Star Tails

[turfpit]

A Google search for Nexstar 4se focal reducer shows problems here that are alluded to by Chuck in the previous post.

A search on Astrobin for Nexstar 4se returns 138 results which are mostly lunar/solar/planetary with very few deep sky images.
https://www.astrobin.com/search/?q=nexstar%204se&page=2

Dave

[Buckrogers]

Hmm interesting,

so my setup is like this


4SE + 1.25 Nose Cone + ASI224MC + SVBONY 0.5 reducer

This gives me a Focal ratio of 6.5 down from 12.99

[Chuckwagon]

So how much distance is between the back of the focal reducer and the image sensor?

I don't see a spacing specification listed for the svbony reducer on their page, but they do state the focal length of the reducer to be 56mm. So that would imply that the .5 reduction will come with a spacing of 28mm between the reducer and the sensor. ZWO lists the sensor depth the ASI224MC as 12.5mm. So you would need to have a spacer of about 15.5mm length between the camera and the reducer. 12.5mm + 15.5mm = 28mm. That would get you pretty close to what should be the proper working spacing for that reducer.

If you had the reducer screwed directly into the camera, (probably hard to do with the different diameter of the threads) then the spacing would be much too short. If it is screwed into the front of the camera's 1.25" nose piece, then the spacing is probably much too long. (My ZWO nose piece is about 27mm long, so that would cause a spacing of about 39.5mm) If your nose piece is shorter it may give the right spacing. But if it's not shorter, it may be difficult to get correct spacing with that setup. You may need to find a shorter nose piece or some way to get creative with your spacers.

Cheers,
Chuck

P.S. - It is possible to use the image you got to calculate what the effective focal length ended up being for that setup. Using the formula F = (206.265 x P x U) / O where P is the size of the object in pixels as captured, U is the pixel size of the camera in microns, O is the true size of the object in arc-seconds, F will be the focal length as calculated from this formula, in mm. I'll see if I can get a minute to try calculating what the FL was for that image. You were shooting the Crescent Nebula, C27, correct?

Edit: I did a little calculating, assuming you didn't scale your image just cropped it, and I come up with a focal length of around 500mm, give or take a little. That's about a .38 reduction from the native 1325mm FL, so much stronger than anticipated. That would indicate too much space between the reducer and the sensor. If the image you posted was scaled at all I'd need the unaltered image to get a good calculation.

[Buckrogers]

Hi Chuck,

Yes I was trying to image C27. I think I have a spacer that I can use (have to check the size). Next clear night, will try so it would be like this then?

Telescope + nose cone + spacer + reducer + camera?

Thanks for helping me out in this.

[Chuckwagon]

You will probably need less space than you had. I just edited my post, so re-read the final part of it to see what I calculated.

Measure the spacing, you want it to be close to 28mm from reducer to sensor. Measure your nose piece, and if it is longer than 16mm or so you will need to get creative.

Chuck

[Chuckwagon]

Here is a link to info that may help you.

https://agenaastro.com/articles/product ... guide.html

Look at section "3.6 Focal Reducers in 1.25" and 2" Barrel Formats" and you will see a picture of an ASI224MC with a GSO .5 reducer and how it attaches. The GSO has a longer focal length than your svbony reducer, so it needs the added spacer to get to the proper working distance of 53.5mm that it wants. But the illustration should give you a good idea of what you want to do. You just need the spacing to be around 28mm instead of the 53.5mm they show. Which may be a little difficult if your nose piece is too long.

Chuck

[Buckrogers]

Thanks Chuck, I'll take a look and will measure the nose piece.

[Buckrogers]

Hi Chuck,

My nose piece is about 25mm.

[Chuckwagon]

I suspected that would be the case. My ZWO nosepiece is around 27mm, so I figured they would be similar. That makes your spacing at least 37.5mm, 12.5mm for the camera and 25mm for the nose, which causes too much reduction. You would need to find either a short nose piece, or some combination of spacers that could connect in place of the nose and get you a total of 28mm spacing. When I looked at svbony's web page for the .5 reducer, their pictures make it seem like they intend it to be attached to the base of an eyepiece. So maybe that's why it has such a short working spacing.

It's odd how there are quite a few .5 reducers on the market, that all look the same, but seem to have such different focal lengths and working spacing. Though that does mean you could try a different reducer that would need more spacing and would work with your nosepiece, like the GSO shown on that Agena page. They say it needs 51mm of working space. So you would need to add about 12.5mm of spacer (1/2") to your nosepiece to get the right spacing.

I have a screwy suggestion you could try. If you have any M42 spacers, see if you have one that is 25mm long. That should screw into the front of the camera where the nosepiece goes. Then see if you can screw the reducer into the bottom of the nosepiece instead of the top. (I can on mine) Then screw the nosepiece into the 25mm M42 spacer. That will put the reducer in the center of the mix, and should put the lens of the reducer at about 27.5mm or so from the sensor. I have attached an image showing how I mocked it up with my equipment. My camera is a 290MC, which has the same size nose as the 224. But my reducer is an Orion, so it's a bit different in size. But the image should give you an idea of how to make it work. Basically, you are trying to get the lens part of the reducer to be about 28mm from the sensor. There is a little space in the reducer from the threads to the lens, so estimate as best you can how large a spacer you would need to place it properly. I guessed about a 25mm spacer with reducer lens sticking into it about 10mm based on the svbony pictures, but you can measure it to be sure.

Maybe that will work.

Good luck,
Chuck

Reducer Config.jpg (67.67 KiB) Viewed 813 times

[Buckrogers]

A very interesting suggestion I believe I do have M42 spacers. Will try this and let you know.