How important is spacing to FF on telescope side?

[Icesheet]

I had my first introduction to the world of spacing last night and like most people's first experience with this it was a nightmare!! No images for me last night

 

I have a SharpStar 60ED F5.5. This is one of the many variants of the TS, Altair, AstroTech compact refractors available on the market, although a tad faster at F5.5 compared to the usual F6 (TS sold the F5.5 version for a while). Anyway, I couldn't source a flattener for it but TS offer the Flat60 which is designed for their F5.5 and F6 versions. That flattener seems to be the same one offered by AltairAstro, so I plumped for that.

 

The optimal working distance on the camera side is stated to be 54.8mm. Typically I couldn't get that with the spacers I have. It was either 56mm or 53mm. So I plumped for 56mm to see how it turned out but try as I might I could not get an image at all. Finally I realised I didn't have enough inward focus, so I switched it out to 53mm from the reducer. Still not enough inward focus!  This confused me a until I realised that there were attachments at the telescope side of the FF. So I fiddled about with them and lo and behold I was able to get focus...of sorts. The best FWHM I could achieve was 17! At 100% crop the stars were like sausages :(. Try as might nothing would improve this.

 

Before I go and spend more money on spacers has anyone experienced this? Should i be  messing around with the adapters at the telescope side? If I get the spacing right now should I expect to see round stars or is there something else at play here? I always read about people having issues with spacing from FF to sensor but never read anything about issues on the other side.

Edit to add. My camera is a ZWO ASI1600mm. 

[Stub Mandrel]

With a 66 ED scope and an OVL flattener I've found that steps of 0.5mm in the camera side spacing make a noticeable difference.

The body of the OVL is about 50.5mm long and it does not change the focal length, so  the total distance from front of the flattener to sensor is about 107mm. Therefore it needs the tube to be wound in a lot compared to an eyepiece.

[symmetal]

The scope has a focal length of 330mm so without using focal reducers or barlows etc. the sensor should be 330mm from the front element. Just putting the field flattener in the optical path the distance from the front element to sensor is now much greater than 330mm so the focuser will need to be racked in to compensate. As you had attachments between the focuser and the flattener which may have been there to allow you to focus before, these would probably need to be removed or the focuser may not have enough inward travel to compensate for the length of the flattener.

The 54.8mm from flattener to sensor is to ensure round stars over the full frame, not to achieve focus so this 54.8mm distance needs to be maintained. At 56mm or 53mm  you should be able to easily achieve focus in the centre of your image but the outer edges of the image will have sausage shape stars.

So with the flattener in place and the sensor at 54.8mm behind it, the distance from the front element to the sensor needs to be around 330mm. It won't be exactly that as the flattener will introduce some optical correction which will modify the exact distance but you need to aim for around that with the focuser not at or very close to an end stop.

Alan

[Waldemar]

The distance FF to sensor is very critical...
According the info of your FF the backfocus is 54,8 mm you say.
Your camera has 6,5 mm backfocus, so 54,8 - 6,5 = 48,3 mm left to fill up with spacers.
This one: https://www.firstlightoptics.com/adapters/baader-varilock-29-lockable-t-2-extension-tube.html in combination with a 1" extension tube will give you all the possibillities you need to reach the sweet spot of your FF. The range will be 45-54 mm. First get that in order (± 49mm), than make sure to get in focus and after that fine tune the Varilock by trial and error untill you have the desired flatness. That may come down to very precise tuning...!

Succes!

[Icesheet]

Thanks folks, this all makes sense and what I was hoping might be the case. I'll remove from the telescope side to allow focus to be achieved and optimise the spacing towards sensor for round stars

 

Was just worried there was perhaps some spacing requirements from the element to the flattener that would make it unsuitable for my scope.

 

24 minutes ago, Waldemar said:

The distance FF to sensor is very critical...
According the info of your FF the backfocus is 54,8 mm you say.
Your camera has 6,5 mm backfocus, so 54,8 - 6,5 = 48,3 mm left to fill up with spacers.
This one: https://www.firstlightoptics.com/adapters/baader-varilock-29-lockable-t-2-extension-tube.html in combination with a 1" extension tube will give you all the possibillities you need to reach the sweet spot of your FF. The range will be 45-54 mm. First get that in order (± 49mm), than make sure to get in focus and after that fine tune the Varilock by trial and error untill you have the desired flatness. That may come down to very precise tuning...!

Succes!

I actually have that extension tube but due to the filter wheel and M48-T2 adapter in the image train, the minimum 29mm of the Baader takes me over unfortunately. Seems like I have some frustration ahead of me though! Is there any way to sort this out indoors rather than waste valuable imaging time with it?

[symmetal]

The Baader extension Waldemar mentions goes from 20 to 29 mm so may suit you. Your spacer at 29mm minimum is the longest of the Baader variable spacers.

Your filters will affect the 54.8mm flattener spacing as the distance is quoted for air spacing. The filters will refract the light rays. Adding 1/3 the thickness of the filters to the air spacing distance gets you the actual spacing distance you need. Using the Zwo filters at 2mm thick will add 0.7mm so your 54.8mm should now be 55.5mm.

There have been suggestions in the past about how to adjust this critical spacing without having to waste star time. One option is to use an artificial star set up at the end of the garden and focus it in the centre, then slew the scope so that the 'star' is at each of the corners and see if you still get round stars. I thought of having a LED lighting panel with a sheet of  aluminium foil over it with loads of tiny pin holes but it was suggested by others that a ball bearing lit by a torch will give a pinpoint reflection like an artificial star.  I did make an A2 size board with hundreds of small ball bearings glued to it to try it out but it was such a palaver trying to get it set up with the scope set on the ground I gave up. I could have left the scope on the mount if the board was hoisted up into a tree but it swinging around wouldn't have been much use.  Four foot tall earthen banks covered in weeds are around the scope, and stop it seeing the ground. I didn't build the banks, they were there as land boundaries, but do make good wind breaks and would also be handy in event of an air attack whilst at the scope. I did try with it set up indoors, looking down the hallway but light reflections off the walls spread the 'artificial star' out so was no good.

You could try something similar but in the end using real stars on moonlit or partly cloudy nights when imaging isn't so good got me there in the end. 

Alan

[Icesheet]

Thanks @symmetal and sorry @Waldemar! I just seen the 29mm and assumed it was the one I had.

 

Ok, I think your suggestion re sorting the issue on a partly cloudy or moonlit night sounds best. The ball bearing method seems like..well a ball ache! Good effort though

 

 

[Waldemar]

Almost everything can be done in daylight. Getting the distance between sensor and FF rreasonably right is a matter of measuring.
Infinity focussing can be done on a remote church. The only thing you need stars for is for final tuning of the distance between your FF and sensor.
like Symmetal said before, add 1/3 of the filterglass thickness to the backfocus distance of your FF. 
So: (54,8 + 1/3 of filterglass) - (filterwheel  + 6,5 backfocus camera) = roughly the distance FF → sensor
final tuning by trial and error is neccessary because it is very critical.

The following little tutorial makes it clear in which direction you should adjust.

FF distance to sensor.docx

[Icesheet]

Thanks, great info!