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Old 28-10-2015, 04:40 PM
E_ri_k (Erik)
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Filter thickness?

Hey guys, I'm working out spacings before I order a couple of adapters from Precise Parts for my flattener. I was informed that I need to take into account the refractive index of the colour filters! Or at least add 1/3 the filter thickness to the length of my adapter.

Is it really necessary? It couldn't amount to a great distance, surely?

Erik
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Old 28-10-2015, 04:48 PM
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Atmos (Colin)
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With most filters it doesn't add much but Astrodon filters are 3mm thick so in that instance it can make a bit of a difference.
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Old 28-10-2015, 05:17 PM
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Just a bit of a warning about getting caught up in the fractions of a millimetre for these adapters etc. I went down that path, calculating backfocus distances, light paths and so on, taking into account manufacturer's tolerances of the camera - worked out 19mm extension tube (give or take a couple of mm depending on the fabrication tolerances) was needed. Stars weren't round. Did some testing with various length extension tubes, and found the actual required distance was 30mm! Over 10mm more than the theoretical calculated distance.

What I'm trying to suggest is that trial and error and testing of the distances first, rather than calculating, before spending money on Precise Parts fixed length extensions would be my recommendation.

I found a handy variable extension tube from Stellarvue that in combination with their other ext tubes allows me to test a wide variety of spacings before buying/making a fixed distance one.
http://www.stellarvue.com/42-mm-extensions/

PS - don't bother with the Orion variable like this one:
http://www.amazon.com/Orion-12-17mm-.../dp/B00I44ETRE
It has nylon grub screws that are very easy to damage, don't hold weight well, and just too much slop in the mechanism.
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Old 28-10-2015, 06:50 PM
E_ri_k (Erik)
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Quote:
Originally Posted by troypiggo View Post

What I'm trying to suggest is that trial and error and testing of the distances first, rather than calculating, before spending money on Precise Parts fixed length extensions would be my recommendation.
Good point Troy, i even think I have an old variable thing laying around the floor in the shed do you suggest simply visually inspecting subs, or I could download ccd inspector?

Erik
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Old 28-10-2015, 07:10 PM
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Hi Erik,

I can't remember if you have PixInsight, but if you do you can use the FWHMEccentricity and SubframeSelector scripts to achieve similar measurements to CCDI.

Cheers,
Rick.
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Old 28-10-2015, 08:22 PM
E_ri_k (Erik)
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Thanks Rick, yes I do have PI, I'll have a look at those functions. I'm 4.2mm short with all my bits and pieces connected! I'll have to improvise lol.
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Old 28-10-2015, 08:43 PM
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Originally Posted by E_ri_k View Post
Good point Troy, i even think I have an old variable thing laying around the floor in the shed do you suggest simply visually inspecting subs, or I could download ccd inspector?

Erik
My rudimentary (but successful) testing was just to start at the calculated length, and take a shot. Note the stars in the corners, and whether the elongation was outwards or inwards. Adjust the spacing in one direction, in my case increasing it, to see if the elongation got better or worse. Keep doing that until round stars. If you notice the elongation direction reverse, start moving in. All done by eye. Keep meaning to go back and measure more precisely, but I'm happy with the results as they are. YMMV.
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Old 29-10-2015, 09:09 AM
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I recently went through this exercise in order to calculate the BF of a TAK TAO-67 for use with my TEC180. The TEC FF BF is too short for my use. I used CCDI and found the results harder to ascertain than I expected. And the results seemed to vary from night to night a bit. I suggest that you find an excellent star field with nothing too bright. Find an exposure that doesn't saturate a lot of stars, but is short enough to not show any tracking errors. Then take at least several exposures for each different length and average the results. I plotted my results and could see a fairly linear progression in FWHM values towards the best position. It took me several nights of work using a micrometer. I could detect a .5mm change in position. Good luck!

Peter
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Old 29-10-2015, 09:27 AM
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Troy,

What equipment was your experience with ?
I find it hard to believe that the manufacturers specified back focal distance was so far out.
Did you include or exclude the actual length for the FF ?

My experience with multiple Tak systems has always been - get all the data - carefully !, add it all into a spreadsheet and assuming you have everything included and correctly sourced the figures will be accurate.

The difficulties that arise is finding a reliable source for the actual critical distances and also sometimes how the distances have been specified as its not always clear if its the flange or the body or something else.

Most FF have a tolerance of about a mm where providing you are within a +/- 0.5mm of theoretical the result will be OK.

So specifying things down to the nearest 0.1mm is generally a good idea - by the time you have 3 or 4 adapters, a MOAG, filter wheel, filter, camera, rotator, focuser, AO and FF it can all add up.
Plus the difference between Optical length and Physical length can get mixed up if you arent careful. eg the filters and AO.
Also with a refracter you will not only have the FF back focal distance, but you will also have the entire back focus distance as well - so that can be tricky to match both, and if you are using a MOAG - you need it to be in the right position otherwise you wont be able to match the two camera focus lengths.

Rally
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Old 29-10-2015, 01:08 PM
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Quote:
Originally Posted by rally View Post
Troy,

What equipment was your experience with ?
I find it hard to believe that the manufacturers specified back focal distance was so far out.
Did you include or exclude the actual length for the FF ?

My experience with multiple Tak systems has always been - get all the data - carefully !, add it all into a spreadsheet and assuming you have everything included and correctly sourced the figures will be accurate.

The difficulties that arise is finding a reliable source for the actual critical distances and also sometimes how the distances have been specified as its not always clear if its the flange or the body or something else.

Most FF have a tolerance of about a mm where providing you are within a +/- 0.5mm of theoretical the result will be OK.

So specifying things down to the nearest 0.1mm is generally a good idea - by the time you have 3 or 4 adapters, a MOAG, filter wheel, filter, camera, rotator, focuser, AO and FF it can all add up.
Plus the difference between Optical length and Physical length can get mixed up if you arent careful. eg the filters and AO.
Also with a refracter you will not only have the FF back focal distance, but you will also have the entire back focus distance as well - so that can be tricky to match both, and if you are using a MOAG - you need it to be in the right position otherwise you wont be able to match the two camera focus lengths.

Rally
G'day mate. Here's the thread where I posted about this at the time. All distances and tolerances were from the manufacturers. Only 4 components to consider - scope TMB92SS, Orion Field Flattener, Astrodon filters, QSI583ws camera. I was surprised at the variance too.
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Old 29-10-2015, 04:40 PM
E_ri_k (Erik)
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The FF I'm using is the TOA67 Flattener, I have been advised it has a back focus distance of 106.2 mm. I have made up a sliding adapter which gives me +\- 10mm or so either side of that figure. This is without my filterwheel because it adds to much length to the system.

I read the TOA67 Flattener has a tolerance of 2mm?

The QHY9's sensor distance is 15mm, 17mm, or 16.3mm depending on were you get your info from I found. Not sure if it's to the body surface or the flange which is about .5mm future away! The 16.3mm measurement is apparently taking into account the refractive index of the CCD chamber glass and the glass on the CCD surface, to the actual silicon layer.

I am still unsure about the colour filters affecting my measurement. I haven't been able to find out how thick QHY's colour filters are by googling, or on the forum. Might have to ask them directly. If the FF does have a tolerance of 2mm perhaps the filters won't make a difference???

I will try what you suggested Peter, find a star field and just experiment!

Erik
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Old 31-10-2015, 01:41 PM
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Hi Erik,
what about trying a VariLock spacer?

http://www.optcorp.com/ba-t2-25y-var...e-20-29mm.html


cheers
Allan
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Old 31-10-2015, 05:29 PM
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Erik,

Are you sure you even need a flattener with a KAF8300 camera? That is a fairly small sensor and field flatteners may only be needed around APS and full frame sensor sizes only in many scopes.

Greg.
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Old 31-10-2015, 05:52 PM
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Quote:
Originally Posted by gregbradley View Post
Erik,

Are you sure you even need a flattener with a KAF8300 camera? That is a fairly small sensor and field flatteners may only be needed around APS and full frame sensor sizes only in many scopes.

Greg.
Certainly needed it with my KAF8300 and TMB92SS triplet.
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Old 31-10-2015, 05:53 PM
E_ri_k (Erik)
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Hi Erik,
what about trying a VariLock spacer?

http://www.optcorp.com/ba-t2-25y-var...e-20-29mm.html


cheers
Allan
Thanks Allan, looks good, I'd still need an adapter made to go either side of it though...

Quote:
Originally Posted by gregbradley View Post
Erik,

Are you sure you even need a flattener with a KAF8300 camera? That is a fairly small sensor and field flatteners may only be needed around APS and full frame sensor sizes only in many scopes.

Greg.
That was my thought also at first, but without it there is quite a noticeable amount of distortion around the image.

I have tried Troys method of inspecting subs at different spacings, and the star shapes do vary between the different lengths. My method for inspection was a little 'lazy' mainly visual, and I managed to get round stars all around the image. I did notice a change in the FWHM also, but I think the seeing may have played a part there also.

The specified 106.2mm seems to work well, but I did get very similar FWHM values at 106.5mm-108.5mm spacings. Which is promising I guess, as there may be a fairly high amount of tolerance? I was using a tape measure, but I'll invest in a pair of callipers and double checking before ordering anything.

I will have some photos to post a bit later!

Erik
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Old 31-10-2015, 06:47 PM
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Atmos (Colin)
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Quote:
Originally Posted by gregbradley View Post
Erik,

Are you sure you even need a flattener with a KAF8300 camera? That is a fairly small sensor and field flatteners may only be needed around APS and full frame sensor sizes only in many scopes.

Greg.
I get about 45% curvature without my flattener with a KAF8300 sensor, definitely needed.
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Old 01-11-2015, 12:36 PM
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Until recently I wasn't using a flattener with my TEC180/KAF8300 combo. It showed about 28% curvature with CCDI. I could see a bit of distortion but I thought it was OK. Then I tried the TAO-67 and the resulting images are far better! My FWHM results are better too. Worth the effort for sure.

Peter
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