LRGB Help

[Oddsocks]

Hi Simon,

Not 100% certain on this but as far as I know the SW Aplanatic Coma Corrector assembly is put together by the optical builder in such a way that the back focus of 55m is measured from the mating face of the metal flange, not the glass optical element, in which case with the 14mm spacer you "should" make the distance of 55.3 which is probably close enough. In the end only an optical test will show whether your scope and this set up will produce acceptable images.

Where I have used a flatteners in the past with a SX camera and Tru-Tech filter wheel I have always gone for the adjustable Baader T2 spacers on the camera side of the wheel as this was easier to get the spacing right, but I was not using such a fast scope as you so this arrangement may introduce some vignetting in the image with the 8300 chip.

http://www.teleskop-express.de/shop/product_info.php/info/p596_Baader-variable-locking-T-2-Extension-12-16mm---33.html

I would say before worrying too much try the set up under "real life" conditions, acquire some images and look at the stars in the corners of the image, if they are round then nothing to worry about, if the stars have slight coma then some further adjustment by a different solution may be necessary.

[Singlin]

The 14mm spacer does screw down completely to the Coma corrector flange , It is 2mm short when srewed in tight.

I have gained .7 of a mm anyway (measure to the flange and not the lens).

[ollypenrice]

The presence of glass in the optical path (in my case just my LRGB or NB filters) is usually said to add a third of their thickness to the focal distance, so that would be about a mm you'd want to add to the spacing over the specified distance.

I would expect the chip distance to be fairly critical at F4 but I don't know in practice. The use of adjustable elements is, as Oddsocks says, an excellent solution. There are times though, as with my TEC140 flattener, when custom parts have to be ordered. Then most people go to Precise Parts but I think FLOdeal with a custom parts maker as well.

Olly

[Singlin]

Thank you Olly.

The custom parts web site is quite slick with it's custom product selection.

[Oddsocks]

Hi Simon,

I had a thought this morning, if you open the filter wheel casing will the M48 to M54 adaptor screw to the filter wheel face plate from the inside of the wheel and still leave enough space for the wheel and filters to rotate?

If so that would lose you the 1.8mm thickness of the flange from the back focus distance by placing it inside the filter wheel housing.

If this is possible you would need to assemble the m48 extension and m54-m48 adaptor together while the filter wheel casing is open so that you can ensure the three parts remain tightly fixed.

[Singlin]

The M48 to M54 flange has yet to arrive from teleskope Express. 

They said 6 weeks.

Good thought though, I will give it a try once I have everything here.

[Singlin]

My m54-m48 adapter arrived today.

The skywatcher coma corrector requires 55mm back focus.

I now have 56.3mm.

Taking into account 1mm extra required for the filters I am only .3 of a mm out.

Should this be sufficiently accurate?

Rgds,

Simon

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[Singlin]

This is what i get with the 383L

What am I doing wrong?

I am depressed at the moment

Inviato dal mio GT-I9300 con Tapatalk 2

[Singlin]

This is another screen shot with the bahtinov mask.

Why am I getting all of this noise?

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[Singlin]

I sent a copy of this image to the DEaler here and he said it was normal for noise and to just take a dark frame.

I find this hard to beleive. I was expecting better performance than with my dslr.

How on earth am I supposed to frame an object.

[ollypenrice]

Nothing is wrong. Relax.   You took that exposure, according to your screen grab, at 0.01 second. That is far, far too short. Try it at 3 seconds. The noise will diminish and the background will darken.

Frame using the L filter and 5 second exposures in bin 3 or somethinkg like that. Use the log stretch slider to get a good preview. (It is screen-only and has no effect on the real data.)

That looks pretty good for 0.01 second!

Olly

[Singlin]

For want of a better word I was S ___ing my self.

Ignorance can be painfull at times.

My clients asked me if everything was alright at the bar las night, I felt stressed but did not think that I looked it.

Thanks.

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[ollypenrice]

For want of a better word I was S ___ing my self.

Ignorance can be painfull at times.

My clients asked me if everything was alright at the bar las night, I felt stressed but did not think that I looked it.

Thanks.

Sent from my SM-T310 using Tapatalk 2

Olly

[Singlin]

I managed to get 1x200 sec sub of M51

It was cloudy.

No darks etc

What does the backfocus accuracy look like?

[Singlin]

This one is slightly less cliped

[Oddsocks]

Hi Simon,

Your image from post 39 is difficult to analyse precisely due to the JPG compression used and there isn't an image attached to post 40? but the post 39 image looks pretty good when zoomed up.

You would need to post an uncompressed TIFF version to certain.

In your post 39 image there is some vertical elongation on all the stars, equally, across the full image, which is probably guiding or flexure and not flattener spacing.

If the flattener spacing is too long then you would still have coma visible on the stars at the edge of the field and the tails of the coma would point towards the centre of the image.

If the flattener spacing is too short then you would see thin squashed stars at the edge of the field that are tangential to the image centre.

Here is a link to a site that shows actual images taken on an Edge HD with the flattener deliberately set too long and too short so that you know what to look for.

http://www.ancientstarlight.com/EdgeHD_First_Lightsb.html

On balance I would say you are pretty well there.

To be certain then when you have a chance take another image of a rich star field with more stars around the edges of the frame and no evidence of guiding error, zoom up the image in your photo processing software until you are looking at the individual pixels and then look for as near perfect round stars in the centre and at the extreme edges of the image. If coma tails are still evident at the edges and are pointing towards the centre then spacing is too long, if the stars are squashed and aligned tangentially to the centre then spacing is too short.

[Singlin]

Here is the link of the original

https://www.dropbox.com/s/ql76dgo806zzc6e/test%20m51_002m53.tif

[Oddsocks]

Hi Simon,

I think you can relax with the flattener spacing and go out for a drink or three...

In your post #42 linked image of M53 taken on 05/04/2014 at 22:38Hrs there is no sign of any under corrected or over corrected coma so the flattener spacing is good.

The only obvious problem is with guiding as all the stars are slightly kidney shaped, your mount is possibly bouncing because of backlash in the RA and DEC axis.

Try balancing the OTA so it is slightly heavy on the camera end and on the East side as well to reduce bouncing between the worm teeth.

[Singlin]

Thank's Bill

I am in the process of re aligning my scope with EQ Align, I have done half of it but am now at the bar as the clients are coming out of the dinning room. I will go up to the scope later.

I will make sure that the balance is adjusted accordingly.

I have an old Eq6 which is very stiff and hard to ascertain when in balance.

[Singlin]

Had  another go at M3 .

This seems better 300 sec.

https://www.dropbox.com/s/tlofsk6ou1mf6te/test%20m3b.tif

[Oddsocks]

Hi Simon,

The latest image of M3 in post #45 shows the mount is tracking much better than before and is the best test image so far, just a little bit of error in the vertical axis this time.

Zooming in to the stars in all four corners you can see that the star shapes show a slight tangential flattening but the stars in the centre are close to round so the flattener spacing is a just a little short.

You need to add a little more length to the flattener to CCD distance but as your scope is quite fast then probably not a lot, try adding about 0.6mm and take another test image when you can.

The thinnest of those Baader Delrin T2 spacer rings on the camera side of the filter wheel would be ideal.

[Singlin]

Thanks William.

I have put the thinnest delrin spacer in this morning, will do another EQAlign this evening when everyone is in the dinning room and then do another image later in the evening.

[Singlin]

Here is another attempt after putting a delrin spacer between the camera and filter wheel.

https://www.dropbox.com/s/8h332cbddo7ixzy/M3%20test%20fits.fit

https://www.dropbox.com/s/3pbh3ge5lklnbj6/M3%20test%20tiff.tiff?m=

[Singlin]

I have lots of rings which are noticeable especially with the fits version. I think these are caused by a dirty primary mirror?

[Oddsocks]

Hi Simon,

I've looked at both the TIFF and FITS images from post #48 in PS and PI, the spacing looks spot on now, the only star shape distortion evident is equal across the whole frame and is down to residual guiding errors.

I couldn't see any rings obvious in these latest images, just some random acquisition noise and some normal vignetting, there were some dust-rings, or water droplets evident in the right side of the image from your earlier post #45.

Dust and dirt on the primary does not usually show up directly in images, it is too far out of focus, the only effect it has when very bad is a reduction in contrast and an apparent reduction in resolution.

Last spring I spent a few weeks imaging with a fast 8" newtonian and there was a huge spider building a web across the primary but it didn't show up in the images, I only got rid of it when it started bringing back things to eat and somebody told me that spider droppings are very corrosive and can damage the mirror coatings.

Small rings caused by dust or water are usually to be found on the filters, corrector or camera window, the bigger they are then the further away from the camera they are, anything on the mirrors just can't be seen.

Looks like your investment in the flattener has been successful, I'm glad your initial spacing worries seem to have been resolved.

When using a flattener on a very fast scope then focussing is critical, if you are a little bit off with the focus then the flattener does not work so well and over or under compensation may creep in at the image edge so watch out for temperature induced focus changes during an imaging session and also double check for par-focality of all your filters.

Flats will remove the vignetting and the dust rings and darks will remove the acquisition noise, you can take bias frames as well but most people don't bother with the SONY CCD chipped cameras as they don't seem to generate much read noise.

I'm looking forward to seeing the finished images with your new kit....