Barlow on 127 Mak please explain

[coppernob]

For observing - the general rule is maximum magnification = 2 x aperture in mm so I was told.

Why then does it seem possible to use a x2 Barlow with my neximage 1 webcam on my 127mm skywatcher Mak. I have seen other threads on here that say it is possible. Surely that would result in a magnification of about 300 assuming that the neximage like over webcams approximates to the same focal length as a 10mm EP?

Sent from my iPad using Tapatalk HD

[ronin]

Suspect the answer is that for imaging via a MAK you get the movie file then throw away say 90% because they are poor quality. You cannot throw away 90% with your eyes, you see all the really awful ones as well. In effect your eye see 9 poor useless images and 1 good one in some random order.

Also a barlow just doubles the image size, and I would not have expected a lens in the webcam otherwise I would have thought you would have needed an eyepiece in to deliver collimated light to the webcam lens for this to focus the light into an image on the sensor. Assumption is you are doing prime focus imaging not afocal.

[coppernob]

Then I suppose there's no problem putting a x3 or x5 Barlow? Will be very difficult to locate the image in the chip though.

Sent from my iPad using Tapatalk HD

[JamesF]

There's a limit to the useful increase in focal length even for imaging.  There's little value for instance in increasing the effective focal length of the scope with a barlow to the point where you have, say, five pixels per arcsecond of sky if the smallest thing that can be resolved with the telescope (which is dependent on aperture) is one arcsecond.  Ideally you match the focal length of the telescope to the camera pixel size and aperture.  As things work out it happens that theoretically optimal arrangement for a given camera pixel size can be specified just as a focal ratio.

So, for a camera such as the SPC900, the theoretically optimal focal ratio is around f/35 to f/40, which with the 127 Mak is perhaps best achieved with a 3x barlow (I actually used a 2.5x barlow and an extension).  With the ASI120MC the pixels are smaller and the optimal focal ratio is actually nearer f/25, so I just use a 2x barlow.

There are other limiting factors such as the seeing.  If the sky won't play ball then there may be very little difference between working at f/30 or f/35.

James

[coppernob]

Thanks James I just happen to have a cheap astroboot x3 Plastic barlow to experiment with as well as the nicer x2

Sent from my iPad using Tapatalk HD

[lensman57]

For observing - the general rule is maximum magnification = 2 x aperture in mm so I was told.

Why then does it seem possible to use a x2 Barlow with my neximage 1 webcam on my 127mm skywatcher Mak. I have seen other threads on here that say it is possible. Surely that would result in a magnification of about 300 assuming that the neximage like over webcams approximates to the same focal length as a 10mm EP?

Sent from my iPad using Tapatalk HD

Hi,

First of  all you can not compare human eye to a CCD or Cmos sensor.  Secondly, the so called magnification relative to the aperture is one of the most misleading jargons that has been spread around by unscrupolus retailers. The quality of what you can see through a telescope depends on many factors, the seeing conditions, humidity in the air, the LP level and also on what type of the telescope you are using, the size of the obstruction if any, the optical quality of the mirror or the lens and the coatings applied and of course the aperture. I have a MaK 127  ( F11.9 ) and even with a sensitive dedicated planetary camera I would struggle to use a 2.5 X powermate on this scope let alone 3X, 5X and so on. What you need with a MAK is a very sensitive camera with small pixel size so that the image on the chip is bright enough for  very fast frame rate capture of a lot of frames. Amongst many bad ones there will be some that are good enough to be stacked and generate an image with good detail that you will develope with processing, the small pixels will help with recording fine planetary detail and this will negate the use of a barlow to enhance the imaage scale.

Regards,

A.G

[JamesF]

Lensman is absolutely right about the prevailing conditions.  They will probably affect the quality of your images more than anything.  I'd recommend getting out ther and having a go any time it's clear regardless of conditions however.  The practice is well worth the time spent, and sometimes the seeing can come and go within tens of minutes.  I did some animations of moon transits across Jupiter last apparition and the changes in seeing during the two hours or so of capture time is quite surprising.

When I was using an SPC900 I did routinely push my 127 Mak to over 3x the native focal length.  It was a real struggle with Saturn last apparition, but on the closer and brighter planets it is quite feasible, especially around opposition.  With the ASI120 there's just no point going that far and I've dropped back to the shorter focal length.  Unfortunately because of the timing of my ASI purchase I don't really have what I'd consider good images of the same target from both cameras in comparable circumstances to illustrate the difference (or lack of it .  If you're going to push things that far though I really would get an illuminated reticle eyepiece.  Getting a target anywhere on a small sensor at an effective focal length of four metres or more is a nightmare otherwise.  You don't need anything expensive.  I think mine was about £35.  I don't even bother focusing to use it, as long as the blurry disc is centred on the cross-hairs.

If I recall correctly however I think the Neximage 1 is based on the same sensor as the SPC900, so here are a some of my SPC900 images of Jupiter and Mars (which are both in the sky this winter) as examples of what ought to be possible with your camera.  Sadly Mars had its "boring side" towards us when I captured these.

James

[ollypenrice]

Attempts equate EP magnification with image scale are inherently absurd. Forget them.

Olly

[coppernob]

wow very impressive pics there James. If only it would stop raining now.

Sent from my iPad using Tapatalk HD

[coppernob]

With clear skies between storms, here are my latest attempts at Jupiter with the neximage 1, x2 Barlow and 127 Mak. I just can't seem to get more detail despite stacking in registax and doing the wavelet thing. Any ideas?

[JamesF]

I think the images might be a little heavy in the green.  Rebalancing the colours to bring out a little more red and blue might help.

Otherwise, IIRC the Neximage 1 has the same sensor as the SPC900 and DFK21AU04.  If that's correct then you might benefit from a higher factor barlow.  I have used a 2.5x barlow with a further 40mm extension between the barlow and camera.  Given a camera with a 5.6um pixel size I think you really want to have the focal length around 4.5m to 5m.  It's easier if you build up to that though, rather than going for it all in one hit.

And don't discount the fact that the seeing has been fairly dire over the UK of late.  That may well be contributing to less-than-desirable image quality.

James