C9.25 focal length with Crayford focuser, then X2 barlow

[Pete Presland]

I am a little confused by the info that Firecapture provides regarding focal length and resolution.

I have fitted a Baader Crayford which I would have thought would increase the focal length by approx. 150mm but the Firecapture info with the filter wheel in position as well is - Focal Length=2300mm and Resolution=0.34" 

With a x2 Televue Barlow in place it jumps to a huge Focal Length=6200mm and Resolution=0.12" Even allowing for the 50mm length of the Barlow this seems a large jump, or are the values more of a guide than exact figures.

[Uplooker]

The focal length and the focal point of your scope has not changed as a result of fitting the focuser. They are dictated by the scope. When you fit the x2 Televue Barlow, you have, in effect, altered the f/l. I would have thought that Firecapture would have said that with the x2 Televue Barlow fitted the f/l would be 6600mm.

Edit: Oops, meant to say f/l changed to 4600mm with the x2 Televue Barlow

[Pete Presland]

Thank you for replying. I though that might be the case about the focal not changing, but I would also have thought with a barlow it would 4700mm FL not the 6200mm.

[Pete Presland]

The reason I ask is because when using the 2x Barlow  it really does seem to much power, even though I know this size Barlow is used regularly with this setup.

[Stu]

As I understand it, using a 2" diagonal does change the focal length of the scope so I would imagine that using a Crayford would also have a similar effect. I think the fact that the eyepiece position has changed, and that the mirror is moved to refocus, the focal length does change. This is specific to compound scopes with moving mirrors.

[Stu]

Some info here

http://stargazerslounge.com/topic/155088-2-diagonal-and-focal-length/

[Pete Presland]

As I understand it, using a 2" diagonal does change the focal length of the scope so I would imagine that using a Crayford would also have a similar effect. I think the fact that the eyepiece position has changed, and that the mirror is moved to refocus, the focal length does change. This is specific to compound scopes with moving mirrors.

I would agree, but Firecapture tells me differently.

[DrRobin]

Hi Pete,

The only way to change the focal length of a scope is to add a lens (or curved mirror), either barlow or FR.  A plain mirror or focuser will not change the focal length.  If you depart from then optimum length behind the visual back you will need to compensate by moving the primary (SCT design) away from the optimum position and this might change the focal length slightly but not to any great extent.  It will however, reduce the performance slightly, so you should set the distance between back of the scope and ccd to be the optimum.

Adding a barlow of course changes this, but if you start in the right place without a barlow you won't be far out.  You can work out the distance to add, for a barlow, or reduce for a focal reducer, try google, or I can dig out the formula for you.

With my C11, I used a barlow and filter wheel, but due to space restrictions, put the barlow on the telescope side of the filter wheel so I was probably using slightly more distance and hence slightly more magnification than quoted by the barlow.  Perhaps this is what Firecapture is doing?

Robin 

[Pete Presland]

Hi Pete,

The only way to change the focal length of a scope is to add a lens (or curved mirror), either barlow or FR.  A plain mirror or focuser will not change the focal length.  If you depart from then optimum length behind the visual back you will need to compensate by moving the primary (SCT design) away from the optimum position and this might change the focal length slightly but not to any great extent.  It will however, reduce the performance slightly, so you should set the distance between back of the scope and ccd to be the optimum.

I have been setting the Crayford in the centre of its travel and then focusing roughly with the scopes focuser.

Adding a barlow of course changes this, but if you start in the right place without a barlow you won't be far out.  You can work out the distance to add, for a barlow, or reduce for a focal reducer, try google, or I can dig out the formula for you.

I do make a large adjustment with the scopes focuser when I add the barlow.

With my C11, I used a barlow and filter wheel, but due to space restrictions, put the barlow on the telescope side of the filter wheel so I was probably using slightly more distance and hence slightly more magnification than quoted by the barlow.  Perhaps this is what Firecapture is doing?

I also have my barlow in this position as well.

Robin 

[DrRobin]

Hi Pete,

The correct way to do it is focus the primary without the Barlow in place and then when you fit the Barlow you only adjust the Crayford out. In reality and depending on how much magnification you are running, you can't get the Crayford out far enough so have to adjust the primary as well, but if you can get away without it means the primary, secondary and the front corrector are all in the correct place.

Robin

[Stu]

Hi Pete,

The correct way to do it is focus the primary without the Barlow in place and then when you fit the Barlow you only adjust the Crayford out. In reality and depending on how much magnification you are running, you can't get the Crayford out far enough so have to adjust the primary as well, but if you can get away without it means the primary, secondary and the front corrector are all in the correct place.

Robin

Robin, I'm by no means an expert in this, but the figures I've seen imply a significant change in focal length for instance between using a 1.25" diagonal and a 2" diagonal. Your experience doesn't seem to match this?

[Merlin66]

I have a spreadsheet for the impact of the normal reducers used on the SCT.

The focal length definitely changes with rear focal distance. On my c11 with the extra distance required for the spectroscope it goes from a f10 to an f11 system.

[Dave In Vermont]

Evidently, you have a TeleVue® 2X Barlow. Normally I try not to plug TV, as this brand tends to be the most expensive gear out there. But in the case of a Barlow-lens, I make an exception. The best Barlow to use in any telescope - or optical-pathway - is one which becomes as close to being invisible. This meaning it doesn't dim-down or distort (cause 'vignetting')the image you see (or image) at the eyepiece. And TeleVue comes about the closest (and a few others I'll let someone else shamelessly plug ) to fitting this description.

I post this to anyone reading this thread who finds themself in a similar quandry and is considering either adding a Barlow to their kit - or subtracting such.

Dave

[ngwillym]

The other thing NOT to forget is that an x2 Barlow is a nominal x2 for use with an eyepiece, when used with cameras the actual magnification factor depends on the barlow lens to chip distance and the distance that the barlow is inside the focal point.

so in your case Firecapture is probably correct that because of the maths your x2 is actually acting as an x2.5 or x3 ish.

this may help to explain:-

http://astunit.com/astunit_tutorial.php?topic=barlow

[Pete Presland]

Thank you for all the comments, it has been very interesting reading them all.

I think the next planetary imaging run I am going to position the barlow between the camera and filter wheel rather than between the scope and the wheel. It will be interesting to see what Firecapture records the focal length at.

Sent from my GT-N7100 using Tapatalk

[Pete Presland]

The position of the Barlow in relation to the distance of the chip does indeed make quite difference to the focal length.

The Barlow fitted between the focuser and filter wheel gave these results - Focal Length=6200mm, Resolution=0.12"

The Barlow when fitted between the filter wheel and camera gave these results - Focal Length=5350mm, Resolution=0.14" certainly food for thought in the future.

[Merlin66]

Due to this effect, the TV telecentric Powermates are sometimes better to use....

There is little or no variation in the amplification factor with varying back focus distance.