When Adding a Barlow Lens...?

Mr_Cellophane

Hey folks, so I've been trying out a new x2 Barlow lens I got. I currently have a Skywatcher Newtonian, that is about 757mm focal length.
When I start the Asi air, it has the focal length of the telescope set for my camera (asi6200mc). My question is when I attach the Barlow should I then change this to then double amount? So because it's a x2 should I think change this in ASI to 1514?

Also after adding a Barlow I'm having a hard time using the auto focuser..nothing seems to be in focus anymore. I went from detecting hundreds of stars to only detexting like 70, and plate solving fails. I tried increasing the exp for each focus attempt but no luck...
Any help is appreciated!!

w7ay

Mr_Cellophane My question is when I attach the Barlow should I then change this to then double amount?

It is double the amount if and only if the back focus is correct (the location of the camera from the metal back of the Barlow). Check the documentation that came with the barlow. You should see something like this web page where the magnification is plotted as a function of the backfocus to the camera (the PowerMates are Telecentric Barlows):

https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app

Notice how the magnification can depend on the backfocus.

If there is no such documentation, return the Barlow -- it is time we stop buying stuff that is not documented, and let those companies go bankrupt.

If your focal length are "close enough," you should be able to do a plate solve in ASIAIR. The plate solve will report the true focal length (with the Barlow).

If ASIAIR Plate Solve fails, it is likely that the entered focal length is wrong, return the Barlow.

If you really want to keep using a cheap Barlow (probably produces stars as horrible blobs), set the focal length in ASIAIR Camera setting to zero, and let ASIAIR try various focal lengths (take longer to solve).

If that still fails, go here to get your plate scale (it takes a while, since you are sharing the server with lots of other people around the world), but astrometry.net will solve images that ASIAIR cannot):

https://nova.astrometry.net/upload

Chen

Mr_Cellophane

w7ay

How is it you answer (and solve) every one of my questions?? You are amazing w7ay, I really appreciate your consistent feedback.
Okay so you really seem leaning towards me sending the barlow back? IS that because it's better to just buy a telescope with the actual focal length that I'm aiming to achieve? Is a Barlow add on sort of like a lesser option than a "naturally high' focal length telescope?

w7ay

Mr_Cellophane Is a Barlow add on sort of like a lesser option than a "naturally high' focal length telescope?

I don't like using Barlows in the first place, although good Barlows will behave as if it were not there, except for providing the magnification. Unfortunately, there are too many Amazon class Barlows. The field is bound to be non-flat for imaging, and it may give you more grief than do what it is supposed to do.

And you lose two f-stops with a 2x Barlow, of course (4x exposure time to collect the same number of photons). You lose one f-stop for a 1.414x Barlow.

Chen

Mr_Cellophane

w7ay
Interesting rabbit hole....so I'd have to pretty much double my exposure time with the Barlow?
The barlow I got is an "Astromania" No idea if that means anything to you. I did have a Televue at one point... but I couldn't achieve ANY focus at all.... I need a good clear night to play around with it, It's been all clouds for a week now.... 😭😭

w7ay

Mr_Cellophane The barlow I got is an "Astromania"

I'll bet no data sheet came with it. Their products are pretty much ZWO class (they source from the same class of low-bidder Chinese OEM).

I try not to use ZWO products if possible. I won't even get near any of their glass filters. I will be continuing my UV-IR measurement project after the solar storm goes away. I will post the results probably within a week or two. You might be interested in how much UV a ZWO UV-IR window for their ASI2600 cameras (D60) block (hint: it is virtually a placebo); and their 2" UV-IR cut filter is not much better. I had bought a ZWO UV/IR and one of their camera windows just to measure and warn users who can ill afford to throw money away at stuff that don't work. You must use an external UV/IR cut (even the cheap Optolong ones) when you use a ZWO ASI2600, for example.

Be very careful what you buy, expecially when there is no way for you to validate the claims of the seller.

At least with a Barlow, you can look at the result (did the star sizes remain as expected? is the field flat?). Buy from the manufacturers who can control the quality, that way, you don't have to constantly replace the junk-- you end up spending more money with the constant upgrading (like what some people do with OTAs).

By the way, with your Televue PowerMate, follow that link I posted earlier. Place the camera sensor at the distance from the metal back (where the 42mm adapter screws into) indicated by the graph. If you do that, you do not need to worry about parfocal position of the PowerMate. The PowerMates are very forgiving with backfocus (anywhere between 0 and 100mm), although some of them will change magnification greatlu when you changel backfocus.

Parfocus is actully quite useful when you can't focus. Place a compatible eyepiece into your OTA's drawtube in the daytime, and try to focus it for your eye -- add/remove spacers until the OTA is in focus. Note the position of the eyepiece -- place the Powermate at precisely that same location.

Chen