using a barlow with spc900

[steelfixer]

Well the clouds have rolled in putting paid to another evenings stargazing so I will spend the time trying to solve a problem.

Like most people I get good images with my spc900 but if I add a barlow lens all I get is a black screen.

I have even tried in daylight but still cannot get any type of image what so ever across the entire range of my focuser.

I know plenty of folks out there use barlow lenses with thier 900's So how are you doing it ????

The barlow I have is an unbranded 3x ACHRO BARLOW

Your help and suggestions please.

Thanks

Graham

[JamesF]

Generally adding a barlow will move the point of focus (sometimes a fair way) and it will also (fairly dramatically in the case of a 3x barlow) reduce the field of view. This can mean that it's very easy to lose the target because it's moved out of the field of view, or that you can't see it because it's so far out of focus. It's perhaps even possible that the point of focus has moved so far that there's insufficient travel in the focuser to get it back.

Once you've located the target without a barlow I'd probably drop the barlow in with a low magnification EP, adjust the focus and re-centre the image, then switch EPs (perhaps more than once) increasing the magnification and refocusing each time until you get to something around a 6mm EP (6mm is supposed to have about the same fov as the camera). Then swap to the camera and see how you do.

The other possibility that crosses my mind is that perhaps the camera can't "see" the target because it isn't getting enough light. Starting at around 10fps with the gain and brightness full up might help you find it. You can then adjust them back afterwards. I'd not really have expected that to be a problem during the day though.

James

[steelfixer]

Thank you for the reply.

It raises an interesting point.

If it reduces the fov could it be possible for the barlow to move the focus point off the chip in the camera hence the black screen.

[KE400]

I had a similar problem like this too, I purchased a flip mirror which holds both an EP and also your webcam. I've found it very usefull and helpfull as you can simply switch between the 2 to see if your target is still central in your FOV while you adjust focus. I try and match both the EP mag and cam mag and focus in with the EP 1st, then when switching over to the cam you can see the image to fine focus for your imaging session.

Does your set up have tracking capabilities? If not that would also be a big help keeping your target central while you change out EP for webcam.

[starship trooper]

Yes it could move it off the chip ,i have a 2x barlow which isnt to bad to use but when I try my 4x powermate I cant get focus at all and that if its on the chip anyway.

I. Do need to mess about with different extenders to see if they will help .

Anyway keep on trying cheers dazz

[steelfixer]

Kevin - I asked that question a short time ago and the general concensis of opion is that a flip mirror on my scope will move the fp too far away and at £120.00 a throw it would be an expensive trial.

As for the tracking I have a home made motorised mount / tracking system using Nema23 stepper motors running through a CNC milling programme and I can track with near perfection over a long time span so no problems there.

Dazz - Because of what James has said I am going to make myself a reversed ep holder. As my scope has a 2" focuser and the barlow is 1.25" I figure I can turn up an ep holder for the barlow that will allow the barlow to sit inside the ota and hopefully negate the lack of focus travel in the focuser.

Time will tell.

Please keep the thoughts comming I need all the ideas and help I can get with this one.

[steelfixer]

A quick update on some further tests.

After taking the advice posted I have realised that the problem was not that the focuser would retract enough but quite the opersite.

I extended the focuser to its limit and found I had an image. A very poor image but a starting point.

I have now discovered the focus point is full entension plus a 6mm spacer ring infront of the barlow lens and I have perfect focus.

The next step is to manufacture an extension tube 75% of the length of the focus tube so I can retract the focus tube to aliveiate the flex in the focuser.

Happy days now maybe I can get that close up ( or as close as I can get ) of Saturn.

Thank you for all your input another problem with a solution.

[JamesF]

I have several barlows, including the nasty Skywatcher one that came with my ST80. Because it's very poor I unscrewed the lens from the end and it now makes a quite acceptable extension tube. If you have something similar or can lay your hands on one for the price of a couple of beers, it may be an easy solution.

James

[steelfixer]

Hi James.

I have the good fortune to own a steel fabrication company so I have all the machines I need to make one. I think the best material to use will be black nylon for the test piece and if it works out as I hope I will make the real thing out of Aluminium.

[Stephen]

200MM (f5?) with SPC880 at 3x = x500 magnification?

[Cornelius Varley]

200MM (f5?) with SPC880 at 3x = x500 magnification?

The term "magnification" is irrelevant when imaging. Cameras don't have any magnification, only a field of view. It is true that a SPC880/900 webcam gives an approximate field of view similar to a 6mm eyepiece but there is no magnification involved.

For planetary and lunar imaging the ideal f number to aim for is between F20 and F30 to get the best resolution. With a 200mm F5 you could go to a 4x (F20) or a 5x (F25) barlow with the right conditions.

Peter

[Stephen]

The term "magnification" is irrelevant when imaging. Cameras don't have any magnification, only a field of view. It is true that a SPC880/900 webcam gives an approximate field of view similar to a 6mm eyepiece but there is no magnification involved.

For planetary and lunar imaging the ideal f number to aim for is between F20 and F30 to get the best resolution. With a 200mm F5 you could go to a 4x (F20) or a 5x (F25) barlow with the right conditions.

Peter

Thanks