weather!

I don't bother using flatteners/reducers with the 314 and never had a problem.

I've just sent off to TS for an M54-M48 adaptor so that I can start using the 383 with 2" filters (looking to buy a quadruplet refractor also).

Best wishes