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gnomus

Atik 460Ex or 383L?

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Thank you for this. I got the advice in an e-mail, which I have re-read. That advice was definitely to lengthen (from 55mm to 56mm). I have the Baader LRGB filters. I assume it would make sense to stick with Baader if/when I get a set of NB filters in light of this backfocus issue?

Given that I have received conflicting advice, is there somewhere where I can double check the information? Here's what I have been told:

  • The normal spacing requirement is 55mm.
  • I need to add 1mm I need to reduce this by 0.66mm
  • The 383 has 17mm of back focus (the 460 has 13 mm of backfocus)
  • EFW 2 with M54 to M42 adapter = 23mm
  • M48 to T2 adapter = 10mm
Total length required = 54.33mm

For 383

17mm + 23mm + 10mm = 50mm; so I require a T2 extension tube 4.33mm long.

For 460

13mm + 23mm + 10mm = 46mm; so I require a T2 extension tube 8.33mm long.

How do I go about finding extension tubes of such odd lengths?

Hi Gnomus,

Yes I know it can be awkward with to different bits of advice to follow.

But the physics is undeniable.

When a medium of higher refractive index is inserted into the light path the light slows down whilst passing through that medium. The wave length changes as it passes through the higher index material. I.e. It is shorter. Once the light emerges from that higher refractive index medium the light wave length returns to the original wavelength. The medium, let's call it the filter, acts to lengthen the light path seen by the photons, by an amount dependant upon the refractive index.

For a refractive index of 1.5 the optical path for a 2mm thick filter is

OP=2/1.5

Therefore the optical path difference will be

OPD=Filt thickness-(2/1.5)

OPD=2-1.33 =0.66mm

Have a look at the QSI website on

www.qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

This gives a rough figure for the three filter manufacturers Astronomik, Baader and Astrodon.

But these figures are just roughly correct. It depends upon the wavelength of the light passing through the filters. The refractive index changes for different wavelengths of light, so any focus correction is only the best average. You could get very close when calculating for Narrowband filters of 3nm, but you would need to get the exact refractive index at that wavelength from the manufacturer and the exact filter thickness.

There are manufacturing tolerances in everything to consider. Each camera manufacturer has tolerances in the placement of the ccd in their cameras.

I hope this helps you.

Derek

Edited by Physopto
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Yes, it is extremely helpful. Thanks again for your assistance.

I have decided to get the 383L.

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Following on from that, I have ordered the Baader spacer rings, which will give me various options for lengthening the T2 'chain'. Assuming that my calculations are correct (see above), I will need 4.33mm in total. Does anyone know of anywhere where I might be able to get hold of a 3mm or 4mm extension tube? I would then be able to get it to 4.4mm. (Is this close enough?)

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Following on from that, I have ordered the Baader spacer rings, which will give me various options for lengthening the T2 'chain'. Assuming that my calculations are correct (see above), I will need 4.33mm in total. Does anyone know of anywhere where I might be able to get hold of a 3mm or 4mm extension tube? I would then be able to get it to 4.4mm. (Is this close enough?)

As stated in the QSI web document being within 1 to 2 mm either side is OK. But this have to be a guess! Remember that there are tolerances to consider. So the closer the better. You may even then find that it is not quite correct, but it is doubtful if you will notice. I have never tried but you would need to expand and measure the "flatness" at each edge of the subs that you take, to see if there is any curvature. Any curvature is also dependant upon the quality of the Field Flattener used. There are tolerances there as well between flatteners from the same manufacturer and match to each telescope used with them. None are perfect. In fact perfection does not exist in any thing in life. Only mathematics can give the perfect solution, but never achieved in real life applications.

I guess you get the point now, it's a compromise at best. It is most likely you will not notice at normal resolutions unless way out.

Derek

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I just had a part made by them. Very well made and unobtainable anywhere else off the shelf. But be aware they are not cheap. It cost me about £150 for the part. But it had special threads cut on each face for Tak fitting and Optec threads.

Beautifully machined and anodised though, could not fault them. Takes about 10 days after the order before posting.

But as I said a little bit out and the money could be wasted as tolerances not quite there. I just would try the cheaper option with off the peg spacers first.

Derek

Edited by Physopto

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By 'sluggish' I didn't mean slow to download, I meant slow to catch photons. However, the comment was based on a few 8300 chipped cameras that I'd seen here. They were from a maker not mentioned on this thread and it seems to me that both the QSI and Atik editions seem to respond pretty well. Good choice. The only thing is filter size. Depending on F ratio I think 1.25 filters are marginal using the Atik F/W which holds the filters further from the chip. At slower F ratios flats should sort it, or you could go for Baader's intermediate filter size aimed at this precise chip.

Olly

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By 'sluggish' I didn't mean slow to download, I meant slow to catch photons. However, the comment was based on a few 8300 chipped cameras that I'd seen here. They were from a maker not mentioned on this thread and it seems to me that both the QSI and Atik editions seem to respond pretty well. Good choice. The only thing is filter size. Depending on F ratio I think 1.25 filters are marginal using the Atik F/W which holds the filters further from the chip. At slower F ratios flats should sort it, or you could go for Baader's intermediate filter size aimed at this precise chip.

Olly

I now have the 36mm filters to go with the camera.

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Good choice made in the end, you'll have a blast with this new camera!

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Good choice made in the end, you'll have a blast with this new camera!

Thank you.  Now I just need to work out whether I should add or subtract.....

Backfocus and filters? Ah, yes, that old chestnut  :smiley:

See here for more info:

http://stargazerslounge.com/topic/236625-do-filters-add-to-back-focus/

Be warned - this thread my confuse you even more!

Regards

John

John - I don't think I'm ever going to be able to thank you properly for sending me to that thread ..... I'm off to lie down for a bit.

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Erm ...  my EFW 2 arrived today.  Can anyone point me towards some instructions as to how to fit the unmounted 36mm filters into the carousel?

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Congrats, it really is an exciting time when you jump into the CCD swimming pool. You will love it and never look back. The red anodized aluminum make you smile everytime you pull it from the box.

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Erm ...  my EFW 2 arrived today.  Can anyone point me towards some instructions as to how to fit the unmounted 36mm filters into the carousel?

OK.  Worked it out.  A bit fiddly, though.

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Thank you.  Now I just need to work out whether I should add or subtract.....

John - I don't think I'm ever going to be able to thank you properly for sending me to that thread ..... I'm off to lie down for a bit.

Hehe - my pleasure :smiley:  I think Andrew's parting shot sums up that thread perfectly:

I have a strong suspicion we are all dancing on the head of a pin but none of us are angels

Regards

John

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OK.  Worked it out.  A bit fiddly, though.

I know. Finger prints a go-go! 

Taking shape then. You'll enjoy this.

Olly

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I know. Finger prints a go-go! 

Taking shape then. You'll enjoy this.

Olly

I'd better!!!!

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Yes I remember fitting those fiddly unmounted filters.  Thank goodness they never have to come out again.

Carole 

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Thanks to those who helped me on this thread.  The camera arrived today.  At first, I had a problem when, despite installing the Artemis software as instructed, the (Windows 8) PC could not identify the camera.  I had to solve this by downloading some 'USB 1/2' drivers from the Atik website.  

This is my first CCD camera so I wasn't sure what to expect.  It's a bit strange having no 'ISO' or no 'Gain'.  It seems I just have length of exposure.  I can see I am back on that steep learning curve again!  I don't suppose someone could suggest some starting exposures for common beginner objects? 

I couldn't take any daytime test exposures (the shortest available exposure time over-exposed to whiteout).   The camera cools down to 40C below ambient.  Unfortunately, in my house the ambient was a toasty 28.3C (I assume there is some heat from the uncooled chip).  The lowest I could get it down to, therefore, was -15C.  What sort of temperature should I be aiming for when out in the field?  

I took a couple of 2 minute darks at this temperature - just so I could do something!  This is the last of these - it's a TIFF file stretched a couple of times in Photoshop.

post-39248-0-88521700-1435240422_thumb.j

I have no idea if this level of white pixels is what would be expected from a camera like this - it seems OK to me.  

A couple of the white pixels did seem a bit "smeary" - I don't know if this crop shows what I am talking about.  I've stretched it a bit more.  The smeared pixels are on the left.  I have no idea what is the significance (if any) of these.  Perhaps taking them in broad daylight does not help.

post-39248-0-94565100-1435240543.jpg

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The smearing is someting that happens to CCDs and it goes away in stacking, so don't worry about it... nor the hot ones.

/per

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Hi Gnomus,

If the manufacturer says delta 40 then it should easily get to -20 deg C out in the field. QSI state delta 40 and give figures at -20 deg C for the  noise. Strange thing is after some one here said that the Kodak chip was only OK down to -5 deg C I checked it out. Kodak performance data pdf says minimum operating  temperature is -10 Deg C.

But I would go with the manufacturers figures as they give the guarantee. If Atik says -20 dec then that's a good temp to aim for. I regularly set mine at -20 deg C, same chip. I've done that for both cameras. To be honest I doubt in practice that you will notice if it is at -10 or -20 once the darks are subtracted or you stack the images. As Per says the noise just drops out with stacking. ( Up to now I've never had enough to stack as Per does, but I trust his expertise).

Derek

Edited by Physopto

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Such an exciting time. I remember the day my first CCD arrived, like it was only 2 months ago...because it was.

Now that you will have 2 weeks of cloudy nights you can spend some time building a dark library at some typical exposure lengths.

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Such an exciting time. I remember the day my first CCD arrived, like it was only 2 months ago...because it was.

Now that you will have 2 weeks of cloudy nights you can spend some time building a dark library at some typical exposure lengths.

Yes. I'm also especially pleased that I have ordered the camera at a time when we are getting only around 2 hours of 'Nautical' darkness, and no 'Astronomical' darkness at all (according to Clear Outside).

I am learning my way around Artemis. It seems quite a nice piece of software, except for the fact that it doesn't seem to allow dithering (I believe you can dither if you have an Atik guidecam - I don't - but I haven't found the button for that yet).

I like the fact that you can sequence L>R>G>B x N in Artemis. I'm not sure if I can set up such a sequence in APT yet. The camera certainly works in APT (and I can dither).

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As regards 'bias' frames, with my Canon I shot these at the fastest available shutter speed. The 383 has a mechanical shutter, and the documentation states that one should never go below 200ms, because there is a risk of severe vignetting. In Artemis, however, I can tick a check-box labelled 'Dark' which stops the shutter from opening when I take the dark frames.

Now, for bias frames, I assume that I should leave the 'Dark' box checked. But what should I choose for exposure time? Do I go down to 0.001s (which seems to be the lowest that I can set in Artemis)?

PS: What would be a reasonable number of frames to take for bias and darks?

Edited by gnomus

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